Subcellular fractionation of a hypercellulolytic mutant, Trichoderma reesei Rut-C30: localization of endoglucanase in microsomal fraction.

The growing mycelia of Trichoderma reesei Rut-C30 are richly endowed with endoplasmic reticula and a variety of pleomorphic subcellular bodies. Mycelia of the culture growing in presence of avicel pH101 was fractionated in sucrose density gradients, and several morphologically and biochemically distinct fractions were isolated. Mycelia were homogenized in a Bead Beater, and the homogenate was freed of nucleus and wall fragments by low-speed centrifugation before fractionation. Organelle-free cytosol, which did not penetrate the gradient, contained (of the total) 72% of the vanadate-sensitive ATPase, 26% of carboxymethyl cellulase (CMCase), 2% of cytochrome c reductase, and 13% of the protein. Significant fractions separated on a gradient were light vesicles containing heavily stained material inside and ribosomes attached to the outside surface, intact vesicles resembling condensing vacuoles, large vesicles derived from the plasma membrane, and heavy vesicles containing crystalline material. The light-vesicle fraction contained a large portion of the cell-bound CMCase activity. The particle-bound ATPase and cytochrome c reductase activities were concentrated in heavy fractions. The fractionation in the presence of MgCl2 improved the preservation of subcellular bodies derived from the endoplasmic reticula. Although the CMCase activity of the light-vesicle fraction was 4 times higher than the activity in the heavy-vesicle fraction, the CMCase antibody-binding capacities of both fractions were about the same. This discrepancy between the catalytic activity and the antibody-binding capacity suggests that the heavy vesicles might have contained considerable amount of inactive CMCase compared with that present in the light vesicles.     

Subcellular fractionation of pig coronary artery smooth muscle

A detailed procedure for subcellular fractionation of the smooth muscle from pig coronary arteries based on dissection of the proper tissue, homogenization, differential centrifugation and sucrose density gradient centrifugation is described. A number of marker enzymes and Ca2+ uptake in presence or absence of oxalate, ruthenium red and azide were studied. The ATP-dependent oxalate-independent azide- or ruthenium red-insensitive Ca2+ uptake, and the plasma membrane markers K+-activated ouabain-sensitive p-nitrophenylphosphatase, 5'-nucleotidase and Mg2+-ATPase showed maximum enrichment in the F2 fraction (15-28% sucrose) which was also contaminated with the endoplasmic reticulum marker NADPH: cytochrome c reductase, and to a small extent with the inner mitochondrial marker cytochrome c reductase, and also showed a small degree of oxalate stimulation of the Ca2+ uptake. F3 fraction (28-40% sucrose) was maximally enriched in the ATP- and oxalate-dependent azide-insensitive Ca2+ uptake and the endoplasmic reticulum marker NADPH: cytochrome c reductase but was heavily contaminated with the plasma membrane and the inner mitochondrial markers. The mitochondrial fraction was enriched in cytochrome c oxidase and azide- or ruthenium red-sensitive ATP-dependent Ca2+ uptake but was heavily contaminated with other membranes. Electron microscopy showed that F2 contained predominantly smooth surface vesicles and F3 contained smooth surface vesicles, rough endoplasmic reticulum and mitochondria. The ATP-dependent azide-insensitive oxalate-independent and oxalate-stimulated Ca2+ uptake comigrated with the plasma membrane and the endoplasmic reticulum markers, respectively, and were preferentially inhibited by digitonin and phosphatidylserine, respectively. This study establishes a basis for studies on receptor distribution and further Ca2+ uptake studies to understand the physiology of coronary artery vasodilation.     

Hepatic subcellular distribution of short-chain beta-ketoacyl coenzyme A reductase and trans-2-enoyl coenzyme A hydratase: 25- to 50-fold stimulation of microsomal activities by the peroxisome proliferator, di-(2-ethylhexyl)phthalate

