Acylation of lysophosphatidylcholine in bovine heart muscle microsomes: purification and kinetic properties of acyl-CoA:1-acyl-sn-glycero-3-phosphocholine O-acyltransferase

New appreciation for an old pathway: the Lands Cycle moves into new arenas in health and disease

The Lands Pathway is a fundamental biochemical process named for its discovery by William EM Lands and revealed in a series of seminal papers published in the Journal of Biological Chemistry between 1958-65. It describes the selective placement in phospholipids of acyl chains, by phospholipid acyltransferases. This pathway has formed a core component of our knowledge of phospholipid and also diglyceride metabolism in mammalian tissues for over 60 years now. Our understanding of how the Lands pathways are enzymatically mediated via large families of related gene products that display both substrate and tissue specificity has grown exponentially since. Recent studies building on this are starting to reveal key roles for the Lands pathway in specific scenarios, in particular inflammation, immunity and inflammation. This review will cover the Lands cycle from historical perspectives first, then present new information on how this important cycle forms a central regulatory node connecting fatty acyl and phospholipid metabolism and how its altered regulation may present new opportunities for therapeutic intervention in human disease.

Member of the membrane-bound O-acyltransferase (MBOAT) family encodes a lysophospholipid acyltransferase with broad substrate specificity

Glycerophospholipids in biological membranes are metabolically active and participate in a series of deacylation-reacylation reactions, which may lead to accumulation of polyunsaturated fatty acids (PUFAs) at the sn-2 position of the glycerol backbone. The reacylation reaction is believed to be catalyzed by acyl-coenzyme A (acyl-CoA):lysophospholipid acyltransferase. Very recently, we have shown that Caenorhabditis elegans mboa-7, which belongs to the membrane-bound O-acyltransferase (MBOAT) family, encodes lysophosphatidylinositol (LPI)-specific acyltransferase (LPIAT). In this study, we found that knockdown of another member of the MBOAT family in C. elegans, named mboa-6, reduced incorporation of exogenous PUFAs into phosphatidylcholine (PC), phosphatidylserine (PS) and phosphatidylethanolamine (PE) in C. elegans. Knockdown of a human mboa-6 homologue, referred to as MBOAT5, also impaired the incorporation of PUFAs into PC, PS and PE in HeLa cells. In in vitro assays, lysoPC (LPC), lysoPS (LPS) and lysoPE (LPE) acyltransferase activities using [(14)C]arachidonoyl-CoA were significantly reduced in the microsomes of MBOAT5 knockdown cells. Conversely, over-expression of MBOAT5 in human embryonic kidney (HEK) 293 cells resulted in great increases in LPC, LPS and LPE acyltransferase activities but not in LPIAT or lysophosphatidic acid (LPA) acyltransferase (LPAAT) activities. These results indicate that human MBOAT5 is a lysophospholipid acyltransferase acting preferentially on LPC, LPS and LPE.

Caenorhabditis elegans mboa-7, a member of the MBOAT family, is required for selective incorporation of polyunsaturated fatty acids into phosphatidylinositol

Phosphatidylinositol (PI) is a component of membrane phospholipids, and it functions both as a signaling molecule and as a compartment-specific localization signal in the form of polyphosphoinositides. Arachidonic acid (AA) is the predominant fatty acid in the sn-2 position of PI in mammals. LysoPI acyltransferase (LPIAT) is thought to catalyze formation of AA-containing PI; however, the gene that encodes this enzyme has not yet been identified. In this study, we established a screening system to identify genes required for use of exogenous polyunsaturated fatty acids (PUFAs) in Caenorhabditis elegans. In C. elegans, eicosapentaenoic acid (EPA) instead of AA is the predominant fatty acid in PI. We showed that an uncharacterized gene, which we named mboa-7, is required for incorporation of PUFAs into PI. Incorporation of exogenous PUFA into PI of the living worms and LPIAT activity in the microsomes were greatly reduced in mboa-7 mutants. Furthermore, the membrane fractions of transgenic worms expressing recombinant MBOA-7 and its human homologue exhibited remarkably increased LPIAT activity. mboa-7 encodes a member of the membrane-bound O-acyltransferase family, suggesting that mboa-7 is LPIAT. Finally, mboa-7 mutants had significantly lower EPA levels in PI, and they exhibited larval arrest and egg-laying defects.

