Acyl-acceptor specificities of 1-acylglycerolphosphate acyltransferase and 1-acylglycerophosphorylcholine acyltransferase resolved from rat liver microsomes

Bioinformatics analysis of targeted metabolomics--uncovering old and new tales of diabetic mice under medication

Metabolomics is a powerful tool for identifying both known and new disease-related perturbations in metabolic pathways. In preclinical drug testing, it has a high potential for early identification of drug off-target effects. Recent advances in high-precision high-throughput mass spectrometry have brought the metabolomic field to a point where quantitative, targeted, metabolomic measurements with ready-to-use kits allow for the automated in-house screening for hundreds of different metabolites in large sets of biological samples. Today, the field of metabolomics is, arguably, at a point where transcriptomics was about 5 yr ago. This being so, the field has a strong need for adapted bioinformatics tools and methods. In this paper we describe a systematic analysis of a targeted quantitative characterization of more than 800 metabolites in blood plasma samples from healthy and diabetic mice under rosiglitazone treatment. We show that known and new metabolic phenotypes of diabetes and medication can be recovered in a statistically objective manner. We find that concentrations of methylglutaryl carnitine are oppositely impacted by rosiglitazone treatment of both healthy and diabetic mice. Analyzing ratios between metabolite concentrations dramatically reduces the noise in the data set, allowing for the discovery of new potential biomarkers of diabetes, such as the N-hydroxyacyloylsphingosyl-phosphocholines SM(OH)28:0 and SM(OH)26:0. Using a hierarchical clustering technique on partial eta(2) values, we identify functionally related groups of metabolites, indicating a diabetes-related shift from lysophosphatidylcholine to phosphatidylcholine levels. The bioinformatics data analysis approach introduced here can be readily generalized to other drug testing scenarios and other medical disorders.

Nitric oxide reduces the palmitoylation of rat myelin proteolipid protein by an indirect mechanism

Brain slices from 20-day-old rats were incubated with [3H]palmitate for 2 hours in the absence or presence of the NO-donors S-nitroso-N-acetyl-penicillamine (SNAP), ethyl-2-[hydroxyimino]-5-nitro-3-hexeneamide (NOR-3), 4-phenyl-3-furoxan carbonitrile (PFC) and sodium nitroprusside (SNP). Each of these drugs reduced the incorporation of [3H]palmitate into myelin proteolipid protein (PLP) in a concentration-dependent manner, SNP being the most active. The effect of SNAP was prevented by the NO-scavenger PTIO (2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide). Furthermore, decayed-SNAP, sodium nitrite and N- nitrosopyrrolidine were inactive, suggesting that free NO and/or some of its direct oxidation products are the active molecular species. The amount of fatty acids bound to PLP and the rate of deacylation were unaffected by NO. Although NO diminished the number of thiols in brain and myelin proteins, with the formation of both nitrosothiols and disulfides, these changes did not parallel those in PLP acylation. In contrast, NO was effective at reducing the palmitoylation of brain and myelin lipids, and this effect along with that of PLP, was ascribed to a decrease in palmitoyl-CoA levels. The NO-induced reduction in acyl-CoA concentration was due to the decline in ATP levels, while the amount of [3H]palmitate incorporated into the tissue, the activity of palmitoyl-CoA ligase and palmitoyl-CoA hydrolase, and the concentration of CoASH were unaltered by the drugs. Experiments with endogenously-synthesized [18O]fatty acids confirmed that NO affects predominantly the ATP-dependent palmitoylation of PLP. In conclusion, the inhibitory action of NO on the fatty acylation of PLP is indirect and caused by energy depletion.

Characterization of a human lysophosphatidic acid acyltransferase that is encoded by a gene located in the class III region of the human major histocompatibility complex

