Analytical characterization and purification of plasma membrane from cultured hepatoma cells (HTC cells)

The plasma membrane of the hepatoma cell line, HTC cells, has been characterized and purified by cell fractionation techniques. In the absence of true 5′-nucleotidase in HTC cells, alkaline phosphodiesterase I has been used as a marker enzyme, following conclusions gained from differential and isopycnic centrifugation studies (Lopez Saura, P., Trouet A. and Tulkens P. (1978) Biochim. Biophys. Acta 543, 430–449). To confirm this localization, HTC cells were exposed to anti-plasma membrane IgG at 4°C and fractionated. Alkaline phosphodiesterase I and IgG showed super imposable distribution patterns in linear sucrose gradients. Alkaline phosphodiesterase I is, however, only poorly resolved from enzyme markers of other organelles, especially NADPH-cytochrome $\text{c}$ reductase (endoplasmic reticulum) and galactosyltransferase (Golgi complex). Maximal purification from the homogenate is only 13-fold, on a protein basis, even when using a microsomal fraction (67 and 13% of alkaline phosphodiesterase I and protein, respectively) as the starting material. Improved resolution can be obtained after the addition of small quantities of digitonin (equimolar with respect to the cholesterol content). Digitonin increases the buoyant density of alkaline phosphodiesterase I by approx. 0.05 g/cm³, whereas the buoyant densities of galactosyltransferase and NADPH-cytochrome $\text{c}$ reductase are increased only by 0.03 and 0.015 g/cm³, respectively. Accordingly, a procedure has been designed which yields a fraction containing 22.8% of alkaline phosphodiesterase I with a purification of 21-fold on a protein basis. The content of NADPH-cytochrome $\text{c}$ reductase and galactosyltransferase is 1.2 and 2.1%, respectively. Electron microscopy shows smooth surface membrane elements and vesicles, with only occasional other recognizable elements.     

Biochemical studies on sub-fractions of rat liver mitochondria

Highly purified mitochondria from rat liver were separated into six sub-fractions by differential centrifugation. The sub-fractions represent a spectrum from “heavy” to “very light” mitochondria. Enzymes representative of mitochondrial compartments were assayed to see whether functional differences occurred among the various mitochondrial sub-fractions. Respiratory control and NADH oxidase activity, both of which are indicators of mitochondrial structural integrity, were also measured. An enzyme marker for endoplasmic reticulum (glucose-6-phosphatase, G-6-Pase) was also assayed. Specific activities for monoamine oxidase (outer membrane marker), cytochrome oxidase (inner membrane marker) and malate-cytochrome c reductase did not vary within experimental error in all sub-fractions; similarly, for respiratory control and NADH oxidase activity. Malate dehydrogenase, a component of malate-cytochrome c reductase is located within the matrix surrounded by the inner membrane. Specific activity of adenylate kinase (located between the outer and inner membrane) decreased markedly from the “heavy” mitochondria to the “very light” fractions. Specific activity for G-6-Pase, very low in the “heavy” fractions, increased markedly in the “light” to “very light” fractions. Isopycnic density centrifugation on a linear sucrose density gradient of each of the fractions indicated that the correlation coefficient for the sucrose concentrations at which cytochrome oxidase and G-6-Pase activities peaked was 0.995. Thus the “light” to “very light” mitochondria may represent mitochondria whose outer membrane is still contiguous with the endoplasmic reticulum. Microsomes containing the endoplasmic reticulum peaked on the gradient at a significantly lower sucrose concentration than any of the mitochondrial sub-fractions. A buoyant effect of endoplasmic reticulum still attached to any of the mitochondrial sub-fractions would be expected to lower the density of attached mitochondria and thus give rise to “light” and “very light” mitochondria.     

