A method for the chemical synthesis of 14C-labeled fatty acyl coenzyme A's of high specific activity

Inhibitory Investigations of Acyl-CoA Derivatives against Human Lipoxygenase Isozymes

Lipid metabolism is a complex process crucial for energy production resulting in high levels of acyl-coenzyme A (acyl-CoA) molecules in the cell. Acyl-CoAs have also been implicated in inflammation, which could be possibly linked to lipoxygenase (LOX) biochemistry by the observation that an acyl-CoA was bound to human platelet 12-lipoxygenase via cryo-EM. Given that LOX isozymes play a pivotal role in inflammation, a more thorough investigation of the inhibitory effects of acyl-CoAs on lipoxygenase isozymes was judged to be warranted. Subsequently, it was determined that C18 acyl-CoA derivatives were the most potent against h12-LOX, human reticulocyte 15-LOX-1 (h15-LOX-1), and human endothelial 15-LOX-2 (h15-LOX-2), while C16 acyl-CoAs were more potent against human 5-LOX. Specifically, oleoyl-CoA (18:1) was most potent against h12-LOX (IC50 = 32 μM) and h15-LOX-2 (IC50 = 0.62 μM), stearoyl-CoA against h15-LOX-1 (IC50 = 4.2 μM), and palmitoleoyl-CoA against h5-LOX (IC50 = 2.0 μM). The inhibition of h15-LOX-2 by oleoyl-CoA was further determined to be allosteric inhibition with a Ki of 82 +/- 70 nM, an α of 3.2 +/- 1, a β of 0.30 +/- 0.07, and a β/α = 0.09. Interestingly, linoleoyl-CoA (18:2) was a weak inhibitor against h5-LOX, h12-LOX, and h15-LOX-1 but a rapid substrate for h15-LOX-1, with comparable kinetic rates to free linoleic acid (kcat = 7.5 +/- 0.4 s-1, kcat/KM = 0.62 +/- 0.1 µM-1s-1). Additionally, it was determined that methylated fatty acids were not substrates but rather weak inhibitors. These findings imply a greater role for acyl-CoAs in the regulation of LOX activity in the cell, either through inhibition of novel oxylipin species or as a novel source of oxylipin-CoAs.

Acute ACAT1/SOAT1 Blockade Increases MAM Cholesterol and Strengthens ER-Mitochondria Connectivity

Cholesterol is a key component of all mammalian cell membranes. Disruptions in cholesterol metabolism have been observed in the context of various diseases, including neurodegenerative disorders such as Alzheimer's disease (AD). The genetic and pharmacological blockade of acyl-CoA:cholesterol acyltransferase 1/sterol O-acyltransferase 1 (ACAT1/SOAT1), a cholesterol storage enzyme found on the endoplasmic reticulum (ER) and enriched at the mitochondria-associated ER membrane (MAM), has been shown to reduce amyloid pathology and rescue cognitive deficits in mouse models of AD. Additionally, blocking ACAT1/SOAT1 activity stimulates autophagy and lysosomal biogenesis; however, the exact molecular connection between the ACAT1/SOAT1 blockade and these observed benefits remain unknown. Here, using biochemical fractionation techniques, we observe cholesterol accumulation at the MAM which leads to ACAT1/SOAT1 enrichment in this domain. MAM proteomics data suggests that ACAT1/SOAT1 inhibition strengthens the ER-mitochondria connection. Confocal and electron microscopy confirms that ACAT1/SOAT1 inhibition increases the number of ER-mitochondria contact sites and strengthens this connection by shortening the distance between these two organelles. This work demonstrates how directly manipulating local cholesterol levels at the MAM can alter inter-organellar contact sites and suggests that cholesterol buildup at the MAM is the impetus behind the therapeutic benefits of ACAT1/SOAT1 inhibition.

