Purification of long-chain acyl-CoA hydrolase from bovine heart microsomes and regulation of activity by lysophospholipids

An improved method to determine serine palmitoyltransferase activity

Serine palmitoyltransferase (SPT) catalyzes the condensation of l-serine and palmitoyl-CoA, which is the rate-limiting step in the de novo synthesis of sphingolipids. SPT activity is commonly measured by monitoring the incorporation of radiolabeled l-serine into 3-ketodihydrosphingosine. In this article, we introduce several adaptations of the established protocol to improve sensitivity, reproducibility, and practicability of the assay. A significant improvement of this new protocol is the possibility to measure SPT activity in total cell lysate instead of microsomes. The assay is furthermore extended by the introduction of a nonradioactive, HPLC-based detection protocol. The suggested HPLC method offers several advantages, most importantly, a 20-fold lower detection limit compared with the radioactive assay and the possibility to use an internal standard to correct for variation in the extraction.

Backbone dynamics of Escherichia coli thioesterase/protease I: evidence of a flexible active-site environment for a serine protease

Escherichia coli thioesterase/protease I (TEP-I) is a member of a novel subclass of the lipolytic enzymes with a distinctive GDSLS motif. In addition to possessing thioesterase and protease activities, TEP-I also exhibits arylesterase activity. We have determined the (15)N nuclear magnetic spin relaxation rates, R(1) and R(2), and the steady state (1)H-(15)N heteronuclear Overhauser effect, measured at both 11.74 T and 14.09 T, of (u-(15)N) TEP-I. These data were analyzed using model-free formalism (with axially symmetric rotational diffusion anisotropy) to extract the backbone dynamics of TEP-I. The results reveal that the core structure of the central beta-sheet and the long alpha-helices are rigid, while the binding pocket appears to be rather flexible. The rigid core serves as a scaffold to anchor the essential loops, which form the binding pocket. The most flexible residues display large amplitude fast (ps/ns time-scale) motion and lie on one stripe whose orientation is presumed to be the ligand-binding orientation. We also detected the presence of several residues displaying slow (microseconds/ms time-scale) conformational exchanging processes. These residues lie around the binding pocket and are oriented perpendicularly to the orientation of the flexible stripe. Two of the putative catalytic triads, Ser10 and His157, and their neighbors show motion on the microseconds/ms time-scale, suggesting that their slow motion may have a role in catalysis, in addition to their possible roles in ligand binding. The presence of a flexible substrate-binding pocket may also facilitate binding to a wide range of substrates and confer the versatile functional property of this protein.

Triacylglycerol synthesis by purified triacylglycerol synthetase of rat intestinal mucosa. Role of acyl-CoA acyltransferase

The activities of the proposed triacylglycerol synthetase complex, acyl-CoA ligase, acyl-CoA acyltransferase (AAT), monoacylglycerol acyltransferase (MGAT), and diacylglycerol acyltransferase (DGAT), coeluted upon Cibacron blue 3GA-agarose affinity chromatography of detergent-solubilized rat intestinal microsomes. The AAT activity is associated with a 54-kDa protein, that binds covalently an acyl group from acyl-CoA via a thiol ester linkage (Lehner, R. and Kuksis, A. (1993) J. Biol. Chem. 268, 24726-24733). Reagents that prevent the acyl-AAT formation inhibit triacylglycerol synthesis as does the removal of AAT from the complex by immunoprecipitation. In the absence of mono- and diacylglycerol acceptors, the acyl group is transferred to water. It is proposed that triacylglycerol synthesis proceeds via a sequential transfer of acyl groups from acyl-CoA ligase to the AAT, from which they are passed to the mono- and diacylglycerol acyltransferases for incorporation into the di- and triacylglycerols depending on the availability of the acyl acceptors.

Phospholipid requirement of progesterone 5 alpha-reductase from gastric mucosa microsomes of guinea pig

Progesterone 5 alpha-reductase partially purified from gastric mucosa microsomes was stimulated by short-chain synthetic phosphatidylcholines (PC), such as dilauroyl PC, but not by various PC from biological sources. Phosphatidylserine (PS) activated the gastric 5 alpha-reductase to a limited extent compared to the liver 5 alpha-reductase described previously [Ichihara, K., and Tanaka, C. (1987) Biochem. Biophys. Res. Commun. 149, 482-487]. In search of more effective phospholipid activators, we tested the effects of various lysophospholipids on 5 alpha-reductase activity. Strongly stimulatory effects were observed when lysophosphatidylcholine (lysoPC) and lysophosphatidylethanolamine (lysoPE) were used instead of PC and phosphatidylethanolamine. Examination of synthetic lysoPC and lysoPE differing in acyl chain lengths showed that fatty chains of 12 to 16 carbons were effective in stimulating the 5 alpha-reductase. By contrast, other lysophospholipids such as lysophosphatidic acid, lysophosphatidylglycerol or lysophosphatidylserine (lysoPS) greatly inhibited 5 alpha-reductase activity. These findings suggest that gastric 5 alpha-reductase may be under dual regulation; lysoPC and lysoPE may play important roles as positive effectors, whereas lysophosphatidic acid, lysophosphatidylglycerol and lysoPS act as negative effectors in progesterone 5 alpha-reductase regulation.

Lysophosphatidylserine enhances the transfer of 22:6n3 to lysophosphatidic acid in rat brain microsomes

Although the acyl groups of phosphatidylserine in brain are uniquely enriched in docosahexaenoic acid (22:6n3), the mechanism for this enrichment is not well understood. When rat brain homogenates and microsomes were incubated in the presence of lysophosphatidylserine (LPS) together with [14C]22:6n3 and cofactors for activation to its acylCoA, very little radioactivity was incorporated into phosphatidylserine (PS). On the other hand, [14C]20:4n6 was more actively incorporated into PS. Addition of LPS (1-10 uM), however, resulted in a 2-5 fold enhancement of the transfer of labeled 22:6n3 and 20:4n6 to phosphatidic acid (PA). Kinetic analysis indicated the ability of LPS to lower the Km and increase the Vmax of the lysophosphatidic acid (LPA) acyltransferase reaction. Among other lysophospholipids tested, lysophosphatidylserine was most effective in enhancing PA biosynthesis. Since PA is an important intermediate for de novo biosynthesis of phospholipids, these results reveal a novel mechanism for promoting synthesis of PA enriched in polyunsaturated fatty acids in brain.

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.