Characterization of membrane-bound electron transport enzymes from castor bean glyoxysomes and endoplasmic reticulum

Membranes purified from castor bean endosperm glyoxysomes by washing with sodium carbonate exhibited integral NADH:ferricyanide and NADH:cytochrome c reductase activities. The enzyme activities could not be attributed to contamination by other endomembranes. Purified endoplasmic reticulum membranes also contained the redox activities; and marker enzyme analysis indicated minimum cross contamination between glyoxysomal and endoplasmic reticulum fractions. The glyoxysomal redox activities were optimally solubilized at detergent to protein ratios (weight to weight) of 10 (Triton X-100), 50 (3-[3-cholamidopropyl)-dimethylammonio]-1-propanesulfonate), and 100 (octylglucoside). Detergent in excess of the solubilization optimum was stimulatory to NADH:ferricyanide reductase and inhibitory to NADH:cytochrome c reductase. Endoplasmic reticulum redox activity solubilization profiles were similar to those obtained for glyoxysomal enzymes using Triton X-100. Purification of the glyoxysomal and endoplasmic reticulum NADH:ferricyanide reductases was accomplished using dye-ligand affinity chromatography on Cibacron blue 3GA agarose. Sodium dodecyl sulfate-polyacrylamide gel electrophoretic analysis of NADH:ferricyanide reductase preparations purified by rate-zonal density gradient centrifugation, affinity chromatography, and nondenaturing electrophoresis of detergent-solubilized glyoxysomal and endoplasmic reticulum membranes consistently displayed 32- and 33-kDa silver-stained polypeptide bands, respectively.     

beta-Oxidation and Glyoxylate Cycle Coupled to NADH: Cytochrome c and Ferricyanide Reductases in Glyoxysomes

Glyoxysomes isolated from castor bean (Ricinus communis L., var Hale) endosperm had NADH:ferricyanide reductase and NADH:cytochrome c reductase activities averaging 720 and 140 nanomole electrons/per minute per milligram glyoxysomal protein, respectively. These redox activities were greater than could be attributed to contamination of the glyoxysomal fractions in which 1.4% of the protein was mitochondrial and 5% endoplasmic reticulum. The NADH:ferricyanide reductase activity in the glyoxysomes was greater than the palmitoyl-coenzyme A (CoA) oxidation activity which generated NADH at a rate of 340 nanomole electrons per minute per milligram glyoxysomal protein. Palmitoyl-CoA oxidation could be coupled to ferricyanide or cytochrome c reduction. Complete oxidation of palmitoyl-CoA, yielding 14 nanomole electrons/per nanomole palmitoyl-CoA, was demonstrated with the acceptors, NAL, cytochrome c, and ferricyanide. Malate was also oxidized by glyoxysomes, if acetyl-CoA, ferricyanide, or cytochrome c was present. Glyoxysomal NADH:ferricyanide reductase activity has the capacity to support the combined rates of NADH generation by β-oxidation and the glyoxylate cycle.     

Development of Endoplasmic Reticulum and Glyoxysomal Membrane Redox Activities during Castor Bean Germination

Redox activities, NADH:ferricyanide reductase, NAD(P)H:cytochrome reductases, and NADH:ascorbate free-radical reductase, are present in endoplasmic reticulum (ER) and glyoxysomal membranes from the endosperm of germinating castor bean (Ricinus comminus L. var Hale). The development of these functions was followed in glyoxysomes and ER isolated on sucrose gradients from castor bean endosperm daily from 0 through 6 days of germination. On a per seed basis, glyoxysomal and ER protein, glyoxysomal and ER membrane redox enzyme activities, and glyoxylate cycle activities peaked at day 4 as did the ER membrane content of cytochrome P-450. NADH:ferricyanide reductase was present in glyoxysomes and ER isolated from dry seed. This activity increased only about twofold in glyoxysomes and threefold in ER during germination relative to the amount of protein in the respective fractions. The other reductases, NADH:cytochrome reductase and NADH:ascorbate free-radical reductase, increased about 10-fold in the ER relative to protein up to 4 to 5 days, then declined. NADPH:cytochrome reductase reached maximum activity relative to protein at day 2 in both organelles. The increases in redox activities during germination indicate that the membranes of the ER and glyoxysome are being enriched with redox proteins during their development. The development of redox functions in glyoxysomes was found to be coordinated with development of the glyoxylate cycle.     

Distribution of enzyme activities in subcellular fractions of bovine retina.