Structures of an unusual 3-hydroxyacyl dehydratase (FabZ) from a ladderane-producing organism with an unexpected substrate preference

The genomes of anaerobic ammonium-oxidizing (anammox) bacteria contain a gene cluster comprising genes of unusual fatty acid biosynthesis enzymes that were suggested to be involved in the synthesis of the unique "ladderane" lipids produced by these organisms. This cluster encodes an acyl carrier protein (denoted as "amxACP") and a variant of FabZ, an ACP-3-hydroxyacyl dehydratase. In this study, we characterize this enzyme, which we call anammox-specific FabZ ("amxFabZ"), to investigate the unresolved biosynthetic pathway of ladderane lipids. We find that amxFabZ displays distinct sequence differences to "canonical" FabZ, such as a bulky, apolar residue on the inside of the substrate binding tunnel, where the canonical enzyme has a glycine. Additionally, substrate screens suggest that amxFabZ efficiently converts substrates with acyl chain lengths of up to eight carbons, whereas longer substrates are converted much more slowly under the conditions used. We also present crystal structures of amxFabZs, mutational studies and the structure of a complex between amxFabZ and amxACP, which show that the structures alone cannot explain the apparent differences from canonical FabZ. Moreover, we find that while amxFabZ does dehydrate substrates bound to amxACP, it does not convert substrates bound to canonical ACP of the same anammox organism. We discuss the possible functional relevance of these observations in the light of proposals for the mechanism for ladderane biosynthesis.

Development of a novel high-throughput screen for the identification of new inhibitors of protein S-acylation

Protein S-acylation is a reversible post-translational modification that modulates the localization and function of many cellular proteins. S-acylation is mediated by a family of zinc finger DHHC (Asp-His-His-Cys) domain–containing (zDHHC) proteins encoded by 23 distinct ZDHHC genes in the human genome. These enzymes catalyze S-acylation in a two-step process involving “autoacylation” of the cysteine residue in the catalytic DHHC motif followed by transfer of the acyl chain to a substrate cysteine. S-acylation is essential for many fundamental physiological processes, and there is growing interest in zDHHC enzymes as novel drug targets for a range of disorders. However, there is currently a lack of chemical modulators of S-acylation either for use as tool compounds or for potential development for therapeutic purposes. Here, we developed and implemented a novel FRET-based high-throughput assay for the discovery of compounds that interfere with autoacylation of zDHHC2, an enzyme that is implicated in neuronal S-acylation pathways. Our screen of >350,000 compounds identified two related tetrazole-containing compounds (TTZ-1 and TTZ-2) that inhibited both zDHHC2 autoacylation and substrate S-acylation in cell-free systems. These compounds were also active in human embryonic kidney 293T cells, where they inhibited the S-acylation of two substrates (SNAP25 and PSD95 [postsynaptic density protein 95]) mediated by different zDHHC enzymes, with some apparent isoform selectivity. Furthermore, we confirmed activity of the hit compounds through resynthesis, which provided sufficient quantities of material for further investigations. The assays developed provide novel strategies to screen for zDHHC inhibitors, and the identified compounds add to the chemical toolbox for interrogating cellular activities of zDHHC enzymes in S-acylation.

Nanodisc scaffold peptide (NSPr) replaces detergent by reconstituting acyl-CoA:cholesterol acyltransferase 1 into peptidiscs

To conduct biochemical studies in vitro, membrane proteins (MPs) must be solubilized with detergents. While detergents are great tools, they can also inhibit the biological activity and/or perturb oligomerization of individual MPs. Nanodisc scaffold peptide (NSPr), an amphipathic peptide analog of ApoA1, was recently shown to reconstitute detergent solubilized MPs into peptidiscs in vitro. Acyl-coenzyme A:cholesterol acyltransferase 1 (ACAT1), also known as sterol O-acyltransferase 1 (SOAT1), plays a key role in cellular cholesterol storage in various cell types and is a drug target to treat multiple human diseases. ACAT1 contains nine transmembrane domains (TMDs) and primarily forms a homotetramer in vitro and in intact cells; deletion of the N-terminal dimerization domain produces a homodimer with full retention in catalytic activity. ACAT1 is prone to inactivation by numerous detergents. Here we pursued the use of NSPr to overcome the detergent-induced inactivation of ACAT1 by generating near detergent-free ACAT1 peptidiscs. Based on native-PAGE analysis, we showed that NSPr reconstitutes ACAT1 into soluble peptidiscs, in which ACAT1 exists predominantly in oligomeric states greater than a homotetramer. The formation of these higher-order oligomeric states was independent of the N-terminal dimerization domain, suggesting that the oligomerization is mediated through hydrophobic interactions of multiple ACAT1 subunits. ACAT1 peptidiscs were still susceptible to heat-mediated inactivation, presumably due to the residual detergent (CHAPS) bound to ACAT1. We then conditioned ACAT1 with phosphatidylcholine (PC) to replace CHAPS prior to the formation of ACAT1 peptidiscs. The results showed, when PC was included, ACAT1 was present mainly in higher-order oligomeric states with greater enzymatic activity. With PC present, the enzymatic activity of ACAT1 peptidiscs was protected from heat-mediated inactivation. These results support the use of NSPr to create a near detergent-free solution of ACAT1 in peptidiscs for various in vitro studies. Our current results also raise the possibility that, under certain conditions, ACAT1 may form higher-order oligomeric states in vivo.

