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H Jamil

Publications and source records attributed to H Jamil.

At least 37 records · Page 2Linked to original sources
PubMed1997

Analysis of the in vitro effect of exogenous nitric oxide on human lymphocytes.

We investigated the role of endogenous or exogenous nitric oxide (NO) on human lymphocyte function. We used sodium nitroprusside, nitroglycerine, S-nitroso-N-acetylpenicillamine, sodium nitrite and S-nitroso-L-glutathione as NO-generating compounds. All agents were used at doses that do not produce direct cytotoxicity as measured by trypan blue exclusion as well as chromium-51 release assay. The immune responses examined were peripheral blood lymphocytes (PBL) proliferation and IL-2 production after activation with OKT3 and PHA; allogeneic mediated proliferation and cell mediated cytotoxicity (CML) in MLR; IgG and IgM production after PBL activation with Con-A; proliferation and expression of IFN-gamma and IL-4 mRNA after activation of allogeneic CD4+T cell clones. Cytokine mRNA expression was measured by reverse transcriptase PCR. Our results show that proliferating lymphocytes do not produce a detectable amount of NO as measured by the Griess reaction. In separate experiments, the addition of NG-monomethyl-L-arginine (L-NMMA) did not affect lymphocyte proliferation. Sodium nitroprusside and nitroglycerine exerted a dose dependent antimitogenic effect, inhibited cytokine production and expression, CML generation and antibody production. DNA gel electrophoresis showed no evidence for enhanced programmed cell death. The antimitogenic effect could not be blocked by the NO scavengers, hemoglobin or methylene blue. In contrast, the other nitric oxide generating compounds did not inhibit lymphocyte mitogenesis. The results suggest that human lymphocytes do not produce appreciable amounts of NO to affect lymphocyte mitogenesis. Sodium nitroprusside and nitroglycerine have a potent but nonspecific immunoinhibitory effect on human lymphocyte function by a mechanism other than NO production. In addition, pharmacological levels of NO do not inhibit human lymphocyte mitogenesis.

Cytokines↗
PubMed1997

Assembly and secretion of VLDL in nondifferentiated Caco-2 cells stably transfected with human recombinant ApoB48 cDNA.

Intestinal cells secrete apoB48-containing very low density lipoproteins (VLDLs) and chylomicrons for the transport of biliary and dietary lipids. The molecular mechanisms regulating the assembly of intestinal lipoproteins are not known due to a lack of reliable and specific cell culture models. Caco-2 (a human colon carcinoma) cells have been used to study intestinal lipid metabolism. These cells have been shown to secrete both apoB100- and apoB48-containing triglyceride (TG)-rich lipoproteins only after differentiation into enterocyte-like cells. To study lipoprotein assembly in nondifferentiated Caco-2 cells, we stably expressed human recombinant apoB48 cDNA under the control of a constitutive cytomegalovirus promoter. Pulse-chase analysis revealed that the majority (> 50%) of apoB48 synthesized was degraded intracellularly in the presence or absence of oleic acid. Transfected nondifferentiated cells secreted lipoproteins with flotation densities similar to those of plasma HDL or LDL when cultured in serum-free or serum-containing media, respectively. Incubation of cells with media containing serum and oleic acid resulted in the secretion of VLDL-like particles. Secretion of VLDL was inhibited (> 80%) by triacsin C due to > 60% inhibition of oleate-induced TG synthesis. However, inhibition of cholesteryl ester synthesis by 70% with an acyl coenzyme A:cholesterol acyltransferase inhibitor did not affect VLDL secretion. Efficient assembly of lipoproteins usually requires the microsomal TG transfer protein (MTP). The presence of MTP in transfected Caco-2 cells was investigated by measuring TG transfer activity in microsomal fractions. Microsomal fractions had 0.2% TG transfer activity per hour per microgram of protein, which corresponds to 30% to 60% of the MTP activity present in liver-derived cells. To determine whether MTP activity was required for lipoprotein assembly, transfected cells were incubated in the presence of the MTP inhibitor CP-10,447. This compound completely abolished the secretion of apoB. These data show that the transfected cell lines secrete lipoproteins of different densities under different culture conditions and that the assembly of larger VLDL particles requires active TG synthesis and MTP activity. Thus, in nondifferentiated Caco-2 cells, the amount of apoB secreted and not the MTP activity is the limiting factor for lipoprotein assembly.