Sequence analysis of cDNA clones corresponding to a number of genes located in the class III region of the human major histocompatibility complex (MHC), in the chromosome band 6p21.3, has shown that the G15 gene encodes a 283-amino acid polypeptide with significant homology over the entire polypeptide with the enzyme lysophosphatidic acid acyltransferase (LPAAT) from different yeast, plant, and bacterial species. The amino acid sequence of the MHC-encoded human LPAAT (hLPAATalpha) is 48% identical to the recently described hLPAAT (Eberhardt, C., Gray, P. W., and Tjoelker, L. W. (1997) J. Biol. Chem. 272, 20299-20305), which is encoded by a gene located on chromosome 9p34.3. LPAAT is the enzyme that in lipid metabolism converts lysophosphatidic acid (LPA) into phosphatidic acid (PA). The expression of the hLPAATalpha polypeptide in the baculovirus system and in mammalian cells has shown that it is an intracellular protein that contains LPAAT activity. Cell extracts from insect cells overexpressing hLPAATalpha were analyzed in different LPAAT enzymatic assays using, as substrates, different acyl acceptors and acyl donors. These cell extracts were found to contain up to 5-fold more LPAAT activity compared with control cell extracts, indicating that the hLPAATalpha specifically converts LPA into PA, incorporating different acyl-CoAs with different affinities. The hLPAATalpha polypeptide expressed in the mammalian Chinese hamster ovary cell line was found, by confocal immunofluorescence, to be localized in the endoplasmic reticulum. Due to the known role of LPA and PA in intracellular signaling and inflammation, the hLPAATalpha gene represents a candidate gene for some MHC-associated diseases.

cDNA cloning and expression of murine 1-acyl-sn-glycerol-3-phosphate acyltransferase

1-Acyl-sn-glycerol-3-phosphate (1-AGP), also known as lysophosphatidic acid (LPA), is an intermediate of de novo biosynthesis of glycerophospholipids and triacylglycerol. LPA is also attracting much attention because of its growth stimulating effects. Here we report cloning of murine cDNA encoding 1-AGP acyltransferase (1-AGPAT), which converts LPA into phosphatidic acid by incorporating acyl moiety at an-2 position. The cDNA contains an open reading frame coding for 285 amino acids, with highly hydrophobic regions in the N-terminal half. The Northern blot analysis using various mouse tissues revealed ubiquitous expressions of two transcripts. The cDNA was expressed in Escherichia coli JC201, a plsC mutant strain deficient in the 1-AGPAT activity, and the enzyme properties were examined. The enzyme utilized both saturated and unsaturated acyl-CoA as an acyl-donor, while it utilized LPA but not other lysophospholipids as an acceptor.

Delta 6-desaturase: improved methodology and analysis of the kinetics in a multi-enzyme system

A new method of assay for the delta 6-desaturation of linoleic acid was developed. This method, which uses HPLC for separation of the fatty acid substrate and product, exhibited a lower coefficient of variation (0.3%) than the reported TLC method (3.5%), and avoided the step of methylation of the saponified fatty acid substrate and product. Using this new method of assay, the kinetics of the delta 6-desaturase in a multi-enzyme system were analysed. A number of factors that could have striking effects on desaturase kinetics were investigated, including the effect of (i) endogenous microsomal linoleic acid on total substrate concentration, and (ii) the pre-reaction catalysed by acyl-CoA synthetase and competing reactions catalysed by lysophospholipid acyltransferase and acyl-CoA hydrolase. Endogenous free linoleate in the hepatic microsomes was found to be 2.9 +/- 1.0 microM (0.5 mg microsomal protein/ml), which was comparable to added substrate concentrations (1.8 to 7.9 microM). The kinetics of the delta 6-desaturase were dissected from the kinetics of the above mentioned pre-reaction and competing reactions through a combination of experimental approaches and computer modeling. From computer modeling, a Km and Vmax of 1.5 microM and 0.63 nmol/min were calculated for the delta 6-desaturase, compared to Km and Vmax of 10.7 microM and 0.08 nmol/min calculated directly from data uncorrected for endogenous substrate. It was concluded that lysophospholipid acyltransferase, acyl-CoA synthetase and endogenous linoleic acid significantly affect the kinetic measurements of hepatic microsomal delta 6-desaturase. These results have implications for kinetic analyses of all desaturates in microsomal systems.