A simple method to disrupt and restore subunit interaction of acyl-CoA:cholesterol acyltransferase 1

Acyl-CoA:cholestereol acyltransferase 1 (ACAT1) is a two-fold dimer (homotetramer) and has two distinct dimerization domains. One domain is in an alpha-helical rich region near the cytoplasmic N-terminus. The other is proposed to be near the C-terminus where multiple transmembrane domains promote hydrophobic interactions between two ACAT1 subunits. The truncation of the ACAT1 N-terminal dimerization domain, Δ1-65, creates a dimer which is fully enzymatically active. It is currently not known how the C-terminal dimerization domain contributes to ACAT1 enzymatic activity. Here we describe a simple method that dissociates ACAT1 dimers through the addition of the non-ionic detergents Triton X-100 or octyl glucoside which disrupt the C-terminal dimerization domain. We also document the protocols for a method to exchange Triton X-100 with CHAPS to restore C-terminal dimerization of the ACAT1 protein, and an optimized liposomal assay to assess ACAT enzymatic activity.

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This method can be applied to dissociate ACAT1 subunits by using Triton X-100 or octyl glucoside.

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ACAT1 dimerization can be restored by exchanging Triton X-100 with CHAPS.

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The liposomal ACAT activity assay conditions have been optimized.

Crosstalk between cellular metabolism and histone acetylation

Dynamic interplay between cellular metabolism and histone acetylation is a key mechanism underlying metabolic control of epigenetics. In particular, the central metabolite acetyl-coenzyme A (acetyl-CoA) acts as the acetyl-donor for histone acetylation in both an enzymatic and non-enzymatic manner. Since members of the family of histone acetyl transferases (HATs) that catalyze the acetylation of histone tails possess a Michaelis constant (Km) within the range of physiological cellular acetyl-CoA concentrations, changing concentrations of acetyl-CoA can restrict or promote enzymatic histone acetylation. Likewise, non-enzymatic histone acetylation occurs at physiological concentrations. These concepts implicate acetyl-CoA as a rheostat for nutrient availability acting, in part, by controlling histone acetylation. Histone acetylation is an important epigenetic modification that controls gene expression and acetyl-CoA dependent changes in both histone acetylation and gene expression have been shown in yeast and mammalian systems. However, quantifying the metabolic conditions required to achieve specific changes in histone acetylation is a major challenge. The relationship between acetyl-CoA and histone acetylation may be influenced by a variety of factors including sub-cellular location of metabolites and enzymes, relative quantities of metabolites, and substrate availability/preference. A diversity of substrates can contribute the two-carbon acyl-chain to acetyl-CoA, a number of pathways can create or degrade acetyl-CoA, and only a handful of potential mechanisms for the crosstalk between metabolism and histone acetylation have been explored. The centrality of acetyl-CoA in intermediary metabolism means that acetyl-CoA levels may change, or be resistant to change, in unexpected ways. Thus, quantification of relevant metabolites is critical evidence in understanding how the nutrient rheostat is set in normal and pathological contexts. Coupling metabolite quantitation with isotope tracing to examine fate of specific metabolites is critical to the crosstalk between metabolism and histone acetylation, including but not limited to acetyl-CoA provides necessary context. This chapter provides guidance on experimental design of quantification with isotope dilution and/or tracing of acetyl-CoA within a targeted or highly multiplexed multi-analyte workflow.