The activities of monocyte lysophosphatidylcholine acyltransferase and coenzyme A-independent transacylase are changed by the inflammatory cytokines tumor necrosis factor alpha and interferon gamma

Alteration of membrane phospholipid fatty acid compositions has been shown to be important for leukocyte inflammatory responses. Such modification of the molecular species of these lipid classes requires deacylation and reacylation reactions and for phosphatidylcholines, lysophosphatidylcholine acyltransferase (LPCAT) and a coenzyme A-independent transacylase (CoAIT) can each be involved. Since previous studies have shown a significant IFNgamma- and TNFalpha-induced modification of phosphatidylcholine species, we have examined whether these inflammatory cytokines alter the activity of reacylation enzymes in the human monocyte cell line MonoMac 6 (MM6). IFN-gamma caused a significant increase in the activity of the LPCAT and CoAIT enzymes in the microsomal fraction at concentrations and over a time-course consistent with an important role for these enzymes in the sensitization (priming) of monocytes. In contrast, TNFalpha was found to significantly increase the activity of the CoAIT by 50% over controls in MM6 cells after 30 min incubation with the cytokine, but decreased LPCAT activity by 65% after 24 h incubation. Such data imply that CoAIT is important for the remodelling of phospholipid composition, which is seen during the acute response of cells to TNFalpha. The results provide further information to emphasise the role of acyltransferases as part of the molecular mechanism underlying inflammation.

Effect of light and protein phosphorylation on photoreceptor rod outer segment acyltransferase activity

Rod outer segments (ROS) exhibit high acyltransferase (AT) activity, the preferred substrate of which being lysophosphatidylcholine. To study factors possibly regulating ROS AT activity purified ROS membranes were assayed under conditions under which protein kinase C (PKC), cAMP-dependent protein kinase (PKA), and phosphatases were stimulated or inhibited. PKC activation produced a significant increase in the acylation of phosphatidylethanolamine (PE) and phosphatidylinositol (PI) with oleate, it inhibited phosphatidylcholine (PC) acylation, and phosphatidylserine (PS) and phosphatidic acid (PA) acylation remained unchanged. ROS PKA activation resulted in increased oleate incorporation into PS and PI while the acylation of PC, PE, and PA remained unchanged. Inhibition of ROS PKC or PKA produced, as a general trait, inverse effects with respect to those observed under kinase-stimulatory conditions. ROS phosphatase 2A was inhibited by using okadaic acid, and the changes observed in AT activity are described. These findings suggest that changes in ROS protein phosphorylation produce specific changes in AT activity depending on the phospholipid substrate. The effect of light on AT activity in ROS membranes was also studied and it is reported that acylation in these membranes remains unchanged independent of the illumination condition used.

Solubilization, partial purification and photolabeling of the integral membrane protein lysophospholipid:acyl-CoA acyltransferase (LAT)