Apolipoprotein B-48↗
PubMed1996

Inhibition of the microsomal triglyceride transfer protein blocks the first step of apolipoprotein B lipoprotein assembly but not the addition of bulk core lipids in the second step.

The microsomal triglyceride transfer protein (MTP) is required for assembly and secretion of the lipoproteins containing apolipoprotein B (apoB): very low density lipoproteins and chylomicrons. Evidence indicates that the subclasses of these lipoproteins that contain apoB-48 are assembled in a distinct two-step process; first a relatively lipid-poor primordial lipoprotein precursor is produced, and then bulk neutral lipids are added to form the core of these spherical particles. To determine if either step is mediated by MTP, a series of clonal cell lines stably expressing apoB-53 and MTP was established in non-lipoprotein-producing HeLa cells. MTP activity in these cells was approximately 30%, and apoB secretion was 7-33% of that in HepG2 cells on a molar basis. Despite having robust levels of triglyceride and phospholipid synthesis, these cell lines, as exemplified by HLMB53-59, secreted >90% of the apoB-53 on relatively lipid-poor particles in the density range of 1.063-1.21 g/ml. These results suggested that coexpression of MTP and apoB only reconstituted the first but not the second step in lipoprotein assembly. To extend this observation, additional studies were carried out in McArdle RH-7777 rat hepatoma cells, in which the second step of apoB-48 lipoprotein assembly is well defined. Treatment of these cells with the MTP photoaffinity inhibitor BMS-192951 before pulse labeling with [35S]methionine/cysteine led to an 85% block of both apoB-48 and apoB-100 but not apoAI secretion, demonstrating inhibition of the first step of lipoprotein assembly. After a 30-min [35S]methioneine/cysteine pulse labeling and 120 min of chase, all of the nascent apoB-48 was observed to have a density of high density lipoproteins (1.063-1.21 g/ml), indicating that only the first step of lipoprotein assembly had occurred. The addition of oleic acid to the cell culture media activated the second step as evidenced by the conversion of the apoB-48 high density lipoproteins to very low density lipoproteins (d < 1.006 g/ml) during an extended chase period. Inactivation of MTP after completion of the first step, but before stimulation of the second step by the addition of oleic acid, did not block this conversion. Thus, inhibition of MTP did not hinder the addition of bulk core lipid to the primordial lipoprotein precursor particles, indicating that MTP is not required for the second step of apoB-48 lipoprotein assembly.

Animals↗
PubMed1996

A novel abetalipoproteinemia genotype. Identification of a missense mutation in the 97-kDa subunit of the microsomal triglyceride transfer protein that prevents complex formation with protein disulfide isomerase.

