Choline Phosphotransferase Research Articles

Lipid composition of Botrytis cinerea and inhibition of its radiolabelling by the fungicide iprodione.

• Botrytis cinerea is an important plant pathogen that causes grey mould in over 200 hosts. It is often controlled by dicarboximides, which have various proposed mechanisms of action, including effects on lipids. Here we have examined the effect of one dicarboximide, iprodione, on lipid metabolism. • B.cinerea, cultured in malt extract media, was challenged with iprodione and its lipids extracted, separated by TLC, and analysed by GLC. Lipid metabolism was followed using [1-14 C]acetate. • Triacylglycerol was the major nonpolar and phosphatidylcholine the main polar lipid in B.cinerea. Linoleate, followed by α-linolenate, were the major fatty acids and most lipid classes had compositions broadly similar to the total fatty acid pattern. Iprodione, at concentrations causing a cessation of growth (5µM) caused a decrease in polar lipid but not total nonpolar lipid labelling. Within the nonpolar lipids, DAG was better labelled. • The data show that iprodione had a selective effect on lipid metabolism. The altered pattern of labelling suggested that choline (ethanolamine) phosphotransferase would be worth investigating as a primary site of action.

Pyrimidine 5' nucleotidase deficiency.

Pyrimidine 5' nucleotidase deficiency.

Differential expression of cholinephosphotransferase in normal and cancerous human mammary epithelial cells

Membrane phospholipids as well as fatty acid profile of cell membrane phospholipids are altered in tumorigenicity and malignancy. Synthesis of total cellular phosphatidylcholine (PC) can be used as a marker for membrane proliferation in neoplastic mammary gland tissues. Cholinephosphotransferase (CPT), the terminal enzyme in the de novo synthesis of PC, has an important role in regulating the acyl group of PC in mammalian cells. In this study, the effect of neoplasia on CPT was examined. The gene shows an elevated expression in cancerous (11-9-14) breast epithelial cell line when compared to that of normal non-tumorigenic (MCF-12A) breast epithelial cell line. Four nucleotide substitutions are observed in the cancer cell line. Of these, three are null mutations, but the third one shows an interesting serine to tyrosine substitution (at amino acid position 89 of our partial sequence which corresponds to position 323 of the CPT sequence reported as NM_020244 in GenBank) in 11-9-14 cells. The tyrosine is present in the right context of KSELYQDT, which directs tyrosine phosphorylation at the tyrosine site. Biochemical approach also reveals a 1.5-fold stimulation in CPT activity in 11-9-14 cells compared to that of the MCF-12A cells.

Hexadecylphosphocholine inhibits phosphatidylcholine biosynthesis and the proliferation of HepG2 cells.

Hexadecylphosphocholine (HePC) is a synthetic lipid representative of a new group of antiproliferative agents, alkylphosphocholines (APC), which are promising candidates in anticancer therapy. Thus we have studied the action of HePC on the human hepatoblastoma cell line HepG2, which is frequently used as a model for studies into hepatic lipid metabolism. Non-toxic, micromolar concentrations of HePC exerted an antiproliferative effect on this hepatoma cell line. The incorporation into phosphatidylcholine (PC) of the exogenous precursor [methyl-14C]choline was substantially reduced by HePC. This effect was not due to any alteration in choline uptake by the cells, the degradation rate of PC or the release of PC into the culture medium. As anaccumulation of soluble choline derivatives points to CTP:phosphocholine cytidylyltransferase (CT) as the target of HePC activity we examined its effects on the different enzymes involved in the biosynthesis of PC via CDP-choline. Treatment with HePC altered neither the activity of choline kinase (CK) nor that of diacylglycerol cholinephosphotransferase (CPT), but it did inhibit CT activity in HepG2 cells. In vitro HePC also inhibited the activity of cytosolic but not membrane-bound CT. Taken together our results suggest that HePC interferes specifically with the biosynthesis of PC in HepG2 cells by depressing CT translocation to the membrane, which may well impair their proliferation.