Triton X-100 or octyl glucoside inactivates acyl-CoA:cholesterol acyltransferase 1 by dissociating it from a two-fold dimer to a two-fold monomer

Cholesterol is an important lipid molecule and is needed for all mammalian cells. In various cell types, excess cholesterol is stored as cholesteryl esters; acyl-CoA:cholesterol acyltransferase 1 (ACAT1) plays an essential role in this storage process. ACAT1 is located at the endoplasmic reticulum and has nine transmembrane domains (TMDs). It is a member of the membrane-bound O-acyltransferase (MBOAT) family, in which members contain multiple TMDs and participate in a variety of biological functions. When solubilized in the zwitterionic detergent CHAPS, ACAT1 can be purified to homogeneity with full enzyme activity and behaves as a homotetrameric protein. ACAT1 contains two dimerization motifs. The first motif is located near the N-terminus and is not conserved in MBOATs. Deletion of the N-terminal dimerization domain converts ACAT1 to a dimer with full catalytic activity; therefore, ACAT1 is a two-fold dimer. The second dimerization domain, located near the C-terminus, is conserved in MBOATs; however, it was not known whether the C-terminal dimerization domain is required for enzyme activity. Here we show that treating ACAT1 with non-ionic detergent, Triton X-100 or octyl glucoside, causes the enzyme to become a two-fold monomer without any enzymatic activity. Detergent exchange of Triton X-100 with CHAPS restores ACAT1 to a two-fold dimer but fails to restore its enzymatic activity. These results implicate that ACAT1 requires hydrophobic subunit interactions near the C-terminus in order to remain active as a two-fold dimer. Our results also caution the use of Triton X-100 or octyl glucoside to purify other MBOATs.

Integrated Analysis of Acetyl-CoA and Histone Modification via Mass Spectrometry to Investigate Metabolically Driven Acetylation

Acetylation is a highly abundant and dynamic post-translational modification (PTM) on histone proteins which, when present on chromatin-bound histones, facilitates the accessibility of DNA for gene transcription. The central metabolite, acetyl-CoA, is a substrate for acetyltransferases, which catalyze protein acetylation. Acetyl-CoA is an essential intermediate in diverse metabolic pathways, and cellular acetyl-CoA levels fluctuate according to extracellular nutrient availability and the metabolic state of the cell. The Michaelis constant (Km) of most histone acetyltransferases (HATs), which specifically target histone proteins, falls within the range of cellular acetyl-CoA concentrations. As a consequence, global levels of histone acetylation are often restricted by availability of acetyl-CoA. Such metabolic regulation of histone acetylation is important for cell proliferation, differentiation, and a variety of cellular functions. In cancer, numerous oncogenic signaling events hijack cellular metabolism, ultimately inducing an extensive rearrangement of the epigenetic state of the cell. Understanding metabolic control of the epigenome through histone acetylation is essential to illuminate the molecular mechanisms by which cells sense, adapt, and occasionally disengage nutrient fluctuations and environmental cues from gene expression. In particular, targeting metabolic regulators or even dietary interventions to impact acetyl-CoA availability and histone acetylation is a promising target in cancer therapy. Liquid chromatography coupled to mass spectrometry (LC-MS) is the most accurate methodology to quantify protein PTMs and metabolites. In this chapter, we present state-of-the-art protocols to analyze histone acetylation and acetyl-CoA. Histones are extracted and digested into short peptides (4-20 aa) prior to LC-MS. Acetyl-CoA is extracted from cells and analyzed using an analogous mass spectrometry-based procedure. Model systems can be fed with isotopically labeled substrates to investigate the metabolic preference for acetyl-CoA production and the metabolic dependence and turnover of histone acetylation. We also present an example of data integration to correlate changes in acetyl-CoA production with histone acetylation.

Stable isotope dilution liquid chromatography/mass spectrometry analysis of cellular and tissue medium- and long-chain acyl-coenzyme A thioesters

RATIONALE

Acyl-Coenzyme A (CoA) thioesters are the principal form of activated carboxylates in cells and tissues. They are employed as acyl carriers that facilitate the transfer of acyl groups to lipids and proteins. Quantification of medium- and long-chain acyl-CoAs represents a significant bioanalytical challenge because of their instability.

