Different effects of sphingosine, R59022 and anionic amphiphiles on two diacylglycerol kinase isozymes purified from porcine thymus cytosol

Porcine thymus cytosol contains two immunologically distinct forms of diacylglycerol kinase (DGK) [Yamada, K. and Kanoh, H. (1988) Biochem. J. 255, 601-608]. These 2 DGK species, having apparent molecular masses of 80 and 150 kDa, were purified from the thymus cytosol. Upon sodium dodecyl sulfate-polyacrylamide gel electrophoresis, the 150-kDa DGK gave 2 polypeptide bands of 50 and 75 kDa, whereas the 80-kDa DGK yielded a single protein band. The 80-kDa DGK was markedly activated by 10-20 microM sphingosine as well as by the known anionic activators such as phosphatidylserine and deoxycholate. In contrast, the 150-kDa DGK was fully active in the absence of the anionic activators and was strongly inhibited by sphingosine (IC50, 20 microM). The putative DGK inhibitor R59022 inhibited the 80-kDa DGK (IC50, 10 microM), but had little effect on the 150-kDa form. It is therefore clear that in the thymus cytosol there are at least 2 DGK isozymes operating under different control mechanisms.     

Characterization of Two Cytosolic Diacylglycerol Kinase Forms

Abstract: Two forms of rat brain cytosolic diacylglycerol kinase (EC 2.7.1.107) were separated by heparin-agarose column chromatography. These forms, designated DGK-I and DGK-II, were not interconvertible as determined by rechromatography. DGK-I and DGK-II had respective molecular masses of 88 and 180 kDa, as measured by Sepharose 6B chromatography. Both forms preferred diacylglycerol over monoacylglycerol and were insensitive to R59022. DGK-II, but not DGK-I, was activated by an activator substance prepared from chicken egg yolk. DGK-II was activated by a rat brain cytosolic activator and was exclusively sensitive to 5'-AMP-mediated inactivation. Further studies revealed that these two forms had the following distinct characteristics: (a) substrate specificity, (b) inhibition by heparin, (c) sensitivity to lysine-containing polyamino acids, and (d) responses to different phospholipids. In general, DGK-II was more responsive to various inhibitors and activators, making it a prime candidate for a regulatable enzyme.     

Porcine 80-kDa diacylglycerol kinase is a calcium-binding and calcium/phospholipid-dependent enzyme and undergoes calcium-dependent translocation.

We attempted to assess the regulatory role of EF-hand motifs recently detected in the primary structure of porcine 80-kDa diacylglycerol kinase (DGK) (Sakane, F., Yamada, K., Kanoh, H., Yokoyama, C., and Tanabe, T. (1990) Nature 344, 345-348). By using 80-kDa DGK purified from porcine thymus cytosol, we found that this isozyme indeed bound 2 mol Ca2+ per mol enzyme with high affinity (apparent dissociation constant, kd = 0.3 microM). The Ca2+ binding was cooperative with a Hill coefficient of 1.4. We next studied the effect of 1 x 10(-5) M Ca2+ on the kinetic properties of DGK employing a beta-octyl glucoside mixed micellar assay system. In the absence of Ca2+, phosphatidylserine, so far used as an enzyme activator in various assay systems, was rather inhibitory, and Ca2+ alone activated enzyme to a limited extent. However, phosphatidylserine plus Ca2+ markedly activated the enzyme, giving approximately 4-fold higher Vmax and 10-fold less Km values for ATP. In contrast, the apparent Km values for diacylglycerol were not significantly affected (approximately 3 mol %). Furthermore, by immunoblotting using anti-80 kDa DGK antibodies we found that the soluble DGK in the homogenate of porcine thymocytes was translocated to membranes in a Ca2(+)-dependent manner. Indeed we noted the presence of a 33-residue amphipathic alpha-helix in the DGK sequence, which may account for the protein-lipid interaction. The results demonstrate that Ca2+ plays a key role in the regulation of DGK action by controlling enzyme interaction with membrane phospholipids.     