Phospholipase C inhibitors and prostaglandins differentially regulate phosphatidylcholine synthesis in rat renal papilla: Evidence of compartmental regulation of CTP:phosphocholine cytidylyltransferase and CDP-choline:1,2-diacylglycerol cholinephosphotransferase

Phosphatidylcholine (PC) is the most abundant phospholipid in mammalian cell membranes. Several lines of evidence support that PC homeostasis is preserved by the equilibrium between PC biosynthetic enzymes and phospholipases catabolic activities. We have previously shown that papillary synthesis of PC depends on prostaglandins (PGs) that modulate biosynthetic enzymes. In papillary tissue, under bradikynin stimulus, arachidonic acid (AA) mobilization (the substrate for PG synthesis) requires a previous phospholipase C (PLC) activation. Thus, in the present work, we study the possible involvement of PLC in PC biosynthesis and its relationship with PG biosynthetic pathway on the maintenance of phospholipid renewal in papillary membranes; we also evaluated the relevance of CDP-choline pathway enzymes compartmentalization. To this end, neomycin, U-73122 and dibutiryl cyclic AMP, reported as PLC inhibitors, were used to study PC synthesis in rat renal papilla. All the PLC inhibitors assayed impaired PC synthesis. PG synthesis was also blocked by PLC inhibitors without affecting cyclooxygenase activity, indicating a metabolic connection between both pathways. However, we found that PC biosynthesis decrease in the presence of PLC inhibitors was not a consequence of PG decreased synthesis, suggesting that basal PLC activity and PGs exert their effect on different targets of PC biosynthetic pathway. The study of PC biosynthetic enzymes showed that PLC inhibitors affect CTP:phosphocholine cytidylyltransferase (CCT) activity while PGD 2 operates on CDP-choline:1,2-diacylglycerol cholinephosphotransferase (CPT), both activities associated to papillary enriched-nuclei fraction. The present results suggest that renal papillary PC synthesis is a highly regulated process under basal conditions. Such regulation might occur at least at two different levels of the CDP-choline pathway: on the one hand, PLC operates on CCT activity; on the other, while PGs regulate CPT activity.

High glucose and advanced glycation end products induce phospholipid hydrolysis and phospholipid enzyme inhibition in bovine retinal pericytes

In the present study, we investigated the possible role of oxidative stress and the modulation of phospholipid turnover in two related models of pericyte injury, i.e., treatment with high glucose or advanced glycation end products (AGEs). Growing microcapillary pericytes from bovine retinas in culture were incubated, for 3 weeks, with 20–50 mM glucose or 2–20 μM AGEs, and peroxidation parameters (malondialdehyde, conjugated diene, hydroperoxide, glutathione (GSH) levels and lactate dehydrogenase (LDH) release) were evaluated. Arachidonate (AA) and choline release from membrane phospholipids was determined in pericytes prelabeled with [1- 14C]arachidonate and [Me- 3H]choline, respectively, and stimulated with elevated glucose or AGEs for 30 min or 2 h. [1- 14C]arachidonate and [Me- 3H]choline incorporation into phospholipids, for 2 h and 3 h respectively, was also studied in conditioned and serum-starved cultures. Finally, lysates of treated and control cells were assayed for cytosolic phospholipase A 2 (cPLA 2), acyl-CoA:1-acyl- sn-glycero-3-phosphocholine O-acyltransferase (AT), CTP:phosphocholine cytidylyltransferase (CT) and microsomal choline phosphotransferase (CPT) enzyme activities. We found that high glucose and AGEs caused neither significant production of reactive oxygen species nor cell toxicity or death, unlike other cell types. Both agents had no significant effect on the cellular ultrastructure, evaluated by light and electron microscopy, AA incorporation and release, cytosolic phospholipase A 2 (cPLA 2) and AT activities. On the contrary, choline incorporation into phosphatidylcholine, CT and CPT activities were significantly reduced either by 50 mM glucose or 20 μM AGEs. Simultaneously, [Me- 3H]choline release was significantly stimulated by both agents. We conclude that prolonged treatments with high glucose or AGEs are not able to induce oxidative injury in bovine retinal capillary pericytes. Nevertheless, they do induce phospholipid hydrolysis and phospholipid enzyme activity inhibition.