Selective channelling of arachidonic and linoleic acids into glycerolipids of rat hepatocytes in primary culture

Rat hepatocytes in primary culture were incubated with a mixture of linoleic and arachidonic acid at various total fatty acid/serum albumin molar ratios. Mixed fatty acids were taken up at the same rate and distributed with the same pattern as fatty acids added separately. The rates of total uptake, incorporation into hepatocyte and secreted triacylglycerols and beta-oxidation were linearly related to the fatty acid/albumin ratios, whereas the rate of incorporation into phospholipids was saturable. Neither the uptake rate nor the distribution of both fatty acids considered together varied with the arachidonic acid/linoleic acid molar ratio. Changes in this ratio and in the uptake rate led to significant variations in the respective fate of the fatty acids. The preferential channelling of arachidonic acid versus linoleic acid into beta-oxidation and phosphatidylinositol was greatest at a low uptake rate and then decreased as the uptake rose. Conversely, the preferential channelling of arachidonic acid versus linoleic acid into phosphatidylcholine, but not phosphatidylethanolamine, increased with the uptake rate. Moreover, both arachidonic acid and linoleic acid were preferentially incorporated into the 1-palmitoyl molecular species of phosphatidylcholine and phosphatidylethanolamine at a low uptake rate, and of phosphatidylcholine at a high uptake rate. This could be related to the synthesis of biliary phosphatidylcholine, of which 1-palmitoyl-2-linoleoyl and 1-palmitoyl-2-arachidonoyl are the main molecular species. Linoleic and arachidonic acid were selectively distributed into distinct metabolic pools of triacylglycerol, the intrahepatocyte pool which preferentially incorporated linoleic acid at a low uptake rate and the secreted pool in which the relative enrichment of arachidonic acid increased with the uptake rate. This strengthens the central role of hepatic secretion in the supply of arachidonic acid to peripheral tissues.

1-Acyl-sn-glycerol-3-phosphate acyltransferase in maturing safflower seeds and its contribution to the non-random fatty acid distribution of triacylglycerol

A 20,000 X g particulate preparation isolated from maturing safflower seeds catalyzed the acylation of 1-acyl-sn-glycerol 3-phosphate with acyl-CoA to form phosphatidate. The specific activity of the reaction exceeded 200 nmol min-1 mg protein-1. Although this preparation was also capable of catalyzing the acylation of sn-glycerol 3-phosphate with acyl-CoA, the hydrolysis of phosphatidate, and the acylation of 1,2-diacylglycerol, phosphatidate was the only major product when the preparation was incubated with 1-acyl-glycerol-3-P and acyl-CoA. The enzyme responsible for this phosphatidate synthesis, 1-acyl-glycerol-3-P acyltransferase, showed a strict acyl-CoA specificity. The relative order of specificity for acyl-CoA was linoleoyl = oleoyl greater than palmitoleoyl greater than elaidoyl greater than cis-vaccenoyl greater than stearoyl = palmitoyl. This observation strongly suggests that the fatty acid composition of position 2 in phosphatidate synthesized in vivo primarily depends on both the acyl-CoA specificity of the 1-acyl-glycerol-3-P acyltransferase and the fatty acid composition of the acyl-CoA pool in the cell. Thus, the absence of saturated fatty acids at position 2 of safflower triacylglycerol may be explained in terms of the acyl-CoA specificity of the 1-acyl-glycerol-3-P acyltransferase. The fatty acid moiety esterified at position 1 of glycerol-3-P also affected the effectiveness of the reaction. The 1-acyl-glycerol-3-P acyltransferase utilized 1-acyl-glycerol-3-P molecular species in the following order of effectiveness: linoleoyl = oleoyl greater than palmitoyl. With a rise in incubation temperature, the initial rates of acylation with unsaturated acyl-CoA species increased more rapidly than those for saturated acyl-CoA species. A similar tendency was observed for saturated and unsaturated acyl acceptors. These data suggest that affinity of the acyltransferase for substrates may vary in response to changes in temperature, and that 1-acyl-glycerol-3-P acyltransferase may be involved in the alteration of the individual fatty acid compositions at positions 1 and 2 of glycerolipids in tissues grown at different temperatures. Based on these findings, further metabolism of 1-acyl-glycerol-3-P acyltransferase products could be the major factor determining the non-random distribution of fatty acids in safflower triacylglycerol.