METHODS

Stable isotope dilution liquid chromatography/selected reaction monitoring-mass spectrometry (LC/SRM-MS) provides the most specific and sensitive method for the analysis of CoA species. However, relevant heavy isotope standards are not available and they are challenging to prepare by chemical synthesis. Stable isotope labeling by essential nutrients in cell culture (SILEC), developed originally for the preparation of stable isotope labeled short-chain acyl-CoA thioester standards, has now been extended to medium-chain and long-chain acyl-CoAs and used for LC/SRM-MS analyses.

RESULTS

Customized SILEC standards with >98% isotopic purity were prepared using mouse Hepa 1c1c7 cells cultured in pantothenic-free media fortified with [(13) C3 (15) N1 ]-pantothenic acid and selected fatty acids. A SILEC standard in combination with LC/SRM-MS was employed to quantify cellular concentrations of arachidonoyl-CoA (a representative long-chain acyl-CoA) in two human colon cancer cell lines. A panel of SILEC standards was also employed in combination LC/SRM-MS to quantify medium- and long-chain acyl-CoAs in mouse liver.

CONCLUSIONS

This new SILEC-based method in combination with LC/SRM-MS will make it possible to rigorously quantify medium- and long-chain acyl-CoAs in cells and tissues. The method will facilitate studies of medium- and long-chain acyl-CoA dehydrogenase deficiencies as well as studies on the role of medium- and long-chain acyl-CoAs in cellular metabolism.

Acyl-coenzyme A:cholesterol acyltransferase 1 - significance of single-nucleotide polymorphism at residue 526 and the role of Pro347 near the fifth transmembrane domain

Acyl-coenzyme A:cholesterol acyltransferases (ACATs), which are members of the membrane-bound O-acyltransferase family, catalyze the conversion of cholesterol to cholesteryl esters. Mammals have two isoenzymes: ACAT1 and ACAT2. Both enzymes are drug targets for treating human diseases. ACAT1 is present in various cell types. It contains nine transmembrane domains (TMDs), with the active site His460 located within TMD7, and the active site Asn421 located within the fourth large cytoplasmic loop. In human ACAT1, a single-nucleotide polymorphism exists for residue 526: the codon is either CAG for Gln, or CGG for Arg. Gln526/Arg526 is present within the C-terminal loop. Its biochemical significance is unknown. In addition, within the C-terminal half of ACAT1, numerous residues conserved with those of ACAT2 are present; the functions of these conserved residues are largely unknown. Here, we performed single-substitution mutagenesis experiments to investigate the roles of individual residues present in the C-terminal loop, including Gln526/Arg526, and the eight conserved Pro residues located near/in various TMDs. The results show that the enzyme activity of ACAT1 with Gln526 is less active than that of ACAT1 with Arg526 by 40%. In addition, several residues in the C-terminal loop are important for maintaining proper ACAT1 protein stability. Other results show that Pro347 plays an important role in modulating enzyme catalysis. Overall, our results imply that the CAG/CGG polymorphism can be utilized to perform ACAT1 activity/human disease susceptibility studies, and that Pro347 located near TMD5 plays an important role in modulating enzyme catalysis.

Cellular pregnenolone esterification by acyl-CoA:cholesterol acyltransferase

Pregnenolone (PREG) can be converted to PREG esters (PE) by the plasma enzyme lecithin: cholesterol acyltransferase (LCAT), and by other enzyme(s) with unknown identity. Acyl-CoA:cholesterol acyltransferase 1 and 2 (ACAT1 and ACAT2) convert various sterols to steryl esters; their activities are activated by cholesterol. PREG is a sterol-like molecule, with 3-β-hydroxy moiety at steroid ring A, but with much shorter side chain at steroid ring D. Here we show that without cholesterol, PREG is a poor ACAT substrate; with cholesterol, the V(max) for PREG esterification increases by 100-fold. The binding affinity of ACAT1 for PREG is 30-50-fold stronger than that for cholesterol; however, PREG is only a substrate but not an activator, while cholesterol is both a substrate and an activator. These results indicate that the sterol substrate site in ACAT1 does not involve significant sterol-phospholipid interaction, while the sterol activator site does. Studies utilizing small molecule ACAT inhibitors show that ACAT plays a key role in PREG esterification in various cell types examined. Mice lacking ACAT1 or ACAT2 do not have decreased PREG ester contents in adrenals, nor do they have altered levels of the three major secreted adrenal steroids in serum. Mice lacking LCAT have decreased levels of PREG esters in the adrenals. These results suggest LCAT along with ACAT1/ACAT2 contribute to control pregnenolone ester content in different cell types and tissues.