The present study demonstrates unequivocally the existence of short-chain trans-2-enoyl coenzyme A (CoA) hydratase and beta-ketoacyl CoA reductase activities in the endoplasmic reticulum of rat liver. Subcellular fractionation indicated that all four fractions, namely, mitochondrial, peroxisomal, microsomal, and cytosolic contained significant hydratase activity when crotonyl CoA was employed as the substrate. In the untreated rat, based on marker enzymes and heat treatment, the hydratase activity, expressed as mumol/min/g liver, wet weight, in each fraction was: mitochondria, 684; peroxisomes, 108; microsomes, 36; and cytosol, 60. Following di-(2-ethylhexyl)phthalate (DEHP) treatment (2% (v/w) for 8 days), there was only a 20% increase in mitochondrial activity; in contrast, peroxisomal hydratase activity was stimulated 33-fold, while microsomal and cytosolic activities were enhanced 58- and 14-fold respectively. A portion of the cytosolic hydratase activity can be attributed to the component of the fatty acid synthase complex. Although more than 70% of the total hydratase activity was associated with the mitochondrial fraction in the untreated rat, DEHP treatment markedly altered this pattern; only 11% of the total hydratase activity was present in the mitochondrial fraction, while 49 and 29% resided in the peroxisomal and microsomal fractions, respectively. In addition, all four subcellular fractions contained the short-chain NADH-specific beta-ketoacyl CoA (acetoacetyl CoA) reductase activity. Again, in the untreated animal, reductase activity was predominant in the mitochondrial fraction; following DEHP treatment, there was marked stimulation in the peroxisomal, microsomal, and cytosolic fractions, while the activity in the mitochondrial fraction increased by only 39%. Hence, it can be concluded that both reductase and hydratase activities exist in the endoplasmic reticulum in addition to mitochondria, peroxisomes, and soluble cytoplasm.     

DIFFERENCES IN THE SUBCELLULAR AND SUBSYNAPTOSOMAL DISTRIBUTION OF THE PUTATIVE ENDOPLASMIC RETICULUM MARKERS, NADPH-CYTOCHROME c REDUCTASE, ESTRONE SULFATE SULFOHYDROLASE AND CDP-CHOLINE DIACYLGLYCEROL CHOLINEPHOSPHOTRANSFERASE IN RAT BRAIN

Abstract— A comprehensive study has been undertaken on the subcellular and subsynaptosomal distribution of a number of markers for subcellular organelles in preparations from rat brain. Although the activity of most enzymatic markers was decreased by freezing and storage at - 70^oC, no significant changes were noted in the distribution of these activities. This demonstrates that contamination of brain fractions by subcellular organelles can be accurately assessed after freezing and thawing. A marked discrepancy was noted between the distribution of three putative markers for endoplasmic reticulum. CDP-choline-diacylglycerol cholinephosphotransferase (EC 2.7.8.1) activity was mainly limited to the microsomal fraction and was present to a lesser extent in the synaptosomal fraction than the other putative markers for endoplasmic reticulum. Estrone sulfate sulfohydrolase (EC 3.1.6.2) activity demonstrated a bimodal distribution between the crude nuclear and microsomal fractions. However, considerable activity was associated with the synaptosomal fraction. NADPH-cytochrome c reductase (EC 2.3.1.15) activity sedimented in the microsomal and the synaptosomal fractions. Calculations based on the relative specific activities of the microsomal and synaptic plasma membrane fraction indicated that the contamination of the synaptic plasma membranes by endoplasmic reticulum was 44.5% (NADPH-cytochrome c reductase), 38.0% (estrone sulfatase) and 9.0% (cholinephosphotransferase). Since it is believed that virtually all of the synthesis of phosphatidylcholine by cholinephosphotransferase occurs in the neuronal and glial cell bodies, it was concluded that cholinephosphotransferase is a satisfactory marker for the endoplasmic reticulum derived from these sources. The results suggest that NADPH-cytochrome c reductase and estrone sulfatase may be present in the smooth endoplasmic reticulum system responsible for the fast transport of macromolecules along the axon to the nerve endings as well as in the endoplasmic reticulum of the cell bodies. The possible relation between that portion of the smooth endoplasmic reticulum involved in fast axonal transport and the GERL (Golgi, Endoplasmic Reticulum, Lysosomes) complex discovered by Novikoff and his coworkers (Novikoff , 1976) is discussed.     

PREPARATION AND CHARACTERIZATION OF A PLASMA MEMBRANE FRACTION FROM ISOLATED FAT CELLS

A rapid method of preparing plasma membranes from isolated fat cells is described. After homogenization of the cells, various fractions were isolated by differential centrifugation and linear gradients. Ficoll gradients were preferred because total preparation time was under 3 hr. The density of the plasma membranes was 1.14 in sucrose. The plasma membrane fraction was virtually uncontaminated by nuclei but contained 10% of the mitochondrial succinic dehydrogenase activity and 25–30% of the RNA and reduced nicotinamide adenine dinucleotide cytochrome c reductase activity of the microsomal fraction. Part of the RNA and NADH-cytochrome c reductase activity was believed to be native to the plasma membrane or to the attached endoplasmic reticulum membranes demonstrated by electron microscopy. The adenyl cyclase activity of the plasma membrane fraction was five times that of Rodbell's "ghost" preparation and retained sensitivity to epinephrine. The plasma membrane ATPase activity was five times that of the homogenate and microsomal fractions. Electron microscopic evidence suggested contamination of the plasma membrane fraction by other subcellular components to be less than the biochemical data indicated.     