Biosynthesis of glycerolipids by hepatoma and liver microsomes. I. Fatty acyl-CoA ligase and acyl-CoA:sn-glycerol-3-phosphate acyltransferase

The intracellular membranes of hepatomas exhibit an altered content and composition of lipid compared to the membranes of normal liver. In order to elucidate the role of lipid biosynthetic enzymes in these membrane differences, we first examined the fatty acyl-CoA ligase and acyl-CoA:sn-glycerol 3-phosphate (Gro-3P) acyltransferase activities and acyl specificities of microsomes from liver, Morris hepatoma 7288C, and hepatoma tissue culture (HTC) cells. Based upon incorporation of fatty acid and Gro-3P, it is concluded that acyl-CoA:sn-Gro-3P acyltransferase activities are markedly elevated (6-30-fold) in the microsomes of Morris Hepatoma 7288C and HTC cells compared to microsomes from liver, whereas the fatty acyl-CoA ligase activity is reduced (30-50-fold). Therefore, the low phospholipid content of these tumor cells does not appear to result from reduced acyltransferase activity. Though diminished ligase activity may play a role, it appears that activation of fatty acid may not be rate-limiting, even at the low levels of fatty acyl-CoA ligase present in the tumor and HTC cells. Preliminary evidence suggests that another factor that may be responsible for the low tumor phospholipid content is the limited availability of Gro-3P, a lipid precursor. The phospholipid in hepatoma 7288C is also characterized by an elevated ratio of monenoic to dienoic fatty acid. We have found that this change does not reflect an altered specificity of acyl-CoA:sn-Gro-3P acyltransferase.

Synthesis, storage and degradation of myocardial triglycerides

In the mammalian myocardium, an active triglyceride synthesis pathway is operating, (re)esterifying activated fatty acids from endogenous or exogenous sources, with the glycolytically derived three-carbon intermediates dihydroxyacetone-phosphate and glycerol-3-phosphate by the so-called Kennedy pathway. The seven enzymes of triglyceride synthesis are membrane bound and located at the sarcoplasmic reticulum. The first enzyme in the glycerol-3-phosphate pathway, glycerol-3-phosphate acyltransferase, is proposed to be rate limiting for triglyceride formation. This microsomal enzyme is regulated by phosphorylation (inactiycation)-dephosphorylation (activation) coupled to the beta-receptor--adenyl cyclase--protein kinase system. Additional regulatory steps in triglyceride formation are the reactions catalyzed by the microsomal phosphatidic acid phosphatase and diglyceride acyltransferase. Intracellular triglycerides occur as free floating cytosolic droplets, membrane-bound particles and lipid-filled lysosomes. No consensus exists about the metabolically active portion of myocardial triglycerides. Various lipases have been proposed to be involved in endogenous lipolysis: the lysosomal acid, microsomal and soluble neutral triglyceride, intracellular lipoprotein lipases and the microsomal di- and monoglyceridase. It has been acknowledged that the bulk of the intracellular neutral lipase represents the precursor of vascular lipoprotein lipase. The presence of a neutral lipase, as distinct from lipoprotein lipase, in the rat heart was recently advocated. Endogenous lipolysis is a hormone-sensitive process. Hormone-sensitivity may involve direct alteration of enzyme activity by protein phosphorylation-dephosphorylation but is also dependent on the removal rate of product fatty acids, since feedback inhibition is a common property of all lipases in the heart.(ABSTRACT TRUNCATED AT 250 WORDS)

Purification and properties of acyl-CoA:1-acyl-sn-glycero-3-phosphocholine-O-acyltransferase from bovine brain microsomes

Acyl-CoA:1-acyl-sn-glycero-3-phosphocholine-O-acyltransferase has been purified approximately 3000-fold from bovine brain microsomes by detergent solubilization followed by ion-exchange and affinity chromatography. Polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate revealed a single protein of molecular weight 43,000. The specificity of the purified enzyme was studied by measuring the catalytic activity with various lysophospholipids and acyl-CoA derivatives. Of the lysophospholipids tested, only lysophosphatidylcholine was a substrate. Less specificity was exhibited toward the acyl-CoA derivatives, although the enzyme showed a clear preference for arachidonoyl-CoA and little or no activity with palmitoyl-CoA or stearoyl-CoA. High concentrations of arachidonoyl-CoA inhibited the enzyme. The velocity was a sigmoidal function of the concentration of lysophosphatidylcholine (LPC) with little activity obtained below 20 microM LPC. The specificity and kinetic properties of the enzyme were altered, however, by incorporation of the enzyme into liposomes composed of a mixture of phospholipids. Decanoyl-CoA and myristoyl-CoA, which were effective substrates for the soluble enzyme, did not serve as acyl donors for the liposome-bound acyltransferase. Furthermore, the liposome-bound enzyme, in contrast to the soluble form of the enzyme, was active at concentrations of LPC below the critical micelle concentration. The liposome-bound enzyme was also substantially less susceptible to thermal denaturation and proteolytic digestion. This modulation of the acyltransferase activity by interaction with phospholipids may relate to the kinetic properties and the regulation of the enzyme in vivo.