Centrifugation of homogenates of bovine retinas to isopycnic equilibrium in sucrose density gradients yielded three partially overlapping bands of particles which were, in the order of increasing density: (a) photoreceptor cell (rod) outer segments; (b) plasma membranes, lysosomes, and large fragments of endoplasmic reticulum; and (c) mitochondria. The only enzyme activity investigated which had a peak coinciding only with outer segment fractions was guanylate cyclase. Enzyme activities with peaks in both the outer segment and denser fractions included 5'-nucleotidase and cyclic GMP phosphodiesterase. Enzyme activities with peaks only in the denser fractions included sodium and potassium ion-activated ATPase ((Na+ + K+)-ATPase), adenylate cyclase, cyclic AMP phosphodiesterase, beta-glucosidase, beta-galactosidase, and succinate-dependent cytochrome c reductase. These results suggest that some of the activities once thought to be present in rod outer segments are actually present in particles from elsewhere in the retina which contaminate rod outer segment preparations.     

The subcellular distribution of carotenoids in Neurospora crassa

The intracellular localization of carotenoids in the fungus Neurospora crassa was examined after completion of photoinduced biosynthesis of these pigments. Differential centrifugation of cell homogenates yielded subcellular fractions which were characterized by activities of several marker enzymes for cell constituents and in part purified by subsequent sucrose density gradient centrifugation. Most (ca 58%) of the carotenoids were found to be localized in lipid globules, but substantial amounts are also associated with two membrane fractions that were rich in membranes of the endoplasmic reticulum as indicated by high activities of NADPH- and NADH—cytochrome c reductase. These results, along with the coincidence in the distribution of both carotenoids and activities of specific marker enzymes in the sucrose density gradients, led to the conclusion that apart from lipid globules, carotenoids are also localized in membranes of the endoplasmic reticulum.     

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.     

ANALYTICAL FRACTIONATION OF HOMOGENATES FROM CULTURED RAT EMBRYO FIBROBLASTS

Homogenates of cultured rat embryo fibroblasts have been assayed for acid phosphatase, N-acetyl-β-glucosaminidase, cathepsin D, acid deoxyribonuclease, cytochrome oxidase, NADH cytochrome c reductase, 5'-nucleotidase, inosine diphosphatase, acid pyrophosphatase, neutral pyrophosphatase, esterase, catalase, cholesterol, and RNA. The validity of the assay conditions was checked. Neutral pyrophosphatase is a readily soluble enzyme. Acid hydrolases, except acid pyrophosphatase, are particle-bound enzymes, which exhibit a high degree of structural latency. They are activated and solubilized in a parallel fashion by mechanical treatments and tensio-active agents. Catalase is also particle-bound and latent; activating conditions stronger than those for hydrolases are required to activate the enzyme. Acid pyrophosphatase, 5'-nucleotidase and inosine diphosphatase are firmly particle-bound, but not latent; they are not easily solubilized. In differential and isopycnic centrifugation, the latent hydrolases, cytochrome oxidase and catalase dissociate largely from each other; this suggests the occurrence of lysosomes and peroxisome-like structures besides mitochondria. The distribution patterns of 5'-nucleotidase and cholesterol are largely similar; digitonin influences their equilibrium density to the same extent; these two constituents are thought to be related to the plasma membrane. Inosine diphosphatase and acid pyrophosphatase are also partially associated with the plasma membrane, although some part of these enzymic activities probably belongs to other structures. NADH cytochrome c reductase is associated partly with the endoplasmic reticulum, partly with mitochondria.     

Comparison of plasma membranes and endoplasmic reticulum fractions obtained from whole white adipose tissue and isolated adipocytes.