The microsomal triglyceride transfer protein (MTP) is a heterodimer composed of the ubiquitous multifunctional protein, protein disulfide isomerase, and a unique 97-kDa subunit. Mutations that lead to the absence of a functional 97-kDa subunit cause abetalipoproteinemia, an autosomal recessive disease characterized by a defect in the assembly and secretion of apolipoprotein B (apoB) containing lipoproteins. Previous studies of abetalipoproteinemic patient, C.L., showed that the 97-kDa subunit was undetectable. In this report, [35S]methionine labeling showed that this tissue was capable of synthesizing the 97-kDa MTP subunit. Electrophoretic analysis showed two bands, one with a molecular mass of the wild type 97-kDa subunit and the other with a slightly lower molecular weight. Sequence analysis of cDNAs from additional intestinal biopsies showed this patient to be a compound heterozygote. One allele contained a perfect in-frame deletion of exon 10, explaining the lower molecular weight band. cDNAs of the second allele were found to contain 3 missense mutations: His297 --> Gln, Asp384 --> Ala, and Arg540 --> His. Transient expression of each mutant showed that only the Arg540 --> His mutant was non-functional based upon its inability to reconstitute apoB secretion in a cell culture system. The other amino acid changes are silent polymorphisms. High level coexpression in a baculovirus system of the wild type 97-kDa subunit or the Arg540 --> His mutant along with human protein disulfide isomerase showed that the wild type was capable of forming an active MTP complex while the mutant was not. Biochemical analysis of lysates from these cells showed that the Arg to His conversion interrupted the interaction between the 97-kDa subunit and protein disulfide isomerase. Replacement of Arg540 with a lysine residue maintained the ability of the 97-kDa subunit to complex with protein disulfide isomerase and form the active MTP holoprotein. These results indicate that a positively charged amino acid at position 540 in the 97-kDa subunit is critical for the productive association with protein disulfide isomerase. Of the 13 mutant MTP 97-kDa subunit alleles described to date, this is the first encoding a missense mutation.

Apolipoproteins B↗
PubMed1996

An inhibitor of the microsomal triglyceride transfer protein inhibits apoB secretion from HepG2 cells.

The microsomal triglyceride (TG) transfer protein (MTP) is a heterodimeric lipid transfer protein that catalyzes the transport of triglyceride, cholesteryl ester, and phosphatidylcholine between membranes. Previous studies showing that the proximal cause of abetalipoproteinemia is an absence of MTP indicate that MTP function is required for the assembly of the apolipoprotein B (apoB) containing plasma lipoproteins, i.e., very low density lipoproteins and chylomicrons. However, the precise role of MTP in lipoprotein assembly is not known. In this study, the role of MTP in lipoprotein assembly is investigated using an inhibitor of MTP-mediated lipid transport, 2-[1-(3, 3-diphenylpropyl)-4-piperidinyl]-2,3-dihydro-1H-isoindol-1-o ne (BMS-200150). The similarity of the IC50 for inhibition of bovine MTP-mediated TG transfer (0.6 microM) to the Kd for binding of BMS-200150 to bovine MTP (1.3 microM) strongly supports that the inhibition of TG transfer is the result of a direct effect of the compound on MTP. BMS-200150 also inhibits the transfer of phosphatidylcholine, however to a lesser extent (30% at a concentration that almost completely inhibits TG and cholesteryl ester transfer). When BMS-200150 is added to cultured HepG2 cells, a human liver-derived cell line that secretes apoB containing lipoproteins, it inhibits apoB secretion in a concentration dependent manner. These results support the hypothesis that transport of lipid, and in particular, the transport of neutral lipid by MTP, plays a critical role in the assembly of apoB containing lipoproteins.

Animals↗
PubMed1996

Crystallization of microsomal triglyceride transfer protein from bovine liver.

The microsomal triglyceride transfer protein (MTP) is a heterodimeric lipid transfer protein required for the assembly of plasma very low density lipoproteins in the liver and chylomicrons in the intestine. Bovine MTP was purified by a modification of a previously published procedure and crystals of MTP were grown reproducibly with polyethylene glycol as a precipitant at pH 7.0. MTP crystals, which diffract to Bragg spacings of better than 3.2 A, have the symmetry of space group P2(1)2(1)2(1) with refined lattice constants of a = 88.7, b = 100.9 and c = 201.1 A, with one heterodimer per asymmetric unit.

Journal Article↗
PubMed1995

Microsomal triglyceride transfer protein. Specificity of lipid binding and transport.