In the present study, we defined experimental conditions that allowed the extraction of the integral membrane protein lysophospholipid:acyl-CoA acyltransferase (LAT, EC 2.3.1.23) from membranes while maintaining the full enzyme activity using the nonionic detergent n-octyl glucopyranoside (OGP) and solutions of high ionic strength. We found that the optimal OGP concentration depended on the ionic strength of the solubilization buffer. Fluorescence measurements with 1,6-diphenyl-1,3,5-hexatriene indicated that the critical micellar concentration (CMC) of OGP decreased with increasing salt concentrations. Analogous studies revealed that the zwitterionic detergent Chaps was ineffective in extracting LAT from membranes in the absence of salt, whereas its solubilization efficiency increased with increasing salt concentrations. Detailed lipid analysis of the different protein/lipid/detergent mixed micelles showed that the protein/lipid/OGP mixed micelles were relatively enriched with sphingomyelin (SPM) compared to protein/lipid/Chaps mixed micelles, indicating that the differences in the solubilization efficiency may be due to the ability to extract more SPM from membranes. When the protein/lipid/OGP mixed micelles were dissociated into protein/detergent and lipid/detergent complexes by the addition of increasing Chaps concentrations, one-tenth of the LAT enzyme activity was preserved making the enzyme accessible to protein purification. Analysis by native PAGE revealed that in the presence of excess Chaps a high molecular mass protein complex migrated into the gel which could be photolabeled by 125I-labelled-18-(4'-azido-2'-hydroxybenzoylamino)-oleyl-CoA. This fatty acid analogue has been shown to be a competitive inhibitor of LAT enzyme activity in the dark, and an irreversible inhibitor after photolysis. Therefore, this protein complex is assumed to contain the LAT enzyme.

Acyl-GPC and alkenyl/alkyl-GPC:acyl-CoA acyltransferases

In mammalian tissues, phosphatidylcholine, or 1,2-diacyl-glycerophosphocholine (GPC), is the most abundant form of choline-containing phospholipids. In some electrically active tissues, a significant portion of the choline-containing phospholipids is 1-alkenyl-2-acyl-GPC (plasmenylcholine). The 1-alkyl-2-acyl-GPC is found in significant amounts in circulating cells such as neutrophils and macrophages but in low amounts in other tissues. Structural studies of phosphatidylcholine indicate that there is an asymmetric distribution of acyl groups on the molecule. Saturated fatty acids are usually esterified at the sn-1 position of the glycerol backbone, whereas unsaturated fatty acids are esterified at the sn-2 position. Similarly, unsaturated acyl groups are usually found in the sn-2 position of plasmenylcholine. The remodelling of the sn-2 acyl group in phosphatidylcholine by the deacylation-reacylation process has been demonstrated in a number of tissues. Phospholipase A2 is responsible for the hydrolysis of the acyl group at the sn-2 position, whereas 1-acyl-GPC:acyl-CoA acyltransferase is responsible for the reacylation reaction. The acyltransferase is located in the microsomal fraction and displays specificity towards the polyunsaturated acyl groups. The enzyme can be solubilized by detergent, but the enzyme activity in soluble form is difficult to maintain. The acyltransferase for the reacylation of 1-alkenyl-GPC is also located in the microsomal fraction and is somewhat specific towards polyunsaturated acyl groups. In guinea pig heart mitochondria, however, a new form of 1-alkenyl-GPC acyltransferase was identified which appeared to be different from the microsomal form. The acyltransferase for the acylation of 1-alkyl-GPC into platelet-activating factor has been studied in several tissues including human neutrophils. At present, the contribution of the acyltransferase in attaining the observed molecular composition of the choline-containing phospholipids in the tissue has not been defined. We postulate that the intrinsic acyl-CoA specificity of the acyltransferase, the flux of 1-acyl-GPC, 1-alkenyl-GPC and 1-alkyl-GPC, as well as the pool size of acyl-CoA are major factors in producing the final composition of the molecular species of the choline-containing phospholipids.

Identification of two different lysophosphatidylcholine:acyl-CoA acyltransferases (LAT) in pig spleen with putative distinct topological localization