Partial Purification and Properties of Human Brain Aldehyde Dehydrogenases

Acetaldehyde and biogenic aldehydes were used as substrates to investigate the subcellular distribution of aldehyde dehydrogenase activity in autopsied human brain. With 10 microM acetaldehyde as substrate, over 50% of the total activity was found in the mitochondrial fraction and 38% was associated with the cytosol. However, with 4 microM 3,4-dihydroxyphenylacetaldehyde and 10 microM indoleacetaldehyde as substrates, 40-50% of the total activity was found in the soluble fraction, the mitochondrial fraction accounting for only 15-30% of the total activity. These data suggested the presence of distinct aldehyde dehydrogenase isozymes in the different compartments. The mitochondrial and cytosolic fractions were, therefore, subjected to salt fractionation and ion-exchange chromatography to purify further the isozymes present in both fractions. The kinetic data on the partially purified isozymes revealed the presence of a low Km isozyme in both the mitochondria and the cytosol, with Km values for acetaldehyde of 1.7 microM and 10.2 microM, respectively. However, the cytosolic isozyme exhibited lower Km values for the biogenic aldehydes. Both isozymes were activated by Mg2+ and Ca2+ in phosphate buffers (pH 7.4). Also, high Km isozymes were found in the mitochondria and in the microsomes.     

Purification and properties of the 5 alpha-dihydrotestosterone 3 alpha(beta)-hydroxysteroid dehydrogenase from human prostatic cytosol.

5 alpha-Dihydrotestosterone 3 alpha(beta)-hydroxysteroid dehydrogenase [3 alpha(beta)-HSDH] [EC 1.1.1.50/EC 1.1.1.51] which catalyses the conversion of 5 alpha-dihydrotestosterone (5 alpha-DHT) to both 5 alpha-androstane-3 alpha,17 beta-diol and 5 alpha-androstane-3 beta,17 beta-diol was purified to an apparent homogeneous state using cytosol of three human hyperplastic prostates by a 4-step purification procedure. After each purification step 3 alpha-HSDH activity was coincident with 3 beta-HSDH activity. On average, specific 3 alpha-HSDH activity was enriched 856-fold, specific 3 beta-HSDH activity 749-fold compared to human prostatic cytosol using anion exchange, hydrophobic interaction, gel filtration and affinity chromatography. Examination of the purified enzyme by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate (SDS) revealed a single protein band with silver staining. The molecular weight of the enzyme was estimated as 33 kDa by SDS-polyacrylamide gel electrophoresis and as 28 kDa by Sephacryl S-200 gel filtration indicating that the native 3 alpha(beta)-HSDH is a monomer. In the presence of the preferred co-factor, NADPH, the purified enzyme had a mean apparent Km for 5 alpha-DHT of 3.9 microM and a Vmax of 93.3 nmol (mg protein)-1 h-1 with regard to 3 alpha-HSDH activity, and a Km of 6.3 microM and a Vmax of 20.6 nmol (mg protein)-1 h-1 with regard to 3 beta-HSDH activity.     

Purification of the NADPH:5 alpha-dihydroprogesterone 3 alpha-hydroxysteroid oxidoreductase from female rat pituitary cytosol