Microsomal triglyceride transfer protein (MTP) is a lipid transfer protein that is required for the assembly and secretion of very low density lipoproteins by the liver and chylomicrons by the intestine. To further elucidate the nature of the lipid molecule binding and transport site on MTP, we have studied the relative rates at which MTP transports different lipid species. Assay conditions were chosen in which there were minimal changes in the physical properties of the substrate membranes so that transfer rates would reflect MTP-lipid interactions at a membrane surface. Lipid transport rates decreased in order of triglyceride > cholesteryl ester > diglyceride > cholesterol > phosphatidylcholine. Changes in the hydrophobic nature of a lipid molecule by the addition of a fatty acid, modulated the ability of MTP to transport it. Addition of one acyl chain from diglyceride to triglyceride, lysophosphatidylcholine to phosphatidylcholine, or cholesterol to cholesteryl ester increased the rate of MTP-mediated transport 10-fold. In contrast, the lipid transport rate was insensitive to the changes in the structure or charge of the polar head group on phospholipid substrates. Zwitterionic, net negative, or net positive charged phospholipid molecules were all transported at a comparable rate. The ability of MTP to transport lipids is strongly correlated to the binding of these lipids to MTP. Thus, MTP has a specific preference for binding and transporting nonpolar lipid compared with phospholipids, and within a class of lipid molecules, a decrease in polarity increases its tendency to be transported.

Animals↗
PubMed1994

Transcriptional regulation of human and hamster microsomal triglyceride transfer protein genes. Cell type-specific expression and response to metabolic regulators.

In order to characterize the molecular mechanisms that dictate microsomal triglyceride transfer protein (MTP) gene transcription in human and hamster, two species with similar plasma lipoprotein profiles, the MTP gene promoters were cloned, sequenced, and functionally characterized by transient transfection analysis. The results presented in this report indicate that the 5' ends of human and hamster MTP genes share similar structural features. The promoter sequences are well conserved and consist of similar functional elements. Transient transfection analysis of MTP promoter-driven luciferase gene expression showed that the promoter is active in liver and intestinal cells but not in epithelial cells, consistent with endogenous MTP gene expression. The -123 to -85 bp region of the human promoter is critical for the expression and contains the consensus recognition sequences for liver cell-specific factors HNF-1 and HNF-4 and activator protein AP-1. The promoter contains a modified sterol response element and a negative insulin response element. The human promoter activity is positively regulated by cholesterol and negatively regulated by insulin. From the functional analysis of MTP promoters, it is concluded that the elements that regulate the cell type-specific expression in human and hamster are well conserved and that insulin and cholesterol can regulate the activity of the MTP promoter in opposite directions.

Animals↗
PubMed1994

Secretion of apolipoprotein B-containing lipoproteins from HeLa cells is dependent on expression of the microsomal triglyceride transfer protein and is regulated by lipid availability.

To elucidate the role of the microsomal triglyceride transfer protein (MTP) in lipoprotein assembly, MTP and apolipoprotein B-53 (apoB 53; the N-terminal 53% of apoB) were expressed in HeLa cells. The results showed that apoB-53 could be expressed in HeLa cells with or without expression of MTP. In contrast, efficient secretion of apoB-53 required expression of MTP. Ultracentrifugal density flotation analysis showed that apoB-53 was secreted predominantly as a particle with the density of high density lipoprotein. An essentially identical apoB-53 particle density distribution was obtained after transient expression of apoB-53 in McArdle RH-7777 rat hepatoma cells. The mass of apoB-53 secreted was greater, and the flotation density was lower, from cells fed lipid, suggesting that apoB secretion in HeLa cells was regulated by lipid availability, similar to what has been described for lipoprotein-producing cell lines. These results indicate that MTP is necessary and sufficient to direct the regulated secretion of apoB-53 in HeLa cells.

Animals↗
PubMed1994

Dephosphorylation of CTP-phosphocholine cytidylyltransferase is not required for binding to membranes.