The influence of the mode of preparation upon some of the characteristics of white adipose tissue plasma membranes and microsomes has been reported. Plasma membrane fractions prepared from mitochondrial pellet were shown to have higher specific activities of (Mg2+ + Na+ + K+)-ATPase than plasma membranes originating in crude microsomes. Isolation of fat cells by collagenase treatment was found to result in a decrease in specific activity of the plasma membrane enzymes; in plasma membranes prepared from isolated fat cells, the specific activity values obtained for (Mg2+ + Na+ +k+)-ATPase and 5'-nucleotidase were only 42% and 6.3% respectively of those obtained in plasma membranes prepared from whole adipose tissue. Purification of whole adipose tissue crude microsomes by hypotonic treatment caused extensive solubilization of the endoplasmic reticulum marker enzymes, NADH oxidase and NADPH cytochrome c reductase. The lability of endoplasmic reticulum marker enzymes, however, was found to be greatly diminished in the preparations from isolated fat cells. The possibility that NADH oxidase and NADPH cytochrome c reductase activities found in the plasma membranes are microsomal enzymes adsorbed by the plasma membranes is discussed. The peptide patterns as well as the NADH oxidase and NADPH cytochrome c reductase activity patterns of plasma membranes and purified microsomes were compared by means of sodium dodecyl sulfate or Triton X-100 polyacrylamide gel electrophoresis.     

Analytical study of microsomes and isolated subcellular membranes from rat liver. 3. Subfractionation of the microsomal fraction by isopycnic and differential centrifugation in density gradients

Rat liver microsomal fractions have been equilibrated in various types of linear density gradients. 15 fractions were collected and assayed for 27 constituents. As a result of this analysis microsomal constituents have been classified, in the order of increasing median density, into four groups labeled a, b, c, and d. Group a includes: monoamine oxidase, galactosyltransferase, 5''-nucleotidase, alkaline phosphodiesterase I, alkaline phosphatase, and cholesterol; group b: NADH cytochrome c reductase, NADPH cytochrome c reductase, aminopyrine demethylase, cytochrome b ₅, and cytochrome P 450; group c: glucose 6-phosphatase, nucleoside diphosphatase, esterase, β-glucuronidase, and glucuronyltransferase; group d: RNA, membrane-bound ribosomes, and some enzymes probably adsorbed on ribosomes: fumarase, aldolase, and glutamine synthetase. Analysis of the microsomal fraction by differential centrifugation in density gradient has further dissociated group a into constituents which sediment more slowly (monoamine oxidase and galactosyltransferase) than those of groups b and c, and 5''-nucleotidase, alkaline phosphodiesterase I, alkaline phosphatase, and the bulk of cholesterol which sediment more rapidly (group a2). The microsomal monoamine oxidase is attributed, at least partially, to detached fragments of external mitochondrial membrane. Galactosyltransferase belongs to the Golgi complex. Group a2 constituents are related to plasma membranes. Constituents of groups b and c and RNA belong to microsomal vesicles derived from the endoplasmic reticulum. These latter exhibit a noticeable biochemical heterogeneity and represent at the most 80% of microsomal protein, the rest being accounted for by particles bearing the constituents of groups a and some contaminating mitochondria, lysosomes, and peroxisomes. Attention is called to the operational meaning of microsomal subfractions and to their cytological complexity.     

Fractionation and characterization of cellular membranes from root tips of garden cress (Lepidium sativum L.)

The first step in the gravitropic reaction chain, i.e. perception, is known to occur in the statenchyma of the root cap. Because of the importance of the root tip in graviperception, a procedure has been developed to isolate root tips from garden cress (Lepidium sativum L.). The root tip fraction contains the tissues of the root cap plus the lower half of the meristem zone, but is clearly separated from the tissues of the elongation zone, the zone of gravitropic response. Membranes from the root tip and root base fractions have been centrifuged on sucrose density gradients and the marker enzyme profiles analyzed. These results show that the marker enzyme profiles for vacuoles, dictyosomes, mitochondria, and plasma membranes are similar in the root tip or root base fractions. The endoplasmic reticulum (ER) has a shoulder of cytochrome c reductase activity at a density of 1.16 g cm^-3 which is distinct from the other enzyme activities and is only observed in root tip preparations. The specific enzyme activity for ER, cytochrome c reductase, was enriched in root tip membranes 1.7 fold. This latter increase is interpreted as at least in part an increased ER content in the root tip.Abbreviations ASG 6-acyl-steryl glucoside - ER endoplasmic reticulum - IDP inosine-5-diphosphate - INT 2-(4-iodophenyl)-3-(4-nitrophenyl)-5-phenyltetrazolium chloride - PM plasma membrane - SG steryl glucoside     