The NADPH:5 alpha-dihydroprogesterone 3 alpha-hydroxysteroid oxidoreductase (3 alpha-HSOR) [EC 1.1.1.50] which catalyzes the reversible conversion of 5 alpha-pregnane-3,20-dione (5 alpha-dihydroprogesterone; 5 alpha-DHP) to 3 alpha-hydroxy-5 alpha-pregnan- 20-one (3 alpha-,5 alpha-tetrahydroprogesterone; 3 alpha,5 alpha-THP) was purified to apparent homogeneity from female rat anterior pituitary cytosol by a three step micro-purification procedure. Specific activity of purified 3 alpha-HSOR was enriched 438-fold from that in pituitary cytosol using successive ion exchange, chromatofocusing and affinity column chromatography purification steps. 3 alpha-HSOR appears to be a monomer with an approximate molecular weight of 36 kDa and an isoelectric point of about 5.75. The purified enzyme appears as a single protein staining band (36 kDa) when examined by polyacrylamide gel electrophoresis and with both silver or Coomassie blue staining. Under non-dissociating electrophoretic conditions, all of the 3 alpha-HSOR activity co-migrated with the 36 kDa protein staining band. The purified enzyme in the presence of the preferred cofactor, NADPH, has an apparent Km for 5 alpha-DHP of 82 nM and a Vmax of 1.2 mumol of 3 alpha,5 alpha-THP formed per mg protein/30 min. The Km for NADPH was 0.71 microM. In the oxidative direction, the enzyme in the presence of NADP+ has a Km for 3 alpha,5 alpha-THP of 1.4 microM and a Vmax of 9.7 mumol of 5 alpha-DHP formed per mg protein/30 min. The Km for NADP+ was 1.6 microM.     

Identification and isolation of a 75-kDa inositol polyphosphate-5-phosphatase from human platelets

We have identified, isolated, and characterized a second inositol polyphosphate-5-phosphatase enzyme from the soluble fraction of human platelets. The enzyme hydrolyzes inositol 1,4,5-trisphosphate (Ins (1,4,5)P3) to inositol 1,4-bisphosphate (Ins(1,4)P2) with an apparent Km of 24 microM and a Vmax of 25 mumol of Ins(1,4,5)P3 hydrolyzed/min/mg of protein. The enzyme hydrolyzes inositol (1,3,4,5)-tetrakisphosphate (Ins(1,3,4,5)P4) at a rate of 1.3 mumol of Ins(1,3,4,5)P4 hydrolyzed/min/mg of protein with an apparent Km of 7.5 microM. The enzyme also hydrolyzes inositol 1,2-cyclic 4,5-trisphosphate (cIns(1:2,4,5)P3) and Ins(4,5)P2. We purified this enzyme 2,200-fold from human platelets. The enzyme has a molecular mass of 75,000 as determined by both sodium dodecyl sulfate-polyacrylamide gel electrophoresis and by gel filtration chromatography. The enzyme requires magnesium ions for activity and is not inhibited by calcium ions. The 75-kDa inositol polyphosphate-5-phosphatase enzyme differs from the previously identified platelet inositol polyphosphate-5-phosphatase as follows: molecular size (75 kDa versus 45 kDa), affinity for Ins(1,3,4,5)P4 (Km 7.5 microM versus 0.5 microM), Km for Ins(1,4,5)P3 (24 microM versus 7.5 microM), regulation by protein kinase C, wherein the 45-kDa enzyme is phosphorylated and activated while the 75-kDa enzyme is not. The 75-kDa enzyme is inhibited by lower concentrations of phosphate (IC50 2 mM versus 16 mM for the 45-kDa enzyme) and is less inhibited by Ins(1,4)P2 than is the 45-kDa enzyme. The levels of inositol phosphates that act in calcium signalling are likely to be regulated by the interplay of these two enzymes both found in the same cell.     

Phosphorylation of the antiviral precursor 9-(1,3-dihydroxy-2-propoxymethyl)guanine monophosphate by guanylate kinase isozymes

Guanylate kinase was purified from human erythrocytes by affinity chromatography using GMP-agarose, and the four isozymes which are present were separated by chromatofocusing. The kinetic properties of each isozyme were analyzed with respect to the natural substrates GMP and dGMP, and the 5'-monophosphate derivatives of the antiviral nucleoside analogs 9-(1,3-dihydroxy-2-propoxymethyl)guanine (DHPG) and 9-(2-hydroxyethoxymethyl)guanine (ACV, Acyclovir). The analysis of substrate kinetics yielded Km values for DHPG 5'-monophosphate which were similar with all isozymes (42-54 microM), and about 3-fold higher than the Km values obtained for GMP. Km values obtained with ACV 5'-monophosphate were 10-20-fold higher than the GMP values and varied nearly 4-fold among isozymes (209-753 microM). GMP produced the highest enzyme velocities with all isozymes, followed by dGMP, DHPG 5'-monophosphate, and ACV 5'-monophosphate, in that order. Differences in maximal velocities among isozymes were generally small. DHPG 5'-monophosphate inhibited the isozymes by a simple competitive mechanism with respect to GMP. In contrast, ACV 5'-monophosphate acted as an apparent hyperbolic mixed-type inhibitor. Similar patterns of inhibition were obtained with all isozymes. It is probable that differences is the reactivity of DHPG 5'-monophosphate and ACV 5'-monophosphate with individual guanylate kinase isozymes do not contribute significantly to differences in their antiviral effects.     