The lysophosphatidylcholine:acyl-CoA acyltransferase (LAT, EC 2.3.1.23) is an integral membrane protein participating in the membrane turnover and the T-cell activation process. Here, we present data that crude membranes of pig spleen contain two different LAT enzyme activities based on topological localization studies and the enzyme specificities towards various acyl-CoAs. When crude membranes are washed with solutions of high ionic strength the supernatant contains a distinct LAT activity that we refer to as peripheral LAT (pLAT). The majority of LAT activity is found in the membrane pellet also after treatment with CHAPS. The CHAPS-insoluble LAT activity is named integral LAT (iLAT) accordingly. While pLAT prefers arachidonoyl-CoA rather than oleoyl-CoA, iLAT shows no specificity towards both unsaturated acyl-CoAs. Further investigations reveal that the CHAPS-insoluble LAT activity in the membranes can be solubilized by n-octyl glucoside and restored to original activity by reconstitution with artificial membranes. The reconstituted iLAT prefers arachidonoyl-CoA rather than oleoyl-CoA. Despite a great deal of effort by several groups little progress has been made so far in LAT purification because of the enzyme instability. We establish experimental conditions that enhance the stability of both enzyme activities and, therefore, allow further protein purification.

Substrate specificity of microsomal 1-acyl-sn-glycero-3-phosphoinositol acyltransferase in rat submandibular gland for polyunsaturated long-chain acyl-CoAs

Microsomal 1-acyl-sn-glycero-3-phosphoinositol (1-acyl-GPI) acyltransferase in the rat submandibular gland showed the highest specific activities for eicosanoid-related polyunsaturated acyl-CoAs, such as arachidonoyl-, bishomo-gamma-linolenoyl- and 5,8,11,14,17-eicosapentaenoyl-CoAs, with low Km values. High activities were also obtained with acyl-CoAs having long (more than 14 carbon atoms) and n - 6 unsaturated (more than 3 double bonds) acyl chains. This enzyme also utilized acyl-CoAs having trans-unsaturated or branched chains, but not short-chains, as substrates, although the activity levels for trans-unsaturated acyl-CoAs were lower than those for cis-unsaturated acyl-CoAs. Chronic administration of isoproterenol induced decreases of this enzyme activity and the content of arachidonic, bishomo-gamma-linolenic and 5,8,11,14,17-eicosapentaenoic acids at the sn-2 position of phosphatidylinositol. These results suggest that enrichment of arachidonic acid in the sn-2 position of phosphatidylinositol is established by the high specificity and affinity of 1-acyl-GPI acyltransferase for arachidonoyl-CoA. On the other hand, the low level of bishomo-gamma-linolenic and 5,8,11,14,17-eicosapentaenoic acids in the sn-2 position of phosphatidylinositol may be explained by their limited availability.

Coenzyme A-dependent cleavage of membrane phospholipids in several rat tissues: ATP-independent acyl-CoA synthesis and the generation of lysophospholipids

Substantial amounts of acyl-CoA were formed when microsomes from several rat tissues were incubated with varying concentrations of free CoA and bovine serum albumin even in the absence of ATP and Mg2+. For instance, 86 nmol of acyl-CoA was produced when microsomes (5 mg protein) were incubated with 300 microM CoA for 30 min. It was calculated that 1.8% of total fatty acyl residues were converted to acyl-CoA during the incubation. No appreciable amount of acyl-CoA was formed from free fatty acid or from boiled microsomes under the same experimental conditions. These observations indicate that acyl-CoA is formed from microsomal lipids by an enzyme activity distinct from previously known long-chain fatty acyl-CoA synthetase. The apparent Km value for CoA and Vmax were 180 microM and 20 nmol/30 min per mg protein, respectively. We found that several species of acyl-CoA such as arachidonoyl-CoA were preferentially synthesized through the reaction and that several types of phospholipids actually act as acyl donors in the formation of acyl-CoA. Phosphatidylinositol and phosphatidylcholine appear to be preferred substrates. We confirmed that lysophosphatidylinositol and lysophosphatidylcholine were generated along with the formation of acyl-CoA. It seems very likely that CoA-mediated cleavage of phospholipids/ATP-independent acyl-CoA synthesis is implicated in the metabolism of certain types of fatty acyl residues of membranous phospholipids in mammalian cells.