Essential fatty acid interconversion during gestation in the rat

The synthesis of arachidonic acid has been investigated in fetal and pregnant rat liver microsomes in the course of the gestation. The delta 5-desaturase activity decreased 2-3 times in rat liver between the 19th and 22nd day of the pregnancy. During this period the delta 5-desaturate activity increased 3-fold in the fetal liver, exceeding the activity of the maternal liver. In contrast, the activity of the fetal delta 6-desaturase was in the same range as in pregnant rat liver and the liver of control animals and did not change between these two stages of the gestation. The elongation rate of linoleic acid in fetal liver was 2-3 times lower than in maternal liver but this increased during the pregnancy. The fatty acid activate rate was always higher than the activity of the desaturases. At the 19th day, the activity of the delta 5-desaturase was apparently the rate limiting step of arachidonic acid synthesis in fetal liver. We did not find any delta 5- and delta 6-desaturase activities or linoleic acid elongation in the placenta microsomes.

De novo biosynthesis of docosahexaenoyl-phosphatidic acid in bovine retinal microsomes

Lysophosphatidic acid stimulated several-fold the formation of docosahexaenoyl-phosphatidic acid from 14C-labeled docosahexaenoic acid (22:6 (n-3] in the bovine retina. 1-Palmitoyl- and 1-oleoyl-sn-glycerol 3-phosphate were the preferred acceptors. Most of the activity was localized in the 105 000 X g microsomal fraction. Despite the very high content of 22:6 in the phospholipids of photoreceptor membranes, only about 1% of the microsomal activity was found in discs isolated from rod outer segments. The newly synthesized docosahexaenoyl-phosphatidic acid was further metabolized to diacylglycerols, triacylglycerols, phosphatidylcholine and phosphatidylserine. The de novo synthesis of docosahexaenoyl-phosphatidylcholine was stimulated by 1 mM CDPcholine. Lysophosphatidic acid and lysophosphatidylcholine up to 50 microM do not compete with each other for 22:6 in the formation of their respective diacylated lipids. This suggests that this fatty acid is introduced into phosphatidic acid and phosphatidylcholine via different acylation systems. We conclude that, in addition to the deacylation-acylation cycle, there is also an active pathway for the acylation of 22:6 into glycerolipids during the de novo biosynthesis of phosphatidic acid.

Evidence for the reversibility of the acyl-CoA:lysophosphatidylcholine acyltransferase in microsomal preparations from developing safflower (Carthamus tinctorius L.) cotyledons and rat liver

Acyl exchange between acyl-CoA and position 2 of sn-phosphatidylcholine occurs in the microsomal preparations of developing safflower cotyledons. Evidence is presented to show that the acyl exchange is catalysed by the combined back and forward reactions of an acyl-CoA:lysophosphatidylcholine acyltransferase (EC 2.3.1.23). The back reaction of the enzyme was demonstrated by the stimulation of the acyl exchange with free CoA and by the observation that the added CoA was acylated with acyl groups from position 2 of sn-phosphatidylcholine. Re-acylation of the, endogenously produced, lysophosphatidylcholine with added acyl-CoA occurred with the same specificity as that observed with added palmitoyl lysophosphatidylcholine. A similar acyl exchange, catalysed by an acyl-CoA:lysophosphatidylcholine acyltransferase, occurred in microsomal preparations of rat liver. The enzyme from safflower had a high specificity for oleate and linoleate, whereas arachidonate was the preferred acyl group in the rat liver microsomal preparations. The rate of the back reaction was 3-5% and 0.2-0.4% of the forward reaction in the microsomal preparations of safflower and rat liver respectively. Previous observations, that the acyl exchange in safflower microsomal preparations was stimulated by bovine serum albumin and sn-glycerol 3-phosphate, can now be explained by the lowered acyl-CoA concentrations in the incubation mixture with albumin and in the increase in free CoA in the presence of sn-glycerol 3-phosphate (by rapid acylation of sn-glycerol 3-phosphate with acyl groups from acyl-CoA to yield phosphatidic acid). Bovine serum albumin and sn-glycerol 3-phosphate, therefore, shift the equilibrium in acyl-CoA:lysophosphatidylcholine acyltransferase-catalysed reactions towards the rate-limiting step in the acyl exchange process, namely the removal of acyl groups from phosphatidylcholine. The possible role of the acyl exchange in the transfer of acyl groups between complex lipids is discussed.