Purification and biochemical characterization of the rabbit pulmonary glutathione S-transferase: stereoselectivity and activity toward pyrene 4,5-oxide

Two homodimeric isozymes, glutathione S-transferase (GST) 25 kDa and GST 27 kDa, in equal proportion comprise the majority (greater than 75%) of the pulmonary cytosolic GST of untreated rabbits. The subunits of GST 25 kDa and GST 27 kDa are distinguishable by electrophoretic mobility (25 and 27 kDa, respectively), apparent isoelectric points (pI 7.4 and pI 9.1, respectively), and immunoreactivity. Immunoblots indicated that these subunits may be minor components in hepatic cytosol. The pulmonary isozymes could not be distinguished by their activities toward chloro-2,4-dinitrobenzene (CDNB) or activity and stereoselectivity toward pyrene 4,5-oxide (PyO). The purified GST fractions represented less than or equal to 16% of the PyO activity for pulmonary cytosol. The stereoselectivity of the cytosolic GST for the pro-S-configured oxirane carbon of PyO was not maintained in the purified preparations which were virtually nonstereoselective. Immunoprecipitation of pulmonary cytosolic GST with anti-GST 27 kDa and anti-GST 25 kDa indicated that at least 84 and 60% of the activity toward CDNB and PyO, respectively, is mediated by the two isozymes. The specific PyO activities of GST 27 kDa, GST 25 kDa, and the rabbit hepatic preparations (approximately 0.2 unit/mg) were similar to that of hepatic GST purified from horse, cow, and pig, and to human placental GST pi (0.02-0.5 unit/mg) but one-tenth that of rat hepatic GST or human hepatic GST mu. However, the activity of the hepatic cytosol from rat and human was similar to that of rabbit. Thus, some GST isozymes may be particularly susceptible to modulation of activity/stereoselectivity that can be discerned with arene oxide substrates such as PyO.     

Purification and properties of three NAD(P)+ isozymes of L-glutamate dehydrogenase of Chlamydomonas reinhardtii.

Three isozymes of glutamate dehydrogenase (GDH) of Chlamydomonas reinhardtii, induced under different trophic and stress conditions, have been purified about 800-1000-fold to electrophoretic homogeneity. They are hexamers of Mr 266,000-269,000 as deduced from gel filtration and sedimentation coefficient data. GDH1 consisted of six identical subunits of 44 kDa each, whereas both GDH2 and GDH3 consisted of six similar-sized monomers (4 of 44 kDa and 2 of 46 kDa). Optimum pH for the three activities with each pyridine nucleotide was identical (8.5 with NADH; 7.7 with NADPH; and 9.0 with NAD+). The isozymes exhibited similar high optimum temperature values (60-62 degrees C) and isoelectric points (7.9-8.1). Activity was enhanced in vitro by Ca2+ ions and strongly inhibited by pyridoxal 5'-phosphate, KCN, o-phenanthroline and EDTA, and to a lesser extent by pHMB and methylacetimidate. In the aminating reaction the three isozymes were inhibited in a concentration-dependent process by both NADH and NADPH, with apparent Km values for NH4+ ranging from 13-53 mM; 0.36-1.85 mM for 2-oxoglutarate and 0.07-0.78 mM for NADH and NADPH. In the deaminating reaction apparent Km values ranged from 0.64-3.52 mM for L-glutamate and 0.20-0.32 for NAD+. In addition, the three isozymes exhibited a non-hyperbolic kinetics for NAD+ with negative cooperativity (n = 0.8).     