Regulation of Cholinephosphotransferase by Thyroid Hormone

We have demonstrated earlier that thyroid hormone (T3) regulates the activity of cholinephosphotransferase (CPT) in guinea pig lung. This effect of T3 is not organ specific because we found T3 also regulates CPT activity in the guinea pig liver. Northern blot analysis using two oligonucleotide probes, one synthesized on the basis of the yeast CPT gene sequence and another on the basis of partial cDNA clone from guinea pig CPT clone, revealed that T3 stimulates the expression of new CPT mRNA. Studies with transcriptional and translational inhibitors indicated that T3 enhanced the translation of the CPT mRNA as well as translocation of preformed CPT enzyme protein from cytosol to mitochondria. Furthermore, it strengthens our previous finding that yeast CPT and guinea pig CPT have high homology in their sequence as both the oligonucleotide probes gave the similar type of Northern blot in the present study.

Turnover of Phosphatidylcholine in Saccharomyces cerevisiae: THE ROLE OF THE CDP-CHOLINE PATHWAY

The regulation of phosphatidylcholine degradation as a function of the route of phosphatidylcholine (PC) synthesis and changing environmental conditions has been investigated in the yeast Saccharomyces cerevisiae. In the wild-type strains studied, deacylation of phosphatidylcholine to glycerophosphocholine is induced when choline is supplied to the culture medium and, also, when the culture temperature is raised from 30 to 37 degrees C. In strains bearing mutations in any of the genes encoding enzymes of the CDP-choline pathway for phosphatidylcholine biosynthesis (CKI1, choline kinase; CPT1, 1, 2-diacylglycerol choline phosphotransferase; PCT1, CTP:phosphocholine cytidylyltransferase), no induction of phosphatidylcholine turnover and glycerophosphocholine production is seen in response to choline availability or elevated temperature. In contrast, the induction of phosphatidylcholine deacylation does occur in a strain bearing mutations in genes encoding enzymes of the methylation pathway for phosphatidylcholine biosynthesis (i.e. CHO2/PEM1 and OPI3/PEM2). Whereas the synthesis of PC via CDP-choline is accelerated when shifted from 30 to 37 degrees C, synthesis of PC via the methylation pathway is largely unaffected by the temperature shift. These results suggest that the deacylation of PC to GroPC requires an active CDP-choline pathway for PC biosynthesis but not an active methylation pathway. Furthermore, the data indicate that the synthesis and turnover of CDP-choline-derived PC, but not methylation pathway-derived PC, are accelerated by the stress of elevated temperature.

Modulation of biosynthesis of phosphatidylcholine via CDP-choline in rat liver: influence of ethanol on the microsomal cholinephosphotransferase activity.

We have studied in vitro the effects of ethanol on the different enzymes involved in the biosynthesis of phosphatidylcholine (PC) via CDP-choline. Ethanol alters neither choline kinase (CK) nor CTP:phosphocholine cytidylyltransferase (CT) activities but, at levels higher than 50 mM, it does significantly inhibit microsomal cholinephosphotransferase (CPT) activity concomitantly with an increase in the ethanol concentration. A study of the kinetics of the reaction catalysed by CPT shows that ethanol decreases Vmax without altering Km, indicating a non-competitive inhibitory effect. An analysis of the thermodependence of CPT activity in the absence of ethanol reveals a break in the Arrhenius plot and thus a straight relationship between enzyme activity and the physico-chemical state of the microsomal membrane. Incubation of microsomes in the presence of ethanol increased the transition temperature from 25.8-28.2 degrees C. Microsomes were also incubated with n-alkanols with chain-lengths of fewer than five carbon atoms at concentrations which, according to their partition coefficients, produce equimolar levels in the membrane. Under these conditions all the alkanols caused the same inhibitory effect. All these results demonstrate that ethanol modulate the PC biosynthesis at the level of CPT activity and does not affect the CT enzyme. The inhibition found on CPT is clearly dependent on the alteration produced by ethanol on the hepatic microsomal membrane.

Inhibition of Phosphatidylcholine Biosynthesis following Induction of Apoptosis in HL-60 Cells

Induction of apoptosis in HL-60 cells, using a variety of cytotoxic drugs, resulted, in all cases, in inhibition of CDP-choline:1, 2-diacylglycerol choline phosphotransferase, leading to an accumulation of its substrate, CDP-choline, and inhibition of phosphatidylcholine biosynthesis. Incubation of the cells with phosphatidylcholine reduced the number displaying an apoptotic morphology following drug treatment, and this was inversely related to the degree to which the drugs inhibited phosphatidylcholine biosynthesis. Inhibition of choline phosphotransferase by two of the drugs, farnesol and chelerythrine, was shown to be due to direct inhibition of the enzyme, while inhibition by the other drugs, etoposide and camptothecin, could be explained by the intracellular acidification that followed induction of apoptosis.