1-Acyl-lysolecithin acyltransferase and synthesis of biliary lecithins in rat liver

The role of lysolecithin acyltransferase activities in biliary lecithin formation was investigated, using livers perfused in the presence of labeled palmitoyl-lysolecithin and albumin, overloaded or not with linoleic acid. At the end of liver perfusion, the lecithins extracted from microsomes, mitochondria and plasma membranes displayed the same specific activity. Double-labeled lysolecithin was used to prove that labeled lecithins were synthesized by lysolecithin acylation. In the absence or presence of a linoleic acid overload, the level of lysolecithin incorporation into linoleyl and arachidonyl containing lecithin was identical. Hence fatty acids did not influence phosphatidylcholine synthesis by the acylation pathway. In vitro the rate of linoleyl lecithin synthesis was the same in plasma membranes, mitochondria and microsomes provided the linoleyl-CoA concentration was lower than 30 microM. Taurocholate was essential to the excretion of lecithin synthesized from lysolecithin and stimulated its synthesis. The specific activities of the two lecithin molecular species excreted in bile (linoleyl and arachidonyl) were not significantly different. These results enabled us to evaluate the contribution of the lysolecithin pathway to the synthesis of lecithin in liver and bile: this contribution in bile was less than 2% under the perfusion conditions used.

The suitability of different acyl acceptors as substrates for the acyl-Coa : 2-acyl-sn-glycero-3-phosphorylcholine acyltransferase in rat liver microsomes

The fatty acid selectivity of the acyl-CoA : 2-acyl-sn-glycero-3-phosphorylcholine acyltransferase towards different acyl acceptors was studied in rat liver microsomes. The individual molecular species of 2-acylglycerylphosphorylcholine tested as enzyme substrates contained either palmitate, stearate, oleate, linoleate, linolenate or arachidonate with an equimolar mixture of [14C]palmitoyl-CoA plus [3H]stearoyl-CoA serving as the acyl donor. At 2-acyl-sn-glycero-3-phosphorylcholine concentrations of 16 or 64 microM, the various acyl acceptors gave generally similar reactivities, although reaction velocities with the linoleoyl, linolenoyl or arachidonoyl species were moderately greater than those with 2-stearoyl-sn-glycero-3-phosphorylcholine. Regardless of the acyl acceptor tested, little preference towards either palmitoyl-CoA or stearoyl-CoA was indicated at 64 microM 2-acyl-sn-glycero-3-phosphorylcholine whereas a distinct preference for stearate over palmitate (by 1.9--2.6-fold) was exhibited when a lower concentration (16 microM) of the acceptor was employed. The results support the potential importance of the acyl-CoA : 2-acyl-sn-glycero-3-phosphorylcholine acyltransferase for the synthesis of 1-stearoyl 2-unsaturated species of phosphatidylcholine. However, it cannot independently account for the varying palmitate : stearate ratios in the 1-position of this phospholipid when different unsaturated fatty acids reside in the 2-position.