Biosynthesis of high density lipoprotein by chicken liver: conjugation of nascent lipids with apoprotein A1

To study the assembly of newly synthesized lipids with apoprotein A1, we administered [2-3H]glycerol to young chickens and determined the hepatic intracellular sites of lipid synthesis and association of nascent lipids with apoprotein A1. [2-3H]glycerol was rapidly incorporated into hepatic lipids, reaching maximal levels at 5 min, and this preceded the appearance of lipid radioactivity in the plasma. The liver was fractionated into rough and smooth endoplasmic reticulum and Golgi cell fractions. The isolated cell fractions were further subfractionated into membrane and soluble (content) fractions by treatment with 0.1 M Na2CO3, pH 11.3. At various times, the lipid radioactivity was measured in each of the intracellular organelles, in immunoprecipitable apoprotein A1, and in materials that floated at buoyant densities similar to those of plasma lipoproteins. Maximal incorporation occurred at 1 min in the rough endoplasmic reticulum, at 3-5 min in the smooth endoplasmic reticulum, and at 5 min in the Golgi cell fractions. The majority (66-93%) of radioactive glycerol was incorporated into triglycerides with smaller (4-27%) amounts into phospholipids. About 80% of the lipid radioactivity in the endoplasmic reticulum and 70% of that in the Golgi cell fractions was in the membranes. The radioactive lipids in the content subfraction were distributed in various density classes with most nascent lipids floating at a density less than or equal to 1.063 g/ml. Apoprotein A1 from the Golgi apparatus, obtained by immunoprecipitation, contained sixfold more nascent lipids than did that from the endoplasmic reticulum. These data indicate that [2-3H]glycerol is quickly incorporated into lipids of the endoplasmic reticulum and the Golgi cell fractions, that most of the nascent lipids are conjugated with apoproteins A1 in the Golgi apparatus, and that very little association of nascent lipid to apoprotein A1 occurs in the endoplasmic reticulum.     

Comparison of the Stereospecificity and Immunoreactivity of NADH-Ferricyanide Reductases in Plant Membranes

The substrate stereospecificity of NADH-ferricyanide reductase activities in the inner mitochondrial membrane and peroxisomal membrane of potato (Solanum tuberosum L.) tubers, spinach (Spinacea oleracea L.) leaf plasma membrane, and red beetroot (Beta vulgaris L.) tonoplast were all specific for the [beta]-hydrogen of NADH, whereas the reductases in wheat root (Triticum aestivum L.) endoplasmic reticulum and potato tuber outer mitochondrial membrane were both [alpha]-hydrogen specific. In all isolated membrane fractions one or several polypeptides with an apparent size of 45 to 55 kD cross-reacted with antibodies raised against a microsomal NADH-ferricyanide reductase on western blots.     

Effect of digitonin on rat myometrium subcellular membrane fractions

Isopycnic centrifugation experiments using sucrose density gradients showed that in digitonin-treated microsomes the distribution of the plasma membrane (PM) marker 5'-nucleotidase was shifted to higher densities. The treatment also caused similar but less pronounced changes in the distribution of protein, the putative endoplasmic reticulum (ER) marker NADPH-dependent cytochrome c reductase, and the inner mitochondrial marker cytochrome c oxidase. Similar experiments using more purified membrane fractions showed that the digitonin treatment led to a comparable increase in the densities of the fractions N1 and N2 previously described as subfractions of plasma membrane and to considerably less increase in the density of the fraction N3B which is enriched in the endoplasmic reticulum and the inner mitochondrial markers. Digitonin inhibited the ATP-dependent Ca uptake by the N1 fraction in a concentration-dependent manner (I50 = 0.3 mg/mL). Digitonin (0.5 mg/mL) inhibited the ATP-dependent azide-insensitive Ca uptake by all the fractions. The results support the hypothesis that (a) N1 and N2 are subfractions of plasma membrane, and (b) ATP-dependent azide-insensitive Ca uptake in rat myometrium is a property of plasma membranes.     