Cyclic nucleotide phosphodiesterase activity in human peripheral blood lymphocytes and monocytes.

cAMP and cGMP phosphodiesterase (PDE) activity was assayed in human peripheral blood lymphocytes purified by isopycnic centrifugation as well as in lymphocyte preparations further purified to remove contaminating platelets and monocytes. The 16,000 X G supernatant from sonicates of each of these cell preparations contained two hydrolytic activities for cAMP with apparent Km of 1.1 to 2.5 microM and 33 to 66 microM, and a single hydrolytic activity for cGMP with an apparent Km of 6 to 25 microM. When lymphocytes were disrupted by Dounce homogenization, there was only a single, low Km cAMP PDE activity in the homogenate; however, the 16,000 X G supernatant demonstrated 2 Km similar to that seen in sonicated lymphocytes. Treatment of the Dounce preparations with 0.5% Triton X-100 or 1.0% NP-40 converted these preparations to activities similar to those seen in sonicated preparations. cGMP hydrolytic activity was low or absent in the Dounce preparations and was not altered by centrifugation; however, it was markedly enhanced by detergent extraction. These data indicate that human peripheral blood lymphocytes and monocytes have PDE activities similar to those seen in other tissues.     

Purification and characterization of D-3-aminoisobutyrate-pyruvate aminotransferase from rat liver

D-3-Aminoisobutyrate-pyruvate aminotransferase (EC 2.6.1.40) was purified 1900-fold from rat liver extract. The purified enzyme showed a molecular mass of 180 kDa by gel-permeation HPLC analysis using a TSK gel G3000SW column. Reductive polyacrylamide gel electrophoresis in sodium dodecyl sulfate resulted in identification of a single band of approx. 50 kDa, indicating that the native enzyme is probably a tetrametric protein. The specific activity of the purified enzyme was 1.14 mumol/min per mg protein. D-3-Aminoisobutyrate and beta-alanine were good amino donors. The Km value for L-3-aminoisobutyrate was 100-times larger than that for the D-isomer. The apparent Km values for D-3-aminoisobutyrate and beta-alanine were 35 and 282 microM, respectively. Pyruvate, glyoxylate, oxalacetate, 2-oxo-n-valerate, and 2-oxo-n-butyrate were good amino acceptors. The apparent Km values for pyruvate and glyoxylate were 32 and 44 microM, respectively.     

Functionally active thrombomodulin is present in human platelets

We found functionally active thrombomodulin in human platelets (60 +/- 18 molecules per platelet). Protein C appeared not to be activated by thrombin with gel-filtered platelets. However, the activation of protein C by thrombin was accelerated by thrombin-stimulated and washed platelets. This cofactor activity of the platelets was neutralized by the anti-lung thrombomodulin-F(ab')2. From the Triton X-extract of platelets, thrombomodulin was partially purified by diisopropylphosphoryl-thrombin-agarose affinity chromatography. The Mr of the predominant platelet thrombomodulin was 78,000 before and 109,000 after reduction on sodium dodecyl sulfate-polyacrylamide gel electrophoresis, values identical to those of placental thrombomodulin. The specific activity of the cofactor activity, apparent Kd (0.4 nM) for thrombin and Km (0.67 microM) for protein C of platelet thrombomodulin were also identical to those of placenta thrombomodulin. Thrombomodulin may play a role in activation of protein C on the surface of platelets.     

Diacylglycerol kinase in the central nervous system--molecular heterogeneity and gene expression.