The subcellular distribution of adenylate and guanylate cyclases in murine lymphoid cells.

Membrane vesicles can be prepared from murine lymphoid cells by nitrogen cavitation and fractionated by sedimentation through nonlinear sucrose density gradients. Two subpopulations of membrane vesicles, PMI and PMII, can be distinguished on the basis of sedimentation rate. The subcellular distribution of adenylate and guanylate cyclases in these membrane subpopulations have been compared with the distribution of a number of marker enzymes. Approximately 20-30% of the total adenylate and guanylate cyclase activity is located at the top of the sucrose gradient (soluble enzyme), the remainder of the activity being distributed in the PMI and PMII fractions (membrane-bound enzyme). More than 90% of the 5'-nucleotidase and NADH oxidase activities detected in lymphoid cell homogenates are located in PMI and PMII fractions, whereas succinate cytochrome c reductase activity is detected only in the PMII fractions. In addition, beta-galactosidase activity is distributed in the soluble and PMII fractions of the sucrose density gradients. On the basis of the fractionation patterns of these various enzyme activities, it appears that PMI fractions contain vesicles of plasma membrane and endoplasmic reticulum, whereas PMII fractions contain mitochondria, lysomes, and plasma membrane vesicles. Approximately 30-40% of the adenylate and guanylate cyclase activities in PMII can be converted to a PMI-like form following dialysis and resedimentation through a second nonlinear sucrose gradient. Adenylate and guanulate cyclases can be distinguished on the basis of sensitivity to nonionic detergents.     

Liver microsomes; an integrated morphological and biochemical study

Rat liver, liver homogenates, and microsome fractions separated therefrom were examined systematically in the electron microscope in sections of OsO(4)-fixed, methacrylate-embedded tissue and pellets. It was found that most microsomes are morphologically identical with the rough surfaced elements of the endoplasmic reticula of hepatic cells. They appear as isolated, membrane-bound vesicles, tubules, and cisternae which contain an apparently homogeneous material of noticeable density, and bear small, dense particles (100 to 150 A) attached to their outer aspect. In solutions of various osmolar concentrations they behave like osmometers. The findings suggest that they derive from the endoplasmic reticulum by a generalized pinching-off process rather than by mechanical fragmentation. The microsome fractions contain in addition relatively few vesicles free of attached particles, probably derived from the smooth surfaced parts of the endoplasmic reticula. Dense, peribiliary bodies represent a minor component of the same fractions. The microsomes derived from 1 gm. wet weight liver pulp contained (averages of 10 experiments) 3.09 mg. protein N, 3.46 mg. RNA (RNA/protein N = 1.12), and 487 microg. phospholipide P. They displayed DPNH-cytochrome c reductase activity and contained an alcohol-soluble hemochromogen. The microsome preparations proved resistant to washing and "aging." Treatment with versene and incubation with ribonuclease (30 minutes at 37 degrees C.) resulted in appreciable losses of RNA and in partial or total disappearance of attached particles. Treatment with deoxycholate (0.3 to 0.5 per cent, pH = 7.5) induced a partial clarification of the microsome suspensions which, upon centrifugation, yielded a small pellet of conglomerated small, dense particles (100 to 150 A) with only occasionally interspersed vesicles. The pellet contained approximately 80 to 90 per cent of the RNA and approximately 20 per cent of the protein N of the original microsomes. The supernatant accounted satisfactorily for the materials lost during deoxycholate treatment. The findings suggest that the microsomal RNA is associated with the small particles whereas most of the protein and nearly all of the phospholipide, hemochromogen, and DPNH-cytochrome c reductase activity are associated with the membrane or content of the microsomes.     