Purification and properties of a peptic heme peptide from cytochrome c1.

Highly purified mouse liver plasma membranes have been used to define the properties of an NADH dehydrogenase activity associated with plasma membrane. The NADH indophenol reductase activity is two-fold stimulated at 5 × 10^?8 M glucagon and the stimulation is inhibited by atebrin. Corresponding activity in endoplasmic reticulum is not stimulated by glucagon. The NADH indophenol reductase is 90% inhibited by insulin at 7 × 10^?11M and shows a return to the original activity at higher insulin concentrations. NADH dehydrogenase activity in endoplasmic reticulum is inhibited up to 50% by insulin at a similar concentration. Triiodothyronine at 10^?7M also inhibits the plasma membrane dehydrogenase whereas thyroxine has little effect. The response of this dehydrogenase to hormones suggests a role in regulation of cellular function.     

Topographic heterogeneity in cholesterol biosynthesis

We have examined the membrane topography of cholesterol biosynthesis in cultured human fibroblasts. We fed the cells with radioacetate and then interrupted the biosynthetic pathway so as to trap labeled intermediates in their subcellular locations. We analyzed homogenates of human fibroblasts labeled biosynthetically from radioacetate by centrifugation to equilibrium on sucrose gradients. The following two methods were used to interrupt cholesterol biosynthesis: incubation at 10 degrees C and treatment with 4,4,10 beta-trimethyl-trans-decal-3 beta-ol, a specific inhibitor of oxidosqualene cyclase. Incubation at 10 degrees C caused the accumulation of radiolanosterol at the expense of cholesterol. The lanosterol appeared predominantly at an unusually buoyant density (20% (w/w) sucrose; d = 1.08 g/cm3) as well as at the density normally labeled at 37 degrees C (30% sucrose; d = 1.13 g/cm3). 4,4,10 beta-Trimethyl-trans-decal-3 beta-ol treatment caused the accumulation of labeled squalene and squalene 2,3-oxide. Reversal of the block permitted the label to progress rapidly as a wave into lanosterol and ultimately into cholesterol. The profiles of the three precursors did not coincide, suggesting that they were mostly in different membranes. Squalene was uniquely confined to a density of 1.18 g/cm3 (40% sucrose) while squalene 2,3-oxide appeared in peaks of density 1.08 g/cm3 and 1.13 g/cm3 (20% and 30% sucrose). Lanosterol was in a peak of density 1.13 g/cm3. Pulse-chase experiments showed that lanosterol synthesized in the membranes at 20% sucrose moved rapidly to the membranes at 30% sucrose where it was converted to cholesterol. The density gradient profiles of the following organelle markers also were monitored: plasma membrane, cholesterol mass; Golgi apparatus, galactosyltransferase; endoplasmic reticulum, RNA, 3-hydroxy-3-methylglutaryl-coenzyme A reductase and cytochrome c reductase; peroxisomes, catalase. None of these markers appeared at the buoyant density of 1.08 g/cm3. We conclude that 1) cholesterol biosynthesis may be topographically heterogeneous and 2) newly synthesized squalene 2,3-oxide resides in a buoyant membrane fraction distinct from markers for the major organelles.     

Cholesterol is not synthesized in membranes bearing 3-hydroxy-3-methylglutaryl coenzyme A reductase