Phosphorylation and Regulation of Choline Kinase fromSaccharomyces cerevisiae by Protein Kinase A

The CKI1-encoded choline kinase (ATP:choline phosphotransferase, EC 2.7.1.32) from Saccharomyces cerevisiae was phosphorylated in vivo on multiple serine residues. Activation of protein kinase A activity in vivo resulted in a transient increase in the phosphorylation of choline kinase. This phosphorylation was accompanied by a stimulation in choline kinase activity. In vitro, protein kinase A phosphorylated choline kinase on a serine residue with a stoichiometry (0.44 mol of phosphate/mol of choline kinase) consistent with one phosphorylation site/choline kinase subunit. The major phosphopeptide derived from the enzyme phosphorylated in vitro by protein kinase A was common to one of the major phosphopeptides derived from the enzyme phosphorylated in vivo. Protein kinase A activity was dose- and time-dependent and dependent on the concentrations of ATP (Km 2.1 microM) and choline kinase (Km 0.12 microM). Phosphorylation of choline kinase with protein kinase A resulted in a stimulation (1.9-fold) in choline kinase activity whereas alkaline phosphatase treatment of choline kinase resulted in a 60% decrease in choline kinase activity. The mechanism of the protein kinase A-mediated stimulation in choline kinase activity involved an increase in the apparent Vmax values with respect to ATP (2.6-fold) and choline (2.7-fold). Overall, the results reported here were consistent with the conclusion that choline kinase was regulated by protein kinase A phosphorylation.

Effect of epidermal growth factor on enzymes of phospholipid biosynthesis in lung and liver of fetal rat in vivo and in vitro.

Epidermal growth factor (EGF), a mitogenic polypeptide that binds to cell surface receptors, is an important regulator of cell differentiation and fetal lung surfactant synthesis, and may be used as a potential novel therapeutic agent in prematurity. Nevertheless, the distinct role in lung development and its mechanisms of action are not well understood. We investigated in vivo the systemic effect of intrafetally administered EGF (200 ng/g fetal body weight) and maternally administered dexamethasone (DEXA; 0.2 and 2.0mg/kg maternal body weight) on the activity of important enzymes of the phospholipid synthesis in the fetal rat lung and liver: choline kinase (EC 2.7.1.32), cholinephosphate cytidyltransferase (EC 2.7.7.15), choline phosphotransferase (EC 2.7.8.2), lysolecithin acyltransferase (EC 2.3.1.23) and glycerolphosphate phosphatidyltransferase (EC 2.7.8.5). Additionally, in vivo and in vitro effects of DEXA on EGF receptor synthesis, and the effects of EGF on protein content and morphogenesis of the fetal rat lung organoid culture, were evaluated. Whereas DEXA induced the activity of all investigated enzymes of phospholipid synthesis and increased EGF receptor synthesis, EGF has no effects on the enzymes, either in vivo or in vitro. EGF enhanced protein synthesis and morphogenesis in vitro. With respect to our data and the literature, we hypothesize that DEXA and EGF may act on different cellular sides. Whereas glucocorticoids induce surfactant phospholipid synthesis, EGF should be more involved in cell proliferation and morphogenesis.

Competitive Inhibition of Choline Phosphotransferase by Geranylgeraniol and Farnesol Inhibits Phosphatidylcholine Synthesis and Induces Apoptosis in Human Lung Adenocarcinoma A549 Cells

We have previously shown that, among various isoprenoids, farnesol and geranylgeraniol specifically induced actin fiber disorganization, growth inhibition, and apoptosis in human lung adenocarcinoma A549 cells (Miquel, K., Pradines, A., and Favre, G. (1996) Biochem. Biophys. Res. Commun. 225, 869-876). Here we demonstrate that isoprenoid-induced apoptosis was preceded by an arrest in G0/G1 phase. The isoprenoid effects were independent of protein prenylation and of mitogen-activated protein kinase activity. Moreover, geranylgeraniol and farnesol induced a rapid inhibition of phosphatidylcholine biosynthesis at the last step of the CDP-choline pathway controlled by choline phosphotransferase and not at the level of CTP:phosphocholine cytidylyltransferase, the key enzyme of the pathway. Inhibition of choline phosphotransferase was confirmed by in vitro assays on microsomal fractions, which clearly showed that the isoprenoids acted by competitive inhibition with the diacylglycerol binding. Exogenous phosphatidylcholine addition prevented all the biological effects of the isoprenoids, including actin fiber disorganization and apoptosis, suggesting that inhibition of phosphatidylcholine biosynthesis might be the primary event of the isoprenoid action. These data demonstrate the molecular mechanism of geranylgeraniol and farnesol effects and suggest that the mevalonate pathway, leading notably to prenylated proteins, might be linked to the control of cell proliferation through the regulation of phosphatidylcholine biosynthesis.

