Showing papers on "Acyl-CoA published in 2000"

The Role of Long-Chain Fatty Acyl-CoA Esters in β-Cell Signal Transduction

Abstract: Glucose-induced insulin secretion is associated with inhibition of free fatty acid (FFA) oxidation, increased esterification and complex lipid formation by pancreatic β-cells. Abundant evidence favors a role for cytosolic long-chain acyl-CoA (LC-CoA), including the rapid rise in malonyl CoA, the inhibitory effect of hydroxy-citrate or acetyl CoA carboxylase knockout, both of which prevent malonyl CoA formation, and the stimulatory effect of exogenous FFA. On the other hand, some evidence opposes the concept, including the fall in total LC-CoA levels in response to glucose, the stimulatory effect of LC-CoA on K ATP channels and the lack of inhibition of glucose-stimulated secretion either by overexpression of malonyl CoA decarboxylase, which markedly lowers malonyl CoA levels, or by triacsin C, which blocks FFA conversion to LC-CoA. Alternative explanations for these data are presented. A revised model of nutrient-stimulated secretion involving two arms of signal transduction that occur simultaneously is proposed. One arm depends on modulation of the K ATP channel evoked by changes in the ATP/ADP ratio. The other arm depends upon anaplerotic input into the tricarboxylic acid cycle, generation of excess citrate, and increases in cytosolic malonyl-CoA. Input from this arm is increased LC-CoA. Signaling through both arms would be required for normal secretion. LC-CoA esters and products formed from them are potent regulators of enzymes and channels. It is hypothesized that their elevations directly modulate the activity of enzymes, genes and various β-cell functions or modify the acylation state of key proteins involved in regulation of ion channels and exocytosis.

The carnitine acyltransferases: modulators of acyl-CoA-dependent reactions.

Abstract: Carnitine and carnitine acyltransferases were thought to be merely a mechanism for the rapid transfer of activated long-chain fatty acids into the mitochondrion for beta-oxidation, until enzymologists came along. By kinetic, physical and localization studies, eight different mammalian carnitine acyltransferases have been characterized. Of these, five have been cloned and sequenced. The carnitine :acylcarnitine exchange carrier, first characterized in mitochondria, has now been demonstrated immunologically in peroxisomal membranes too. This cell-wide carnitine system consisting of at least six proteins linking at least four intracellular pools of acyl-CoA that supply a multitude of lipid metabolic pathways is clearly more complex than was first thought. In this article, I describe the location and properties of the components to show how they can modulate acyl-CoA-dependent reactions in the cell.

Long-Chain Acyl-CoA–Dependent Regulation of Gene Expression in Bacteria, Yeast and Mammals

Abstract: Fatty acyl-CoA thioesters are essential intermediates in lipid metabolism. For many years there have been numerous conflicting reports concerning the possibility that these compounds also serve regulatory functions. In this review, we examine the evidence that long-chain acyl-CoA is a regulatory signal that modulates gene expression. In the bacteria Escherichia coli, long-chain fatty acyl-CoA bind directly to the transcription factor Fad R. Acyl-CoA binding renders the protein incapable of binding DNA, thus preventing transcription activation and repression of many genes and operons. In the yeast Saccharomyces cerevisiae, genes encoding peroxisomal proteins are activated in response to exogenously supplied fatty acids. In contrast, growth of yeast cells in media containing exogenous fatty acids results in repression of a number of genes, including that encoding the Δ9-fatty acid desaturase (OLE1). Both repression and activation are dependent upon the function of either of the acyl-CoA synthetases Faa1 p or Faa4p. In mammals, purified hepatocyte nuclear transcription factor 4a (HNF-4a) like E. coli FadR, binds long chain acyl-CoA directly. Coexpression of HNF-4a and acyl-CoA synthetase increases the activation of transcription of a fatty acid-responsive promoter, whereas coexpression with thioesterase decreases the fatty acid-mediated response. Conflicting data exist in support of the notion that fatty acyl-CoA are natural ligands for peroxisomal proliferator-activated receptor a (PPARa).

Long-chain acyl-CoA as a multi-effector ligand in cellular metabolism.

Abstract: Fatty acyl-CoA esters have the ability to bind at specific sites on certain proteins through their CoA moiety, thereby acting as modulators of cellular metabolism. In some cases at least, the acyl-CoA competes with cofactors (nucleotides) for binding to the proteins and results in either their activation or inhibition of catalytic activity. Photolabeling derivatives of acyl-CoA permit covalent binding of the esters to the proteins, which should lead to determination of amino acid residues required for ligand binding, if a common binding motif exists. On the basis of the accumulation of published results, there is now evidence to implicate acyl-CoA esters in the regulation of a variety of biological processes, ranging from mitochondrial metabolism to gene transcription to insulin secretion and signaling.