Diacylglycerol (DAG) is one of the important second messengers, which serves as an activator of protein kinase C (PKC). DAG kinase (DGK) phosphorylates DAG to generate phosphatidic acid, thus DGK is considered to be a regulator of PKC activity through attenuation of DAG. Recent studies have revealed molecular structures of several DGK isozymes from mammalian species, and showed that most of the isozymes are expressed in the brain in various amounts. We have cloned four DGK isozyme cDNAs from rat brain library (DGK alpha, -beta, -gamma, and -zeta) (previously also designated DGK-I, -II, -III, and -IV, respectively) and examined their mRNA expressions in rat brain by in situ hybridization histochemistry. Interestingly, it is revealed that the mRNA for each isozyme is expressed in a distinct pattern in the brain; DGK alpha is expressed in oligodendrocytes, glial cells that form myelin; DGK beta in neurons of the caudate-putamen; DGK gamma predominantly in the cerebellar Purkinje cells; and DGK zeta in the cerebellar and cerebral cortices. Molecular diversity and distinct expression patterns of DGK isozymes suggest a physiological importance for the enzyme in brain function. Furthermore, functional implications of these DGK isozymes are briefly discussed.     

Arabidopsis AtDGK7, the smallest member of plant diacylglycerol kinases (DGKs), displays unique biochemical features and saturates at low substrate concentration: the DGK inhibitor R59022 differentially affects AtDGK2 and AtDGK7 activity in vitro and alters plant growth and development

Diacylglycerol kinase (DGK) regulates the level of the second messenger diacylglycerol and produces phosphatidic acid (PA), another signaling molecule. The Arabidopsis thaliana genome encodes seven putative diacylglycerol kinase isozymes (named AtDGK1 to -7), structurally falling into three major clusters. So far, enzymatic activity has not been reported for any plant Cluster II DGK. Here, we demonstrate that a representative of this cluster, AtDGK7, is biochemically active when expressed as a recombinant protein in Escherichia coli. AtDGK7, encoded by gene locus At4g30340, contains 374 amino acids with an apparent molecular mass of 41.2 kDa. AtDGK7 harbors an N-terminal catalytic domain, but in contrast to various characterized DGKs (including AtDGK2), it lacks a cysteine-rich domain at its N terminus, and, importantly, its C-terminal DGK accessory domain is incomplete. Recombinant AtDGK7 expressed in E. coli exhibits Michaelis-Menten type kinetics with 1,2-dioleoyl-sn-glycerol as substrate. AtDGK7 activity was affected by pH, detergents, and the DGK inhibitor R59022. We demonstrate that both AtDGK2 and AtDGK7 phosphorylate diacylglycerol molecular species that are typically found in plants, indicating that both enzymes convert physiologically relevant substrates. AtDGK7 is expressed throughout the Arabidopsis plant, but expression is strongest in flowers and young seedlings. Expression of AtDGK2 is transiently induced by wounding. R59022 at approximately 80 mum inhibits root elongation and lateral root formation and reduces plant growth, indicating that DGKs play an important role in plant development.     

Evidence for two closely related isozymes of arylamine N-acetyltransferase in human liver

Acetyl CoA-dependent arylamine N-acetyltransferase (EC 2.3.1.5) is the target of a genetic polymorphism in the metabolism of drugs and carcinogens. N-Acetyltransferase was purified 1000-fold from cytosol of human liver and its identity was verified by amino acid sequence homology of two of its tryptic peptides with published rabbit and chicken N-acetyltransferase sequences. Enzyme activity correlated with the presence of two proteins, NAT-1 and NAT-2, with indistinguishable molecular masses (31 kDa). NAT-1 and NAT-2 could be separated by anion-exchange chromatography and were functionally distinguished by their different apparent affinities for the acceptor amine sulfamethazine (SMZ). Antibodies raised against NAT-1 were able to recognize both isozymes on Western blots.     