Expression, Purification, and Characterization of Choline Kinase, Product of the CKI Gene from Saccharomyces cerevisiae

In the yeast Saccharomyces cerevisiae, choline kinase (ATP:choline phosphotransferase, EC 2.7.1.32) is the product of the CKI gene. Choline kinase catalyzes the committed step in the synthesis of phosphatidylcholine by the CDP-choline pathway. The yeast enzyme was overexpressed 106-fold in Sf-9 insect cells and purified 71.2-fold to homogeneity from the cytosolic fraction by chromatography with concanavalin A, Affi-Gel Blue, and Mono Q. The N-terminal amino acid sequence of purified choline kinase matched perfectly with the deduced sequence of the CKI gene. The minimum subunit molecular mass (73 kDa) of purified choline kinase was in good agreement with the predicted size (66.3 kDa) of the CKI gene product. Native choline kinase existed in oligomeric structures of dimers, tetramers, and octomers. The amounts of the tetrameric and octomeric forms increased in the presence of the substrate ATP. Antibodies were raised against the purified enzyme and were used to identify choline kinase in insect cells and in S. cerevisiae. Maximum choline kinase activity was dependent on Mg2+ ions (10 mM) at pH 9.5 and at 30 degrees C. The equilibrium constant (0.2) for the reaction indicated that the reverse reaction was favored in vitro. The activation energy for the reaction was 6.26 kcal/mol, and the enzyme was labile above 30 degrees C. Choline kinase exhibited saturation kinetics with respect to choline and positive cooperative kinetics with respect to ATP (n = 1.4-2.3). Results of the kinetic experiments indicated that the enzyme catalyzes a sequential Bi Bi reaction. The Vmax for the reaction was 138.7 micromol/min/mg, and the Km values for choline and ATP were 0.27 mM and 90 microM, respectively. The turnover number per choline kinase subunit was 153 s-1. Ethanolamine was a poor substrate for the purified choline kinase, and it was also poor inhibitor of choline kinase activity. ADP inhibited choline kinase activity (IC50 = 0.32 mM) in a positive cooperative manner (n = 1.5), and the mechanism of inhibition with respect to ATP and choline was complex. The regulation of choline kinase activity by ATP and ADP may be physiologically relevant.

Phospholipid-metabolizing enzymes in Alzheimer's disease: increased lysophospholipid acyltransferase activity and decreased phospholipase A2 activity.

Damage to brain membrane phospholipids may play an important role in the pathogenesis of Alzheimer's disease (AD); however, the critical metabolic processes responsible for the generation and repair of membrane phospholipids affected by the disease are unknown. We measured the activity of key phospholipid catabolic and anabolic enzymes in morphologically affected and spared areas of autopsied brain of patients with AD and in matched control subjects. The activity of the major catabolic enzyme phospholipase A2 (PLA2), measured in both the presence and absence of Ca2+, was significantly decreased (-35 to -53%) in parietal and temporal cortices of patients with AD. In contrast, the activities of lysophospholipid acyltransferase, which recycles lysophospholipids into intact phospholipids, and glycerophosphocholine phosphodiesterase, which returns phospholipid catabolites to be used in phospholipid resynthesis, were increased by approximately 50-70% in the same brain areas. Brain activities of enzymes involved in de novo phospholipid synthesis (ethanolamine kinase, choline kinase, choline phosphotransferase, phosphoethanolamine cytidylyltransferase, and phosphocholine cytidylyltransferase) were either normal or only slightly altered. The activities of PLA2 and acyltransferase were normal in the degenerating cerebellum of patients with spinocerebellar atrophy type 1, whereas the activity of glycerophosphocholine phosphodiesterase was reduced, suggesting that the alterations in AD brain were not nonspecific consequences of neurodegeneration. Our data suggest that compensatory phospholipid metabolic changes are present in AD brain that reduce the rate of phospholipid loss via both decreased catabolism (PLA2) and increased phospholipid resynthesis (acyltransferase and glycerophosphocholine phosphodiesterase).