A comparison of vesicles derived from terminal cisternae and longitudinal tubules of sarcoplasmic reticulum isolated from rabbit skeletal muscle

Sarcoplasmic reticulum vesicles were separated into heavy (derived from terminal cisternae) and light (derived from longitudinal tubules) fractions, according to Meissner [Biochim. Biophys. Acta, 389, 51-68 (1975)]. The similar Ca2+ sensitivities of phosphoprotein formation, ATPase activity and calcium uptake, and the similar phosphoprotein turnover rates (ATPase/phosphoprotein formation) of both fractions indicate that the same ATPase enzyme is present in the terminal cisternae and longitudinal sarcoplaxmic reticulum. The higher V for Ca2+-activated ATPase activity and calcium uptake in the light fraction correlated with the higher concentration of ATPase enzyme per mg of membrane protein in this fraction. In both the presence and absence of calcium-precipitating anions, the light fraction stored more calcium than the heavy. The Ca2+ dependence of calcium release after addition of EGTA appeared similar in both fractions, but the rate of calcium release was more rapid in the light fraction. These findings suggest that calcium release may occur more rapidly from longitudinal than terminal cisternae portions of the sarcoplasmic reticulum and that calcium release, like calcium uptake, may be mediated by the ATPase enzyme in the sarcoplasmic reticulum membrane. Although the activation energies for Ca2+-activated ATPase activity above and below the transition temperature were significantly different for the heavy and light fractions, their transition temperatures were similar. Partial purification of the ATpase enzyme by deoxycholate treatment modified the activation energies of the light but not the heavy fraction and caused the activation energies to become similar. The phosphoprotein levels of heavy and light vesicles did not become similar after deoxycholate treatment, although gel electrophoretograms indicated both samples contained > 90% ATPase protein. These results indicate the protein-lipid associations in these two fractions may be different.     

LIVER MICROSOMES : AN INTEGRATED MORPHOLOGICAL AND BIOCHEMICAL STUDY

Rat liver, liver homogenates, and microsome fractions separated therefrom were examined systematically in the electron microscope in sections of OsO₄-fixed, methacrylate-embedded tissue and pellets. It was found that most microsomes are morphologically identical with the rough surfaced elements of the endoplasmic reticula of hepatic cells. They appear as isolated, membrane-bound vesicles, tubules, and cisternae which contain an apparently homogeneous material of noticeable density, and bear small, dense particles (100 to 150 A) attached to their outer aspect. In solutions of various osmolar concentrations they behave like osmometers. The findings suggest that they derive from the endoplasmic reticulum by a generalized pinching-off process rather than by mechanical fragmentation. The microsome fractions contain in addition relatively few vesicles free of attached particles, probably derived from the smooth surfaced parts of the endoplasmic reticula. Dense, peribiliary bodies represent a minor component of the same fractions. The microsomes derived from 1 gm. wet weight liver pulp contained (averages of 10 experiments) 3.09 mg. protein N, 3.46 mg. RNA (RNA/protein N = 1.12), and 487 µg. phospholipide P. They displayed DPNH-cytochrome c reductase activity and contained an alcohol-soluble hemochromogen. The microsome preparations proved resistant to washing and "aging." Treatment with versene and incubation with ribonuclease (30 minutes at 37°C.) resulted in appreciable losses of RNA and in partial or total disappearance of attached particles. Treatment with deoxycholate (0.3 to 0.5 per cent, pH = 7.5) induced a partial clarification of the microsome suspensions which, upon centrifugation, yielded a small pellet of conglomerated small, dense particles (100 to 150 A) with only occasionally interspersed vesicles. The pellet contained ~80 to 90 per cent of the RNA and ~20 per cent of the protein N of the original microsomes. The supernatant accounted satisfactorily for the materials lost during deoxycholate treatment. The findings suggest that the microsomal RNA is associated with the small particles whereas most of the protein and nearly all of the phospholipide, hemochromogen, and DPNH-cytochrome c reductase activity are associated with the membrane or content of the microsomes.     

Comparison of the properties of active Ca2+ transport by the islet-cell endoplasmic reticulum and plasma membrane

The properties of active or ATP-dependent calcium transport by islet-cell endoplasmic reticulum and plasma membrane-enriched subcellular fractions were directly compared. These studies indicate that the active calcium transport systems of the two membranes are fundamentally distinct. In contrast to calcium uptake by the endoplasmic reticulum-enriched fraction, calcium uptake by islet-cell plasma membrane-enriched vesicles exhibited a different pH optimum, was not sustained by oxalate, and showed an approximate 30-fold greater affinity for ionized calcium. A similar difference in affinity for calcium was exhibited by the Ca²⁺-stimulated ATPase activities which are associated with these islet-cell subcellular fractions. Consistent with the effects of calmodulin on calcium transport, calmodulin stimulated Ca²⁺-ATPase in the plasma membranes, but did not increase calcium-stimulated ATPase activity in the endoplasmic reticulum membranes. The physiological significance of the differences observed in calcium transport by the endoplasmic reticulum and plasma membrane fractions relative to the regulation of insulin secretion by the islets of Langerhans is discussed.     