The questionable role of a microsomal °8 acyl-CoA-dependent desaturase in the biosynthesis of polyunsaturated fatty acids

Abstract: Several experimental approaches were used to determine whether rat liver and testes express an acyl-CoA-dependent δ8 desaturase. When [1-14C]5, 11, 14-eicosatrienoic acid was injected via the tail vein, or directly into testes, it was incorporated into liver and testes phospholipids, but it was not metabolized to other labeled fatty acids. When [1-14C]11, 14-eicosadienoic acid was injected, via the tail vein or directly into testes, or incubated with microsomes from both tissues, it was only metabolized to 5,11, 14-eicosatrienoic acid. When ethyl 5,5,11,11,14,14-d6-5,11,14-eicosatrienoate was fed to rats maintained on a diet devoid of fat, it primarily replaced esteri-fied 5,8,11-eicosatrienoic acid, but not arachidonic acid. No labeled linoleate or arachidonate were detected. Dietary ethyl linoleate and ethyl 19,19,20,20-d4-1,2-13C-11,14-eicosadienoate were about equally effective as precursors of esterified arachidonate. The doubly labeled 11,14-eicosadienoate was metabolized primarily by conversion to 17,17,18,18-d4-9,12-ocatdeca-dienoic acid, followed by its conversion to yield esterified arachidonate, with a mass four units greater than endogenous arachidonate. In addition, the doubly labeled substrate gave rise to a small amount of arachidonate, six mass units greater than endogenous arachidonate. No evidence was obtained, with the radiolabeled substrates, for the presence of a δ8 desaturase. However, the presence of an ion, six mass units greater than endogenous arachidonate when doubly labeled 11, 14-eicosa-dienoate was fed, suggests that a small amount of the substrate may have been metabolized by the sequential use of δ8 and δ5 desaturases.

Functional characterization of mammalian mitochondrial carnitine palmitoyltransferases I and II expressed in the yeast Pichia pastoris

Abstract: Mitochondrial carnitine palmitoyltransferases I and II (CPTI and CPTII), together with the carnitine carrier, transport long-chain fatty acyl-CoA from the cytosol to the mitochondrial matrix for beta-oxidation. Recent progress in the expression of CPTI and CPTII cDNA clones in Pichia pastoris, a yeast with no endogenous CPT activity, has greatly facilitated the characterization of these important enzymes in fatty acid oxidation. It is now well established that yeast-expressed CPTI is a catalytically active, malonyl CoA-sensitive, distinct enzyme that is reversibly inactivated by detergents. CPTII is a catalytically active, malonyl CoA-insensitive, distinct enzyme that is detergent stable. Reconstitution studies with yeast-expressed CPTI have established for the first time that detergent inactivation of CPTI is reversible, suggesting that CPTI is active only in a membrane environment. By constructing a series of deletion mutants of the N-terminus of liver CPTI, we have mapped the residues essential for malonyl CoA inhibition and binding to the conserved first six N-terminal amino acid residues. Mutation of glutamic acid 3 to alanine abolished malonyl CoA inhibition and high affinity malonyl CoA binding, but not catalytic activity, whereas mutation of histidine 5 to alanine caused partial loss in malonyl CoA inhibition. Our mutagenesis studies demonstrate that glutamic acid 3 and histidine 5 are necessary for malonyl CoA inhibition and binding to liver CPTI, but not catalytic activity.

A plant long chain fatty acid biosynthetic enzyme

Abstract: In various aspects, the present invention provides nucleic acid sequence encoding all or part of a new plant long chain fatty acid condensing enzyme (a fatty acid elongase), designated herein as KCS2 (for beta-ketoacyl-coenzyme A synthase 2). In some embodiments, KCS2 may mediate the biosynthesis of C18, C20, C22 and C24 fatty acids. The activity of the enzyme is typically characterized by two carbon (malonyl-CoA) additions to C16, C18, C20 and C22 moieties (C16-C22 acyl CoA molecules), i.e. condensation of malonyl-CoA with a C16, C18, C20 or C22 acyl-CoA. The fatty acids produced by the enzyme may for example be saturated 18:0, 20:0, 22:0 and 24:0 fatty acids. The invention includes recombinant nucleic acid molecules comprising a heterologous nucleic acid coding sequence encoding the plant long chain fatty acid condensing enzyme.

Acyl-CoA thioesterases belong to a novel gene family of peroxisome proliferator-regulated enzymes involved in lipid metabolism

Abstract: Acyl-CoA thioesterases hydrolyze acyl-CoAs to the corresponding free fatty acid plus coenzyme A. The activity is strongly induced in rat and mouse liver after feeding the animals peroxisome proliferators (PPs). To elucidate the role of these enzymes in lipid metabolism, the authors have cloned the cDNAs corresponding to the inducible cytosolic and mitochondrial type I enzymes (CTE-I and MTE-I), and studied tissue expression and nutritional regulation of expression of the mRNAs in mice. The constitutive expression of both mRNAs was low in liver, with CTE-I expressed mainly in kidney and brown adipose tissue, and MTE-I expressed in brown adipose tissue and heart. As expected, the expression in liver of both the CTE-I and MTE-I mRNAs were strongly induced (> 50-fold) by treatment with clofibrate. A similar level of induction was observed by fasting and a time-course study showed that the CTE-I and MTE-I mRNAs were increased already at 6 h after removal of the diet. Refeeding normal chow diet to mice fasted for 24 h normalized the mRNA levels with a T1/2 of about 3-4 h. Feeding mice a fat-free diet further decreased the expression, possibly indicating repression of expression. The strong expression of MTE-I and CTE-I in the heart was increased about 10-fold by fasting. To further characterize these highly regulated enzymes, the authors have cloned the corresponding genes and promoter regions. The structures of the two genes were found to be very similar, consisting of three exons and two introns. Exon-intron borders conform to general consensus sequences, and, especially, the first exon appears to be highly conserved. The promoter regions of both the CTE-I and MTE-I genes contain putative PP response elements, suggesting an involvement of PP-activated receptors in the regulation of these genes.