Acylation of lysophosphatidylcholine by brain membranes

Brain microsomes catalyze the acylation of lysophosphatidylcholine (lysoPtdCho) in the presence and absence of added CoA derivatives. The catalytic activity is distributed widely in various subcellular fractions from rat or bovine cerebral cortex as measured by the conversion of 1-[14C]palmitoyl-sn-glycero-3-phosphocholine to [14C]PtdCho. Analysis of this latter compound revealed that the dipalmitoyl derivative is the predominant molecular species, which is formed in this reaction by transacylation between two [14C]lysoPtdCho molecules. This lysoPtdCho: lysoPtdCho transacylation reaction was enhanced several-fold by the addition of oleoyl-CoA, which also is an effective donor of acyl groups in the acyl-CoA: lysoPtdCho acyltransferase-catalyzed reaction. Measurements of the initial velocity of the transacylation reaction were used to determine kinetic constants. Apparent Km values for lysoPtdCho in the presence and absence of oleoyl-CoA were 29 microM and 104 microM, respectively, and the corresponding maximal velocities were 0.11 and 1.06 nmol.min-1.mg-1, respectively. Oleoyl-CoA at 4 microM produced half-maximal stimulation of the transacylation reaction. CoA also stimulated the rate of conversion of [14C]lysoPtdCho to [14C]PtdCho, either in the presence or absence of oleoyl-CoA, with a half-maximal effect of CoA at 80 microM. These results may be important in understanding the regulation of PtdCho synthesis and the mechanism by which acyl group composition of this compound is controlled.

Effects of lysophospholipids and diacylglycerols on the transfer of arachidonic acid to phospholipids and triacylglycerols in rat brain membranes

Brain membranes catalyze the acylation of lysophospholipids and diacylglycerols (DAG) to form the respective phospholipids and triacylglycerols (TAG). These acylation reactions were examined using brain plasma membrane-enriched fractions by measuring the incorporation of [14C]arachidonic acid into TAG and individual phospholipids under a variety of conditions. In the absence of added lipid substrates, the amount of [14C]arachidonic acid incorporated into TAG in the presence of ATP, Mg2+, and CoA was approx twice the amount incorporated into phosphatidylositol (PtdIns), and more than 10 times the amount incorporated into phosphatidylcholine (PtdCho), phosphatidylethanolamine (PtdEtn) and phosphatidylserine (PtdSer). These results suggest the presence of endogenous DAG, lysoPtdIns, and the required enzymes in the membrane preparations for acylation reactions. The addition of DAG, lysoPtdCho or lysoPtdIns to the incubation system resulted in a 2-20-fold increase in the rate of incorporation of labeled arachidonic acid into TAG, PtdCho or PtdIns, respectively. LysoPtdEtn and lysoPtdSer were poor substrates for the synthesis of PtdEtn and PtdSer. On the other hand, the addition of lysoPtdSer stimulated the incorporation of [14C]arachidonic acid into TAG and into most phospholipids, especially phosphatidic acid, the synthesis of which was enhanced more than 10-fold. Exogenous lysoPtdCho and lysoPtdIns inhibited the incorporation of [14C]arachidonate into TAG in the presence of DAG, and DAG inhibited the incorporation of [14C]arachidonic acid into phospholipids in the presence of lysophospholipids. In general, [14C]palmitic acid was less effectively incorporated into lipids than arachidonic acid. These results suggest reciprocal regulatory effects of DAG and lysophospholipids on acyltransfer to phospholipids and triacylglycerol in brain membranes.

Solubilization and modulation of acyl-CoA:1-acyl-glycerophosphocholine acyltransferase activity in rat liver microsomes