FRACTIONATION OF OLFACTORY TISSUE HOMOGENATES. ISOLATION OF A CONCENTRATED PLASMA MEMBRANE FRACTION

Abstract— Differential and sucrose-density-gradient centrifugation techniques were used for studies on the separation of subcellular particles from rabbit brain and olfactory tissue. Comparisons were made among various fractions from the two types of tissue. These comparisons included protein concentration and enzyme activities of the individual fractions as well as their distribution in subfractions from density gradient separations. In tissue whole homogenates, the percentage of total ATPase activity as ouabain sensitive Na⁺-K⁺ ATPase activity was about 4 times greater in brain cortex (63 per cent) than in olfactory tissue (17 per cent). Cytochrome oxidase and Na⁺-K⁺ ATPase activities were used to indicate the presence and the concentration of mitochondria and of the plasma membranes. A fraction with properties similar to the mitochondria plus nerve ending fraction from brain homogenates (fraction B) was obtained from olfactory tissue. Nerve ending concentration subfractions (B₂) were prepared from the B primary fractions. Plasma membrane subfractions were obtained by osmotic shock treatment of B₂, In the fraction of plasma membrane from olfactory tissue (E₂), 56 per cent of the total ATPase activity was Na⁺-K⁺ ATPase activity. In E₂ from brain 71 per cent was Na⁺-K⁺ ATPase activity. Deoxycholate (DOC)-treated fractions containing nerve endings from brain preparations showed much greater increase in cytochrome oxidase activity than did similar fractions from olfactory tissue. DOC treatment increased the NADH cytochrome c reductase activity of all fractions and subfractions from brain, while it decreased activity in all but one fraction from olfactory tissue. DOC treatment decreased both the Mg²⁺ and Na⁺-K⁺ ATPase activities in both types of tissue. Electron photomicrographs of olfactory B₂, B₃, E₂ and E₃ show clear morphological differences among these subfractions. The presence of possible cilia and basal bodies on vesicles in B₂ gives morphological evidence for the presence of terminal swellings in this subtraction in agreement with enzyme marker activity results.     

The lateral organization of components of the membrane skeleton and superoxide generation in the plasma membrane of stimulated human neutrophils

Studies were performed to examine the lateral organization of the NADPH oxidase system in the plasma membrane of human neutrophils. Analysis of the subcellular fractionation of human neutrophils by isopycnic sedimentation of cavitated cell lysates suggested that there may be more than one population of plasma membrane vesicles formed upon cell disruption. One population (30-32% sucrose) contained surface accessible wheat germ agglutinin binding sites, alkaline phosphatase activity, and cytochrome b. Another population (34-36% sucrose) contained membrane-bound flavin and, when the cells were prestimulated with phorbol myristate acetate (PMA), NADPH-dependent superoxide generating activity. Approximately 25% of the neutrophil cytochrome b cosedimented with the heavy population, confirming our previous hypothesis (Parkos et al. (1985) J. Biol. Chem. 260, 6541-6547) that only a fraction of the total cellular cytochrome b is involved in superoxide production. The heavy plasma membrane fraction was also enriched in membrane associated actin and fodrin as detected by Western blot analysis. After extraction of the plasma membrane vesicles with detergent cocktails, the majority of superoxide generating activity remained associated with the detergent insoluble pellet. Western blot analysis demonstrated that the pellets were also enriched in actin. Further analysis of these pellets using rate-zonal detergent-containing sucrose density gradients indicated that the superoxide generating complex had an approximate sedimentation coefficient of 80 S, suggesting that the neutrophil superoxide generating system may form a complex on the plasma membrane which is associated with or somehow organized by the membrane skeletal matrix. This organization may be of functional relevance not only to the actual production of superoxide, but also to the targeting of microbicidal oxidants.     