We have shown previously that newly synthesized lanosterol and cholesterol in homogenates of cultured human fibroblasts do not have the same equilibrium buoyant density as the 3-hydroxy-3-methylglutaryl-CoA reductase (HMG-CoA reductase) in the smooth endoplasmic reticulum (SER) (Lange, Y., and Steck, T. L. (1985) J. Biol. Chem. 260, 15592-15597). This finding suggested two alternative and novel hypotheses: (a) that lanosterol and cholesterol might be transported rapidly from the SER to other internal membranes or (b) that synthesis of the sterols is not associated with the SER, or at least not with that portion of this organelle bearing HMG-CoA reductase. We therefore compared the subcellular distribution of HMG-CoA reductase with that of enzymes which convert lanosterol to cholesterol. The two activities studied were the consumption of exogenous [3H]lanosterol and the conversion of exogenous radiolanosterol to radiocholesterol. Differential centrifugation, rate zonal centrifugation, and equilibrium sucrose gradient centrifugation of rat liver homogenates all showed that these enzyme activities did not comigrate with HMG-CoA reductase. The subcellular distribution of newly synthesized sterols also was examined in cultured human fibroblasts. Cells were incubated with radioactive acetate to label endogenous sterols biosynthetically, homogenized, and spun to equilibrium on sucrose gradients. The buoyant density profiles of radioactive cholesterol and lanosterol both had a peak at 1.12 g/cm3. Digitonin treatment shifted both sterols to higher densities, strong evidence that they resided in cholesterol-rich membranes. Pretreatment of intact cells with cholesterol oxidase, which selectively oxidizes plasma membrane cholesterol, abolished the digitonin shift of lanosterol but not of intracellular cholesterol. These findings provide support for the hypothesis that newly synthesized cholesterol and lanosterol are not in the same membrane.     

Gonadotropin and prostaglandins binding sites in rough endoplasmic reticulum and Golgi fractions of bovine corpora lutea.

Highly purified rough endoplasmic reticulum and three subfractions of golgi were prepared from 105,000g pellet of the homogenate by centrifugation in floatation and sedimentation discontinuous sucrose gradients. Highly purified plasma membranes were also prepared from 9,000g pellet of the same homogenates for assessment under the same experimental conditions. Although 5′-nucleotidase, a marker for plasma membranes, was markedly enriched in plasma membranes, very little or none of this enzyme activity was found in other fractions. Very little or no NADH cytochrome c reductase activity, a marker for rough endoplasmic reticulum, was found in fractions other than rough endoplasmic reticulum. Galactosyl transferase, a marker for golgi, was found and enriched in all the fractions; however, enrichment in golgi fractions was higher than in other fractions. Very little or no lysosomal marker activity, i.e., acid phosphatase, was found in rough endoplasmic reticulum or golgi fractions as compared to lysosomes. These marker enzyme data suggested that rough endoplasmic reticulum and golgi fractions were relatively pure with little or no cross contamination with other organelles. The [¹²⁵I]human choriogonadotropin ([¹²⁵I]hCG), [³H]prostaglandin (PG)E₁, and [³H]PGF_2a specifically bound to rough endoplasmic reticulum and golgi fractions in addition to plasma membranes. The enrichments of binding in the former two fractions, in some cases, were as high as plasma membranes itself. The specific binding of some of the ligands was found to be partially latent in rough endoplasmic reticulum and golgi fractions but not in plasma membranes. Marker enzyme data, ratio between bindings and marker enzyme activities (an index of organelle contamination), and partial latency of binding suggest that rough endoplasmic reticulum and golgi fractions intrinsically contain gonadotropin and PGs binding sites.     

Isolation and characterization of the Neurospora crassa endoplasmic reticulum

The endoplasmic reticulum from Neurospora crassa was identified by monitoring the activity of the putative enzyme marker phosphatidylcholine glyceride transferase. After differential centrifugation of a cell homogenate, phosphatidylcholine glyceride transferase activity initially copurified with plasma membrane H+-ATPase. However, isopycnic centrifugation of the whole-cell homogenate on a linear sucrose gradient separated the two enzyme activities into different fractions. The lighter membrane fraction exhibited characteristics that have been associated with the endoplasmic reticulum in other organisms: (i) the inclusion of magnesium caused this light membrane fraction to shift to a higher density on the gradient; (ii) it was highly enriched in cytochrome c reductase, an endoplasmic reticulum marker in other systems; and (iii) the morphology of the light fraction with and without added magnesium was clearly distinguishable from that of the plasma membrane fraction by electron microscopy. A reinvestigation of the location of chitin synthetase confirmed its association with the plasma membrane fraction even after separation of the lighter fractions.