The acylation of 1-acyl-glycerophosphocholine is an important mechanism for the maintenance of the asymmetrical distribution of acyl groups in phosphatidylcholine. The majority of acyl-CoA:1-acyl-glycerophosphocholine acyltransferase is located in the microsomal fraction. In this study, the rat liver microsomes were incubated with various detergents, and the solubilized enzyme was separated from the remainder by centrifugation. Sodium cholate, sodium deoxycholate and octylglucopyranoside caused the solubilization of 14-25% of the enzyme activity. The acyl specificity of the solubilized enzyme was similar to the insoluble enzyme, indicating that there was no selective solubilization of any acyl specific acyltransferase. The solubilized enzyme did not display any lipid requirement, and its activity was inhibited by phosphatidylcholine, phosphatidylethanolamine and 1,2-diacylglycerol. Kinetic studies with varying concentrations of acyl-CoAs revealed that the inhibition by 1,2-diacylglycerol was essentially uncompetitive. The modulation of acyltransferase activity by 1,2-diacylglycerol may be an important mechanism for controlling the acylation of lysophosphatidylcholine.

IL-1 alpha increases arachidonyl-CoA: lysophospholipid acyltransferase activity and stimulates [3H]arachidonate incorporation into phospholipids in rat mesangial cells

The proinflammatory cytokine interleukin-1 alpha is a potent stimulus of prostaglandin synthesis. We have previously shown that IL-1 amplifies mesangial cell prostaglandin synthesis by inducing synthesis of a non-pancreatic phospholipase A2. Phospholipase A2 activation results in the formation of lysophospholipids and free fatty acids. We now investigate the effects of IL-1 alpha on reacylation of lysophospholipids. Incubations with IL-1 alpha for 24 hours significantly stimulated mesangial cell [3H]arachidonic acid incorporation but not [3H]oleic acid incorporation into phosphatidylinositol and phosphatidylethanolamine. Lysophospholipid acyltransferase activity was measured in vitro. Cytokine treatment increased enzyme activity when lysophosphatidylcholine, lysophosphatidylethanolamine and lysophosphatidylinositol were used as exogenous substrates. We conclude that IL-1 promotes cellular phospholipid remodeling by stimulating the deacylation and reacylation of phospholipids.

Phospholipid acylation by mouse sciatic nerve microsomes

The partition of 0.3 nmol of [1-14C]oleoyl-CoA in the microsomes (10 micrograms proteins) from mouse sciatic nerves is unaffected by the presence of lysophospholipids and is about 45% of the total oleoyl-CoA (77% of the acylglycerophosphocholine partition in the membrane). The concentration of both oleoyl-CoA and acylglycerophosphocholine is over 1 mM in the membrane. There is a selective acyl transfer from acyl-CoA to lysolipid acceptors (oleoyl greater than myristoyl, palmitoyl, stearoyl much greater than eicosanoyl greater than docosanoyl, tetracosanoyl). The exogenous acyl acceptors are acylglycerophosphocholine and acylglycerophosphoinositol and to a lesser extent acylglycerophosphoethanolamine, but not acylglycerophosphoserine. A PC formation from acylGPC in the absence of exogenous acyl donors or from oleoyl-CoA in the absence of exogenous acyl acceptor was also observed.

Subcellular localization of rat gastric phospholipase A2

In the present study, we have performed experiments to gain some insight into the subcellular localization and biochemical properties of gastric mucosal phospholipase A2. After classical subcellular fractionation of whole glandular stomach mucosa, we found that gastric phospholipase A2 was essentially enriched in the 105,000 x g pellet that contains microsomes and plasma membranes. Except for the cytosol, all the subcellular fractions exhibited similar phospholipase A2 activity (i.e., optimum of pH, calcium dependence, apparent Km and positional specificity). The high-speed pellet was further characterized by ultracentrifugation on a sucrose gradient. Data showed that the sedimentation profile of phospholipase A2 was quite similar to those of plasma membrane markers and more specifically to an apical membrane marker. These results, taken together, showed that a gastric phospholipase A2 is distributed among the various subcellular fractions (as a result of cross-contamination) together with the membrane fraction on which it is associated. It is proposed that this fraction is the apical plasma membrane which would be the main site of phospholipase A2 action for arachidonic acid release. Lysophospholipase showed the same sedimentation profile as phospholipase A2, whereas acyl CoA-lysophosphatidylcholine: acyltransferase mainly sedimented with heavy microsomes. The substrate specificity of the enzyme was assessed by endogenous hydrolysis of gastric mucosal phospholipids. We were able to show that the enzyme acts at nearly the same rate on two major gastric membrane phospholipids, namely phosphatidylcholine and phosphatidylethanolamine.