Resolution and reconstitution of succinate-cytochrome c reductase: preparations and properties of high purity succinate dehydrogenase and ubiquinol-cytochrome c reductase

An improved method was developed to sequentially fractionate succinate-cytochrome c reductase into three reconstitutive active enzyme systems with good yield: pure succinate dehydrogenase, ubiquinone-binding protein fraction and a highly purified ubiquinol-cytochrome c reductase (cytochrome b-c1 III complex). An extensively dialyzed succinate-cytochrome c reductase was first separated into a succinae dehydrogenase fraction and the cytochrome b-c1 complex by alkali treatment. The resulting succinate dehydrogenase fraction was further purified to homogeneity by the treatment of butanol, calcium phosphate gel adsorption and ammonium sulfate fractionation under anaerobic condition in the presence of succinate and dithiothreitol. The cytochrome b-c1 complex was separated into chtochrome b-c1 III complex and ubiquinone-binding protein fractions by careful ammonium acetate fractionation in the presence of deoxycholate. The purified succinate dehydrogenase contained only two polypeptides with molecular weights of 70 000 anbd 27 000 as revealed by the sodium dodecyl sulfate polyacrylamide gel electrophoretic pattern. The enzyme has the reconstitutive activity and a low Km ferricyanide reductase activity of 85 mumol succinate oxidized per min per mg protein at 38 degrees C. Chemical composition analysis of cytochrome b-c1 III complex showed that the preparation was completely free of contamination of succinate dehydrogenase and ubiquinone-binding protein and was 30% more pure than the available preparation. When these three components were mixed in a proper ratio, a thenoyltrifluoroacetone- and antimycin A-sensitive succinate-cytochrome c reductase was reconstituted.     

Developmental changes in the distribution of 3-hydroxy-3-methylglutaryl coenzyme A reductase among subcellular fractions of rat brain.

—The distribution of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase (EC 1.1.1.34) relative to that of several biochemical markers has been studied in subcellular fractions prepared from the brains of rats, aged 4 days to adult, by differential centrifugation. In the brains of 10-day-old animals fractions which sedimented at 800 g (P₁ and 9000 g (P₂) contained 28% and 65% respectively of the total reductase activity. A similar distribulion of the microsomal marker, NADPH-cytochrome c reductase, suggested that the HMG-CoA reductase activity in the low-speed pellets was due to substantial contamination of these fractions with endoplasmic reticulum. When P₂ was fractionated on a discontinuous sucrose gradient, the distributions of protein, RNA and NADPH-cytochrome c reductase paralleled that of HMG-CoA reductase, indicaling a non-specific association of endoplasmic reliculum and HMG-CoA reductase with all of the structures sedimenting in P₂. As brain maturation proceeded and a greater percentage of total brain protein (primarily associated with myelin) sedimenled in P₁, the subcellular distributions of HMG-CoA reductase and the microsomal marker changed in a parallel way. By 21 days P₁ contained nearly all of the reductase activity. Because the specific activity of HMG-CoA reductase in P₁ decreased steadily between 4 and 21 days, while the specific activity of 2′:3′-cyclic nucleotide 3′-phosphohydrolase in this fraction increased in a coordinate fashion, we conclude that the reductase is not an integral component of myelin, and probably is associated exclusively with the endoplasmic reticulum included in P₁. In view of the developmental changes in the distribution of HMG-CoA reductase among subcellular fraclions, we suggest that whole homogenates (or comparable tissue extracts) should be utilized to evaluate reductase activity in the developing brain.     

Studies on the heterogeneity of sarcoplasmic reticulum vesicles.

Isolated sarcoplasmic reticulum vesicles from rabbit white muscle were separated into a light (15--20% of total microsomes) and a heavy (80--85%) fraction by density gradient centifugation. The ultrastructure, chemical composition, enzymic activities and localization of membrane components in the vesicles of both fractions were investigated. From the following results it was concluded that both fractions are derived from the membranes of the sarcoplasmic reticulum system of the muscle: (i) The protein pattern of both fractions is essentially the same, except for different ratios of acidic, Ca2+-binding proteins. (ii) The 105000 dalton protein of the light fraction cross-reacts immunologically with the Ca2+-dependent ATPase of the heavy fraction. (iii) Ca2+-dependent ATPase, although of different specific activity, is found in both fractions. After rendering the vesicles leaky, specific activities in both fractions reach the same value. The light fraction was found to consist of "inside-out" vesicles by the following criteria: (i) No Ca2+ accumulation can be measured and the Ca2+-dependent ATPase activity is low and variable. (ii) The rate of trypsin digestion is lower and, compared to the heavy microsomes, a different ratio of degradation products is obtained. (iii) The sarcoplasmic reticulum membrane has a highly asymmetrical lipid distribution. This distribution of aminophospholipids is opposite to that in vesicles of heavy fraction. The light sarcoplasmic reticulum fraction has a higher phospholipid to protein ratio than the heavy one. This is consistent with the possibility that the two fractions derive from different parts of the sarcoplasmic reticulum system.     