The sequence of the reversible phosphorylation and activation of CTP:phosphocholine cytidylyltransferase was investigated. Treatment of primary rat hepatocytes with oleic acid or phospholipase C caused a significant increase in the activity and amount of particulate cytidylyltransferase which correlated with decreased cytidylyltransferase activity and protein in the cytosol. The increase in membrane-associated cytidylyltransferase is accompanied by a decrease in the phosphorylation of the enzyme. Reversal of membrane association resulted in an increased amount of phosphorylated cytidylyltransferase in the cytosol. We wished to determine if dephosphorylation of the enzyme were a prerequisite for its translocation from the cytosol to the membranes. In vitro studies with membranes from oleic acid- or phospholipase C-treated cells showed that phosphorylated cytosolic cytidylyltransferase associated with these membranes with negligible dephosphorylation. Incubation of hepatocytes with oleic acid for different periods of time demonstrated that cytidylyltransferase associated with membranes in an active, phosphorylated form and was subsequently dephosphorylated. This result was supported by comparison of phosphopeptide maps of 32P-labeled cytidylyltransferase obtained from cytosolic, as well as membrane fractions of control, oleic acid-treated, or phospholipase C-treated cells. These studies revealed dephosphorylation on some sites and phosphorylation on other sites. Our data strengthen the hypothesis that a change in the lipid composition of membranes can mediate the initial binding of cytidylyltransferase to the membrane and that subsequently the enzyme becomes dephosphorylated.

Animals↗
PubMed1994

Atrial myxoma: report of 24 operations using the biatrial approach.

Between October 1978 and June 1993, 24 consecutive operations for atrial myxoma were performed on 23 patients at the Damascus University Cardiovascular Surgical Center, using the biatrial approach. Fifteen of the patients were female; ages ranged between 4 and 71 years. Nineteen patients had symptoms of left and 3 of right atrioventricular obstruction. One patient presented with a cerebrovascular accident, and another with recurrent emboli to the lower extremities. Ten patients had constitutional manifestations. Diagnosis was established preoperatively in all patients by echocardiography with or without angiocardiography. Twenty-one patients had left atrial myxoma, 1 of whom had a double lesion. Two patients had right atrial myxoma; in 1 of them it extended to the left atrium. This last patient had the only recurrence in the series, which was removed 7 years after the initial operation. All tumors were excised with a wide margin of full-thickness septum. Four patients had concomitant mitral valve reconstruction. There was no hospital mortality, and follow-up did not reveal any other complication related to the operation.

Adolescent↗
PubMed1993

Evidence that binding of CTP:phosphocholine cytidylyltransferase to membranes in rat hepatocytes is modulated by the ratio of bilayer- to non-bilayer-forming lipids.

The mechanism by which phospholipase C (PLC) digestion of cultured cells mediates binding of CTP:phosphocholine cytidylyltransferase to cellular membranes was investigated. Incubation of choline-depleted rat hepatocytes with PLC caused a translocation of enzyme from cytosol to membranes concomitant with a decrease in the concentration of phosphatidylcholine with no effect on the concentration of other phospholipids. Removal of PLC and supplementation with choline restored the amount of phosphatidylcholine in the cells and translocated cytidylyltransferase to the cytosol. However, when phosphatidylcholine levels were decreased by incubation with phospholipase A2 (PLA2), there was no significant redistribution of cytidylyltransferase activity. With PLA2 the concentration of phosphatidylethanolamine, as well as of phosphatidylcholine, was significantly decreased. Since PLC, but not phospholipase A2, raised the cellular concentration of diacylglycerol, possibly diacylglycerol mediated the binding of cytidylyltransferase to membranes. This possibility was examined, but is unlikely, since addition of lysophosphatidylcholine to PLC-treated cells restored the concentration of phosphatidylcholine and released cytidylyltransferase into the cytosol, but did not lower diacylglycerol levels to normal values. Studies in vitro, incubations of cells with choline analogues and a survey of the literature suggested that the over-riding common factor in regulation of cytidylyltransferase binding to membranes may be the ratio of bilayer to non-bilayer lipids in that membrane.

Animals↗
PubMed1992

On the mechanism of the okadaic acid-induced inhibition of phosphatidylcholine biosynthesis in isolated rat hepatocytes.