Role of platelet-activating factor in two-kidney, one-clip hypertension.

Platelet-activating factor (PAF) is a bioactive phospholipid which is a potent hypotensive agent. To investigate the role of PAF in renovascular hypertension, we determined the PAF concentration and its production level assessed by the activity of cholinephosphotransferase (CPT) in renal tissue and examined the effect of a PAF antagonist on the mean arterial pressure (MAP) in control and two-kidney with one clipped (2K1C) hypertensive rats. The concentration of PAF and CPT in the renal medulla and cortex were determined by radioassay. Also, the effect of a PAF antagonist, CV-6209, on MAP was also examined in both 2K1C hypertensive and normal control rats. The PAF concentration and CPT activity were significantly higher in the medulla than in the cortex in both 2K1C hypertensive and normal control rats, and both values in the medulla were also significantly higher in the clipped kidney than in the contralateral unclipped kidney or in control rat kidneys. We also observed a significant negative correlation between the PAF concentration in the medulla, and the medulla weight in the clipped kidney of 2K1C hypertensive rats. Infusion of the PAF antagonist, CV-6209, did not affect MAP in 2K1C hypertensive rats, but was significantly increased (P < 0.05) in control rats. These findings suggest that PAF, whose production is induced by renal ischemia due to renal artery stenosis, plays an important role in the renomedullary vasodepressor system, but the effect of PAF as a vasodilator in the peripheral vessels is limited in 2K1C hypertension.

Activities of enzymes in platelet activating factor biosynthetic pathways in the gerbil model of cerebral ischemia.

The activities of enzymes in platelet activating factor (PAF) biosynthetic pathways were analyzed in hippocampal and cerebral cortical regions of normal and ischemic gerbil brain to assess changes in enzyme activities and potential modulators that could explain the accentuated production of PAF seen in ischemia. Global forebrain ischemia was produced by bilateral carotid artery ligation, and the effectiveness of the ligation was shown by free fatty acid release and ATP depletion. Specific activities of 1-alkyl-2-acetyl-sn-glycerol (AAG) choline phosphotransferase, 1-alkyl-sn-glycero-3-phosphate (AGP) acetyl transferase, and 1-alkyl-sn-glycero-3-phosphocholine (lyso PAF) acetyl transferase in tissue homogenates were in the ratio 4:1:0.1, respectively. Sham-operated and ischemic or ischemic-reperfused tissues showed similar activities for individual enzymes, indicating that enzyme levels or activation states did not change in ischemic or reperfused tissues. However, small metabolites (relevant to ischemia) added to the in vitro assays did modify enzyme activities. Physiological concentrations of MgATP severely inhibited AGP acetyl transferase activity, and this resulted in the ratio of AGP acyl transferase to AGP acetyl transferase activities changing from 48:1 in the presence of 2.5 mM MgATP to 6:1 in the absence of MgATP. This suggests that falling ATP levels in cerebral ischemia may promote the de novo pathway of PAF biosynthesis by releasing inhibition of AGP acetyl transferase. Lyso PAF acetyl transferase was much less active than AGP acetyl transferase and was also inhibited by MgATP. AAG choline phosphotransferase was not inhibited by MgATP but was inhibited by calcium. However the superior specific activity of the choline phosphotransferase in comparison with the AGP acetyl transferase suggested that the lowered choline phosphotransferase activity in the presence of rising intracellular calcium would not seriously compromise the synthesis of PAF by the de novo route. Both acetyl transferase enzymes were also inhibited by oleoyl CoA.