The latency of rat liver microsomal protein disulphide-isomerase.

Protein disulphide-isomerase (PDI) activity was not detectable in freshly prepared rat liver microsomes (microsomal fraction), but became detectable after treatments that damage membrane integrity, e.g. sonication, detergent treatment or freezing and thawing. Maximum activity was detectable after sonication. Identical latency was observed in microsomes prepared by gel filtration and in those prepared by high-speed centrifugation. PDI activity was latent in all particulate subcellular fractions, but not latent in the high-speed supernatant. When all fractions were sonicated to expose total PDI activity, PDI was found at highest specific activity in the microsomal fraction and co-distributed with marker enzymes of the endoplasmic reticulum. Washing of microsomes under various conditions that removed peripheral proteins and, in some cases, bound ribosomes did not remove significant quantities of PDI, nor did it affect the latency of PDI activity. Treatment of microsomes with proteinases, under conditions where the permeability barrier of the microsomal vesicles was maintained intact, did not inactivate PDI significantly or affect its latency. PDI was very readily solubilized from microsomal vesicles by low concentrations of detergents, which removed only a fraction of the total microsomal protein. In all these respects, PDI resembled nucleoside diphosphatase, a marker peripheral protein of the luminal surface of the endoplasmic reticulum, and differed from NADPH: cytochrome c reductase, a marker integral protein exposed at the cytoplasmic surface of the membrane. The data are compatible with a model in which PDI is loosely associated with the luminal surface of the endoplasmic reticulum, a location consistent with the proposed physiological role of the enzyme as catalyst of formation of native disulphide bonds in nascent and newly synthesized secretory proteins.     

Isolation of highly purified fractions of plasma membrane and tonoplast from the same homogenate of soybean hypocotyls by free-flow electrophoresis

A procedure is described whereby highly purified fractions of plasma membrane and tonoplast were isolated from hypocotyls of dark-grown soybean (Glycine max L. var Wayne) by the technique of preparative free-flow electrophoresis. Fractions migrating the slowest toward the anode were enriched in thick (10 nanometers) membranes identified as plasma membranes based on ability to bind N-1-naphthylphthalamic acid (NPA), glucan synthetase-II, and K⁺-stimulated, vanadate-inhibited Mg²⁺ ATPase, reaction with phosphotungstic acid at low pH on electron microscope sections, and morphological evaluations. Fractions migrating farthest toward the anode (farthest from the point of sample injection) were enriched in membrane vesicles with thick (7-9 nanometers) membranes that did not stain with phosphotungstic acid at low pH, contained a nitrate-inhibited, Cl-stimulated ATPase and had the in situ morphological characteristics of tonoplast including the presence of flocculent contents. These vesicles neither bound NPA nor contained levels of glucan synthetase II above background. Other membranous cell components such as dictyosomes (fucosyltransferase, latent nucleosidediphosphate phosphatase), endoplasmic reticulum vesicles (NADH- and NADPH- cytochrome c reductase), mitochondria (succinate-2(p-indophenyl)-3-p-nitrophenyl)-5-phenyl tetrazolium-reductase and cytochrome oxidase) and plastids (carotenoids and monogalactosyl diglyceride synthetase) were identified on the basis of appropriate marker constituents and, except for plastid thylakoids, had thin (<7 nanometers) membranes. They were located in the fractions intermediate between plasma membrane and tonoplast after free-flow electrophoretic separation and did not contaminate either the plasma membrane or the tonoplast fraction as determined from marker activities. From electron microscope morphometry (using both membrane measurements and staining with phosphotungstic acid at low pH) and analysis of marker enzymes, both plasma membrane and tonoplast fractions were estimated to be about 90% pure. Neither fraction appeared to be contaminated by the other by more than 3%.     