Purification and characterization of membrane-bound and cytosolic forms of diacylglycerol kinase from rat brain

Two different types of diacylglycerol kinase (DGK) have been purified 10,455-fold (DGK I) and 7,410-fold (DGK IV) from the cytosol and membrane fractions of rat brain, respectively. The cytosolic DGK was purified by successive chromatographies on Affi-Gel Blue, Q-Sepharose F.F., Mono Q, hydroxylapatite, and ATP-agarose. The membrane-bound DGK was purified from the 2 M NaCl extract of membranes by chromatography on Affi-Gel Blue, phenyl-Superose, hydroxylapatite, and ATP-agarose. The resultant preparations contained homogeneous enzymes with a Mr of 110,000 (DGK I) and 150,000 (DGK IV) as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. These enzymes both phosphorylate 1,2-dioleoyl glycerol at rates of 11.5 mumol/min/mg protein for DGK I and 5.2 mumol/min/mg protein for DGK IV. Both enzymes require divalent cations and ionic detergents for activity. Magnesium is the most potent cation for both enzymes, but Ca2+ was also found to be fairly effective. Manganese is less effective than Mg2+ or Ca2+. Anionic detergents such as sodium deoxycholate or sodium cholate stimulate the activities of both enzymes, although DGK IV is stimulated more markedly than DGK I at lower concentrations. The optimal pH for the two enzymes was found to be the same, pH 7.4. Some phospholipids such as phosphatidylserine and phosphatidylinositol elevate the kinase activities of these kinases even in the absence of detergents. DGK IV is activated more significantly than DGK I by low amounts of phospholipids. The two enzymes also show structural differences. DGK I and DGK IV give different peptide maps after digestion with Staphylococcus aureus V8 protease or alpha-chymotrypsin. The results suggest that these enzymes are different forms of DGK and may be involved in different biological processes.     

Purification and characterization of glutathione peroxidase from human blood platelets. Age-related changes in the enzyme

Platelet glutathione peroxidase (GPx) is known to play a pivotal role in controlling the level of lipid hydroperoxides, especially those resulting from the 12-lipoxygenase activity. GPx was purified fromm the cell cytosol by more than 700-fold using an exchange chromatography, FPLP, gel filtration and covalent fixation. Isoelectric focusing revealed a peak activity at pH 5.1. The molecular mass of enzyme was found between 90 and 100 kDa by gel filtration, and was approximating at 23kDa by SDS-PAGE. A polyclonal antibody raised against commercial bovine erthrocyte GPx recognized the human platelet enzyme. It is concluded that human platelet GPx is likely a homotetramer of 92 kDa as described for most sources. We have also found that the decreased platelet GPx activity observed in platelets from elederly people is associated with a lower content of the immunoreactive enzyme.     

Purification and reconstitution of phosphatidylinositol 4-kinase from human erythrocytes.

A membrane-bound phosphatidylinositol 4-kinase (PtdIns kinase) has been purified to apparent homogeneity from human erythrocytes. Enzyme activity was solubilized from urea-KCl-stripped, inside-out membrane vesicles by 3% Triton X-100. Purification to apparent homogeneity was accomplished by cation-exchange chromatography on phosphocellulose, followed by heparin-acrylamide chromatography. This resulted in a nearly 3900-fold purification of PtdIns kinase activity to a specific activity of 44 nmol min-1 mg-1. The purified enzyme has an Mr of 59,000 on silver-stained SDS-PAGE; however, many preparations also contain 54 kDa and 50 kDa proteins which are related to the 59 kDa protein and have PtdIns kinase activity. Kinetic analysis of the PtdIns kinase indicate apparent Km values of 40 and 35 microM for phosphatidylinositol and ATP, respectively. The purified enzyme has been reconstituted into phospholipid liposomes and shown to phosphorylate phosphatidylinositol.     

Membrane-bound lipoxygenase of rat cerebral microvessels

The microvessels isolated from rat cerebral cortex has arachidonate lipoxygenase activity, which was not due to possible contamination of the platelets. The major product was identified to be 12-hydroxyeicosatetraenoic acid. After homogenization and sonication of the microvessel preparations, the lipoxygenase activity was recovered both in the membrane- and the cytosol-fractions, whereas that in the platelets was recovered in the cytosol fraction. Membrane-bound lipoxygenase of the microvessels has apparent Km value of 3.8 microM for arachidonic acid, which was corresponded to 1/5 of that in the platelet enzyme. Microvessel lipoxygenase was inhibited by nordihydroguaiaretic acid but not by indomethacin.