The mechanism of inhibition of phosphatidylcholine biosynthesis by okadaic acid was investigated in suspension cultures of isolated rat hepatocytes. Cells were pulsed with [methyl-3H]choline and chased in the absence or presence of 1 microM okadaic acid for up to 120 min. Phosphatidylcholine biosynthesis was inhibited after 15 min of chase. To see if okadaic acid altered the degree of phosphorylation of cytidylyltransferase (CT), hepatocytes were incubated with 32P(i) and chased in the absence or presence of okadaic acid. Okadaic acid caused a rapid (within 15 min) increase in the phosphorylation state of the cytosolic enzyme. Two-dimensional peptide map analysis revealed an increase in the phosphorylation of several peptides in okadaic acid-treated hepatocytes compared with controls. After 15 min of incubation of hepatocytes with okadaic acid, membrane CT activity was decreased and a corresponding increase in cytosolic CT activity was observed. In hepatocytes incubated with okadaic acid and oleate a correlation between membrane CT activity, diacylglycerol level, and phosphatidylcholine biosynthesis was observed. These data suggest that the concentration of diacylglycerol is responsible for the increase in membrane CT activity and subsequently phosphatidylcholine biosynthesis in oleate-treated cells. We postulate that the okadaic acid-induced decrease in phosphatidylcholine biosynthesis is due to an increase in the phosphorylation state of CT which promotes a translocation of CT activity from the membranes to the cytosol.

Animals↗
PubMed1992

Evidence that cyclic AMP-induced inhibition of phosphatidylcholine biosynthesis is caused by a decrease in cellular diacylglycerol levels in cultured rat hepatocytes.

The mechanism by which glucagon and cAMP analogues inhibit phosphatidylcholine biosynthesis was investigated in rat hepatocytes. The studies were facilitated by preparation of an antibody to a synthetic peptide (D-F-V-A-H-D-D-I-P-Y-S-S-A) corresponding to residues 164-176 of CTP:phosphocholine cytidylyl-transferase. The antibody, which was purified by affinity chromatography, quantitatively immunoprecipitated cytidylyltransferase from rat liver cytosol. Various analogues of cAMP had no effect on the labeling of cytidylyltransferase with 32Pi in rat hepatocytes. Nor did the cAMP analogues have any effect on the distribution of cytidylyltransferase between cytosol and membranes. These results indicate that the supply of CDP-choline does not limit phosphatidylcholine biosynthesis in hepatocytes treated with cAMP analogues. A decreased supply of diacylglycerol was considered as an alternative mechanism for inhibition of phosphatidylcholine biosynthesis. An approximately 30% decrease in diacylglycerol concentration was observed in hepatocytes treated with the cAMP analogues or glucagon, compared with controls. A similar decrease of phosphatidylcholine biosynthesis was observed. The cAMP-mediated decrease in diacylglycerol levels and inhibition of phosphatidylcholine biosynthesis were reversed by addition of 0.5-1.5 mM oleic acid to the treated hepatocytes. A correlation coefficient of 0.93 was calculated between the levels of diacylglycerol and the rate of phosphatidylcholine biosynthesis. In another approach, the diacylglycerol levels were increased by an inhibitor of diacylglycerol lipase (U-57908) which also reversed the cAMP effects on diacylglycerol levels and phosphatidylcholine biosynthesis. We conclude that the cAMP-mediated inhibition of phosphatidylcholine biosynthesis was not due to an effect on the phosphorylation of cytidylyltransferase. Instead, phosphatidylcholine biosynthesis appears to be inhibited due to a decreased level of diacylglycerol, a substrate for CDP-choline: 1,2-diacylglycerol cholinephosphotransferase.

8-Bromo Cyclic Adenosine Monophosphate↗
PubMed1991

Diacylglycerol signals the translocation of CTP:choline-phosphate cytidylyltransferase in HeLa cells treated with 12-O-tetradecanoylphorbol-13-acetate.