Acylation of endogenous phospholipids and added lysoderivatives by rat liver plasma membranes

Phospholipid acyltransferase activities of plasma membranes have been investigated with various acyl-CoA thioesters (palmitoyl, stearoyl, oleoyl, linoleoyl and arachidonoyl) with and without added lysoderivatives. Different patterns of incorporation were observed for each acyl-CoA into endogenous phosphatidylcholine and phosphatidylethanolamine. The turnover rates calculated with tracer amounts of 10 microM acyl-CoA thioesters were five times faster for the polyunsaturated than for the saturated acyl moieties of phosphatidylethanolamine and phosphatidylcholine. Arachidonoyl-CoA was the best acyl donor at low concentrations and the maximal turnover rate was observed at about 25 microM. No saturation appeared at up to 100 microM linoleoyl-CoA. Linoleoyl-CoA transacylase acylated the lyso-compounds in the following order: lysophosphatidylcholine greater than lysophosphatidylserine and lysophosphatidylinositol, while lysophosphatidylethanolamine inhibited linoleate incorporation into the phosphatidylethanolamine itself. Linoleoyl-CoA transacylation was not affected by the fatty acyl moiety at the 1-position of the lysophosphatidylcholine. The results support the view that the plasma membrane acyltransferase activity might contribute to the formation of bile phosphatidylcholines.

Triacylglycerol metabolism in isolated rat kidney cortex tubules

Triacylglycerol metabolism has been studied in kidney cortex tubules from starved rats, prepared by collagenase treatment. Triacylglycerol was determined by a newly developed fully enzymic method. Incubation of tubules in the absence of fatty acids led to a decrease of endogenous triacylglycerol by about 50% in 1h. Addition of albuminbound oleate or palmitate resulted in a steady increase of tissue triacylglycerol over 2h. The rate of triacylglycerol synthesis was linearly dependent on oleate concentration up to 0.8mm, reaching a saturation at higher concentrations. Triacylglycerol formation from palmitate was less than that from oleate. This difference was qualitatively the same when net synthesis was compared with incorporation of labelled fatty acids. Quantitatively, however, the difference was less with the incorporation technique. Gluconeogenic substrates, which by themselves had no effect on triacylglycerol concentrations, stimulated neutral lipid formation from fatty acids. Glucose and lysine did not have such a stimulatory effect. Inhibition of gluconeogenesis from lactate by mercaptopicolinic acid likewise inhibited triacylglycerol formation. This inhibitory effect was seen with oleate as well as with oleate plus lactate. When [2-(14)C]lactate was used the incorporation of label into triacylglycerol was found in the glycerol moiety exclusively. Addition of dl-beta-hydroxybutyrate (5mm) to the incubation medium in the presence of oleate or oleate plus lactate led to a significant increase in triacylglycerol formation. In contrast with the gluconeogenic substrates, dl-beta-hydroxybutyrate had no stimulatory effect on fatty acid uptake. The results suggest that renal triacylglycerol formation is a quantitatively important metabolic process. The finding that gluconeogenic substrates, but not glucose, increase lipid formation, indicates that the glycerol moiety is formed by glyceroneogenesis in the proximal tubules. The effect of ketone bodies seems to be caused by the sparing action of these substrates on fatty acid oxidation. The decrease of triacylglycerol in the absence of exogenous substrates confirms previous conclusions that endogenous lipids provide fatty acids for renal energy metabolism.

Synthesis of 1-palmitoyl and 1-stearoyl phosphatidylcholines from mixtures of acyl acceptors via acyl-CoA:1-acyl-sn-glycero-3-phosphorylcholine acyltransferase in liver microsomes

The fatty acid selectivity of the acyl-CoA:1-acyl-sn-glycero-3-phosphorylcholine acyltransferase in rat liver microsomes was studied using a mixture of the [1-(3)H]palmitoyl plus [1-(14C)stearoyl molecular species of 1-acylglyceryl-phosphorylcholine. At a 1-acyl-sn-glycero-3-phosphorylcholine concentration of 0.16 mM, the enzyme exhibited a selectivity of 3.5-fold for the 1-palmitoyl over the 1-stearoyl species of the acyl acceptor and reaction velocities with linoleoyl- and arachidonoyl-CoA were 38--47% greater than with oleoyl-CoA. Lowering the acceptor concentration to 0.016 mM gave reaction rates with the polyenoic thiolesters which were 174--187% greater than with oleoyl-CoA and the 1-palmitoyl-sn-glycero-3-phosphorylcholine was preferred by 2.2, 1.6, and 1.6-fold with oleoyl-, linoleoyl- and arachidonoyl-CoA, respectively. The results support the potential importance of the fatty acid selectivities of the acyl-CoA:1-acyl-sn-glycero-3-phosphorylcholine acyltransferase towards both acyl acceptor and donor in regulating the phosphatidylcholine species formed by the reaction in vivo.