Purification of lysophosphatidylcholine transacylase from bovine heart muscle microsomes and regulation of activity by lipids and coenzyme A

Heart muscle microsomes catalyze the transacylation of lysophosphatidylcholine (lyso PC) to produce phosphatidylcholine (PC). The enzyme which catalyzes this reaction, lyso PC:lyso PC transacylase, has been isolated and characterized from bovine heart muscle microsomes. The purification of the enzyme was achieved by a procedure involving extraction with 3-[3-cholamidopropyl)dimethylammonio)-1-propanesulfonate (CHAPS) detergent and chromatography on DEAE-cellulose, Reactive blue agarose, and Matrex gel green A. The purified enzyme was nearly homogeneous and consisted of a single molecular species of 128 kDa as determined by polyacrylamide gel electrophoresis in the presence of dodecyl sulfate. The catalytic activity of the enzyme was dependent on the presence of either CoA or acyl-CoA, both of which maximally stimulated at concentrations of approx. 10 microM. Analysis of the PC produced in the reaction showed that the enzyme catalyzed a transacylation in which both acyl groups arose from lyso PC. Furthermore, the enzyme did not possess acyl-CoA:lyso PC acyltransferase activity, lysophospholipase or acyl-CoA hydrolase activity, nor did it catalyze transacylation from lyso PC to lysophosphatidylethanolamine, lysophosphatidylinositol or lysophosphatidylserine. Although transacylation was highly specific for lyso PC as the substrate, various unsaturated fatty acyl-CoA derivatives served as activators. Palmitoyl-CoA and stearoyl-CoA did not significantly activate, although acetyl-CoA was an effective activator. Further modulation of activity was produced by palmitic acid and PC, both of which further activated the enzyme in the presence of oleoyl-CoA, whereas arachidonic acid, oleic acid, phosphatidylethanolamine and phosphatidylserine had no effect on activity. The high activity of this transacylase and its regulation by lipids suggests an important role for disaturated PC species in membranes and a mechanism for controlling the metabolism of lyso PC.

Purification and kinetic properties of lysophosphatidylinositol acyltransferase from bovine heart muscle microsomes and comparison with lysophosphatidylcholine acyltransferase

The enzyme acyl-CoA:1-acyl-sn-glycero-3-phosphoinositol acyltransferase (LPI acyltransferase, EC 2.3.1.23) was purified approximately 11,000-fold to near homogeneity from bovine heart muscle microsomes. The purification was effected by extraction with the detergent 3-((3-cholamidopropyl)dimethylammonio)-1-propanesulfonate, followed by chromatography on Cibacron blue agarose, DEAE-cellulose, and Matrex gel green A. The isolated enzyme was a single protein of 58,000 Da as measured by polyacrylamide gel electrophoresis in the presence of dodecyl sulfate. This purification procedure also allows isolation of the related enzyme lysophosphatidylcholine (LPC) acyltransferase, which was separated from LPI acyltransferase at the final chromatographic step. The purified LPI acyltransferase exhibits an absolute specificity for LPI as the acyl acceptor. Broader specificity was found for acyl-CoA derivatives as substrates, although the preferred substrates are long-chain, unsaturated derivatives: measured reactivities were in the order arachidonoyl-CoA greater than oleoyl-CoA greater than eicosadienoyl-CoA greater than linoleoyl-CoA. Little activity was found with palmitoyl-CoA or stearoyl-CoA as potential substrates. These properties are consistent with a role of the enzyme in controlling the acyl group composition of phosphoinositides. Comparison of LPC acyltransferase and LPI acyltransferase shows that these two enzymes have distinct kinetic and physical properties and are affected differently by local anesthetics, which are potent inhibitors.