The mechanism of 12-O-tetradecanoylphorbol-13-acetate (TPA)-stimulated phosphatidylcholine biosynthesis in HeLa cells was investigated. TPA caused a 3-fold increase in particulate CTP:phosphocholine cytidylyltransferase activity in HeLa cells which correlated with decreased cytidylyltransferase activity in the cytosol. The increase in membrane-associated cytidylyltransferase was confirmed by immunoblotting. Immunoprecipitation studies suggested that TPA had no effect on the phosphorylation state of cytidylyltransferase. Enhanced binding of cytidylyltransferase to diacylglycerol-enriched membranes has previously been shown. Diacylglycerol levels in TPA-treated HeLa cells increased approximately 2-fold (2.29 to 4.02 nmol/mg of protein) after 1 h of TPA treatment. A time course experiment showed a temporal relationship in which production of diacylglycerol appeared to signal translocation of cytidylyltransferase to membranes followed by a stimulation of phosphatidylcholine biosynthesis. Diacylglycerol was further evaluated as a translocator of cytidylyltransferase by depleting HeLa cells of protein kinase C and incubating with dioctanoylglcerol. This treatment increased both membrane-associated cytidylyltransferase activity and the rate of phosphatidylcholine biosynthesis approximately 2-fold. A time course experiment with dioctanoylglycerol showed a strong positive correlation (r2 = 0.89) between the amount of particulate cytidylyltransferase activity and the rate of phosphatidylcholine biosynthesis. Therefore, TPA stimulates phosphatidylcholine biosynthesis by causing a translocation of cytidylyltransferase from the cytosol to membranes, which appears to be mediated by increased diacylglycerol.

Cell Membrane Permeability↗
PubMed1991

Substrate specificity of CTP:phosphocholine cytidylyltransferase.

The specificity of CTP:phosphocholine cytidylyltransferase from rat liver for phosphorylated bases has been investigated. The apparent Km for phosphocholine was 0.17 mM. As the number of methyl substituents on the phospho-base decreased, the apparent Km increased: 4.0 mM for phosphodimethylethanolamine, 6.9 for phosphomonomethylethanolamine and 68.4 for phosphoethanolamine. The Vmax for the reaction was similar for phosphocholine (12.6 mumol/min per mg protein), phosphomonomethylethanolamine (13.5 mumol/min per mg protein) and phosphoethanolamine (9.2 mumol/min per mg protein). When phosphodimethylethanolamine was the substrate, the Vmax was 3-fold higher (40.3 mumol/min per mg protein). Phosphoethanolamine, phosphomonomethylethanolamine and phosphodimethylethanolamine were competitive inhibitors of the cytidylyltransferase when phosphocholine was used as substrate with Ki values of 18.5 mM, 9.3 mM and 1.5 mM, respectively. The results show that the cytidylyltransferase is highly specific for phosphocholine.

Animals↗
PubMed1991

Phosphatidylcholine metabolism in rat liver after partial hepatectomy. Evidence for increased activity and amount of CTP:phosphocholine cytidylyltransferase.

The effect of partial (70%) hepatectomy on phosphatidylcholine (PC) synthesis in rat liver was investigated during the first 4 post-operative days. Between 4 and 96 h after partial hepatectomy, the mass of PC increased from 30% to 80% of sham-operation values, being comparable with the restoration of total liver mass after partial hepatectomy. Relative to control (sham-operation), the incorporation in vivo of [3H]choline into PC was stimulated 2.6-fold at 22 h after partial hepatectomy. Moreover, CTP:phosphocholine cytidylyltransferase (EC 2.7.7.15) activity was significantly enhanced, and the pool size of phosphocholine decreased at 22 and 48 h after partial hepatectomy, whereas the activity of choline kinase (EC 2.7.1.32) was augmented at a later stage of liver regeneration (48 and 96 h). Stimulation of CTP:phosphocholine cytidylyltransferase activity by partial hepatectomy occurred in both the microsomal and cytosolic fractions. The stimulatory effect in the cytosolic fraction was mainly due to an increase in the number of enzyme molecules, as demonstrated by immunotitration of the amount of cytosolic cytidylyltransferase protein.

Animals↗