Phosphatidylcholine synthesis-related enzyme activities of bovine brain microvessels exhibit susceptibility to peroxidation

In microvessels isolated from bovine brain, microsomal enzyme activities involved in phosphatidylcholine biosynthesis and degradation were determined. The microvessels possessed acyl-CoA:1-acyl- sn-glycero-3-phosphocholine (AT) and glycerophosphocholine phosphodiesterase (GroPChoPDE) activity at a higher level compared with bovine and rat brain or rat liver microsomes whereas they expressed CTP: phosphocholine cytidylyltransferase (CT) and choline phosphotransferase (CPT) activity at a lower level. Each enzyme has been characterized in terms of response to inhibitors or activators revealing properties very similar to those in brain and liver microsomes. In the homogenate prepared from t-butylhydroperoxide-treated microvessels (10 min exposure to 10 μM up to 1 mM concentrations), AT and CPT activities exhibited a significant dose-dependent inhibition. In contrast, GroPChoPDE activity was unaffected. CT was inhibited only at 1 mM concentration. Short treatment of microvessels with Fe 2+ (20 μM)-ascorbate (0.25 mM) or 100 μM linoleate hydroperoxide did not have any effect on the activity of the four enzymes. Strong inhibition of all enzymes was noted when the linoleate hydroperoxide system was fortified by Fe 2+ ions (100 μM). AT inactivation was also found when oxidized low density lipoprotein was preincubated with microvessels. On the other hand, oxidized LDL left unchanged CPT and GroPChoPDE activities whereas it promoted a slight stimulation of cytidylyltransferase activity. Overall, the results suggest a link between oxygen radical generation and the perturbation of the microvessel membrane structure in which the four enzymes are incorporated, coupled to a direct sulfhydryl protein modification.

Oleate desaturase activity in sunflower ( Helianthus annuus) seeds and its relation to associated constitutents during seed development

The locus and specific acitivity of oleate desaturase (ODS) activity of developing sunflower seeds has been studied using differential centrifugation. The ratio of activity recovered in the crude pellet, oil body and microsomal fractions was consistently about 3:1:2 throughout seed development. Microsomes always contained the highest specific activity of the enzyme, reaching a maximum at 20–25 days after flowering and declining rapidly thereafter. The change in desaturase specific activity (in vitro) with seed development slightly preceded linoleate accumulation (in vivo) but was synchronous with the changing levels of cytochrome b. Although the associated metabolic enzymes CDP-choline:1,2-diacylglycerol cholinephosphotransferase (CPT) and acyl-CoA:lysophosphatidylcholine acyltransferase (LPCAT) reached maximum activities at the same time as the desaturase enzyme, these activities decreased less markedly during subsequent seed maturation. An unrelated microsomal enzyme, NADPH-cytochrome c-reductase (NCR), did not reach maximal activities until 40 days after flowering.

Phospholipid and cation activation of chimaeric choline/ethanolamine phosphotransferases

The Saccharomyces cerevisiae CPT1 and EPT1 genes encode for a cholinephosphotransferase (CPT) and choline/ethanolaminephosphotransferase, respectively. Both Cpt1p and Ept1p activities display an absolute requirement for cations and phospholipids. A mixed-micelle assay was employed to determine cation and lipid activators of parental and chimaeric Cpt1p/Ept1p enzymes to gain insight into their mechanism(s) of activation. Mg2+, Mn2+ and Co2+ were the only cations capable of activating Cpt1p and Ept1p in vitro. Kinetic data revealed that only Mg2+ is present in appropriate amounts to activate CPT activity in vivo. Kinetic data revealed that only Mg2+ is present in appropriate amounts to activate CPT activity in vivo. The two enzymes displayed distinct activation profiles on the basis of their relative affinities for Mg2+, and Mn2+ and Co2+. This allowed the use of chimaeric enzymes to determine the mechanism of cation activation. Cations do not activate Cpt1p or Ept1p by complexing with the substrate, CDP-choline, but instead bind to disparate regions within the enzymes themselves. Cpt1p and Ept1p also displayed distinct phospholipid activation profiles. Phospholipid activation required a phosphate and/or glycero-phosphoester linkage, with the phospho-head group moiety positioned at the surface of the micelle. Assays with parental and chimaeric Cpt1p/Ept1p constructs revealed that the phospholipid binding/activation domains are not located within linear segments of the protein, but instead are contained within distinct and separate regions of the proteins that require an intact tertiary structure for formation. Phosphatidylcholine (and its structural analogue sphingomyelin) were the best lipid activators of Cpt1p, the main biologically relevant CPT activity in S. cerevisiae. Hence CPT displays product activation. Because phosphatidylcholine is an efficient activator of CPT activity (and hence Cpt1p is not subject to feedback inhibition by its product), Cpt1p is incapable of functioning as a direct monitor of membrane phosphatidylcholine composition.