Calcium Transport in Sealed Vesicles from Red Beet (Beta vulgaris L.) Storage Tissue : I. Characterization of a Ca-Pumping ATPase Associated with the Endoplasmic Reticulum

Calcium transport was examined in microsomal membrane vesicles from red beet (Beta vulgaris L.) storage tissue using chlorotetracycline as a fluorescent probe. This probe demonstrates an increase in fluorescence corresponding to calcium accumulation within the vesicles which can be collapsed by the addition of the calcium ionophore A23187. Calcium uptake in the microsomal vesicles was ATP dependent and completely inhibited by orthovanadate. Centrifugation of the microsomal membrane fraction on a linear 15 to 45% (w/w) sucrose density gradient revealed the presence of a single peak of calcium uptake which comigrated with the marker for endoplasmic reticulum. The calcium transport system associated with endoplasmic reticulum vesicles was then further characterized in fractions produced by centrifugation on discontinous sucrose density gradients. Calcium transport was insensitive to carbonylcyanide m-chlorophenylhydrazone indicating the presence of a primary transport system directly linked to ATP utilization. The endoplasmic reticulum vesicles contained an ATPase activity that was calcium dependent and further stimulated by A23187 (Ca(2+), A23187 stimulated-ATPase). Both calcium uptake and Ca(2+), A23187 stimulated ATPase demonstrated similar properties with respect to pH optimum, inhibitor sensitivity, substrate specificity, and substrate kinetics. Treatment of the red beet endoplasmic reticulum vesicles with [gamma-(32)P]-ATP over short time intervals revealed the presence of a rapidly turning over 96 kilodalton radioactive peptide possibly representing a phosphorylated intermediate of this endoplasmic reticulum associated ATPase. It is proposed that this ATPase activity may represent the enzymic machinery responsible for mediating primary calcium transport in the endoplasmic reticulum linked to ATP utilization.     

The localization of the anion-sensitive ATPase activity in corneal endothelium

The localization of the anion-sensitive ATPase (EC 3.6.1.3) of bovine corneal endothelium has been investigated. Homogenates were fractionated by differential and density gradient centrifugation, into fractions enriched in plasma membranes and mitochondria. (Na+ + K+)-ATPase (EC 3.6.1.3) and cytochrome oxidase (EC 1.9.3.1) were used as marker enzymes for these two cell components, and glucose-6-phosphatase (EC 3.1.3.5) was used to identify endoplasmic reticulum. 5'-Nucleotidase (EC 3.1.3.5) was also measured but was found not to be exclusively associated with any one cell component. The activity of the anion-sensitive ATPase (HCO3--ATPase) was measured in suspensions that were frozen and thawed before assay in order to expose latent enzyme activity. The fraction containing the greatest amount of (Na+ + K+)-ATPase (35%) contained only 6% of the cytochrome oxidase and HCO3--ATPase. Conversely, the mitochondrial fraction, containing 40% of the cytochrome oxidase, contained about 40% of the HCO3--ATPase, but only 7% of the (Na+ + K+)-ATPase. The recoveries and relative degree of purification of the cytochrome oxidase and HCO3--ATPase were also nearly identical in the other fractions examined. It was concluded that the anion-sensitive ATPase activity of the corneal endothelium is located solely in the mitochondria and not in the plasma membrane. Consequently, any role that the enzymes may have in the transport of bicarbonate across this tissue, which had been suggested in earlier studies, must be an indirect one.     

Subcellular distribution and properties of carbonyl reductase in guinea pig lung

On subcellular fractionation, carbonyl reductase (EC 1.1.1.184) activity in guinea pig lung was found in the mitochondrial, microsomal, and cytosolic fractions; the specific activity in the mitochondrial fraction was more than five times higher than those in the microsomal and cytosolic fractions. Further separation of the mitochondrial fraction on a sucrose gradient revealed that about half of the reductase activity is localized in mitochondria and one-third in a peroxidase-rich fraction. Although carbonyl reductase in both the mitochondrial and microsomal fractions was solubilized effectively by mixing with 1% Triton X-100 and 1 M KCl, the enzyme activity in the mitochondrial fraction was more highly enhanced by the solubilization than was that in the microsomal fraction. Carbonyl reductases were purified to homogeneity from the mitochondrial, microsomal, and cytosolic fractions. The three enzymes were almost identical in catalytic, structural, and immunological properties. Carbonyl reductase, synthesized in a rabbit reticulocyte lysate cell-free system, was apparently the same in molecular size as the subunit of the mature enzyme purified from cytosol. These results indicate that the same enzyme species is localized in the three different subcellular compartments of lung.