Purification of recombinant acyl-coenzyme A:cholesterol acyltransferase 1 (ACAT1) from H293 cells and binding studies between the enzyme and substrates using difference intrinsic fluorescence spectroscopy

Acyl-coenzyme A:cholesterol acyltransferase 1 (ACAT1) is a membrane-bound enzyme utilizing long-chain fatty acyl-coenzyme A and cholesterol to form cholesteryl esters and coenzyme A. Previously, we had expressed tagged human ACAT1 (hACAT1) in CHO cells and purified it to homogeneity; however, only a sparse amount of purified protein could be obtained. Here we report that the hACAT1 expression level in H293 cells is 18-fold higher than that in CHO cells. We have developed a milder purification procedure to purify the enzyme to homogeneity. The abundance of the purified protein enabled us to conduct difference intrinsic fluorescence spectroscopy to study the binding between the enzyme and its substrates in CHAPS/phospholipid mixed micelles. The results show that oleoyl-CoA binds to ACAT1 with K(d) = 1.9 μM and elicits significant structural changes of the protein as manifested by the significantly positive changes in its fluorescence spectrum; stearoyl-CoA elicits a similar spectrum change but much lower in magnitude. Previously, kinetic studies had shown that cholesterol is an efficient substrate and an allosteric activator of ACAT1, while its diastereomer epicholesterol is neither a substrate nor an activator. Here we show that both cholesterol and epicholesterol induce positive changes in the ACAT1 fluorescence spectrum; however, the magnitude of spectrum changes induced by cholesterol is much larger than epicholesterol. These results show that stereospecificity, governed by the 3β-OH moiety in steroid ring A, plays an important role in the binding of cholesterol to ACAT1.

Two human ACAT2 mRNA variants produced by alternative splicing and coding for novel isoenzymes

Acyl coenzyme A:cholesterol acyltransferase 2 (ACAT2) plays an important role in cholesterol absorption. Human ACAT2 is highly expressed in small intestine and fetal liver, but its expression is greatly diminished in adult liver. The full-length human ACAT2 mRNA encodes a protein, designated ACAT2a, with 522 amino acids. We have previously reported the organization of the human ACAT2 gene and the differentiation-dependent promoter activity in intestinal Caco-2 cells. In the current work, two human ACAT2 mRNA variants produced by alternative splicing are cloned and predicted to encode two novel ACAT2 isoforms, named ACAT2b and ACAT2c, with 502 and 379 amino acids, respectively. These mRNA variants differ from ACAT2a mRNA by lack of the exon 4 (ACAT2b mRNA) and exons 4-5 plus 8-9-10 (ACAT2c mRNA). Significantly, comparable amounts of the alternatively spliced ACAT2 mRNA variants were detected by RT-PCR, and Western blot analysis confirmed the presence of their corresponding proteins in human liver and intestinecells. Furthermore, phosphorylation and enzymatic activity analyses demonstrated that the novel isoenzymes ACAT2b and ACAT2c lacked the phosphorylatable site SLLD, and their enzymatic activities reduced to 25%-35% of that of ACAT2a. These evidences indicate that alternative splicing produces two human ACAT2 mRNA variants that encode the novel ACAT2 isoenzymes. Our findings might help to understand the regulation of the ACAT2 gene expression under certain physiological and pathological conditions.

Investigating the allosterism of acyl-CoA:cholesterol acyltransferase (ACAT) by using various sterols: in vitro and intact cell studies

ACAT1 (acyl-CoA:cholesterol acyltransferase 1) is thought to have two distinct sterol-binding sites: a substrate-binding site and an allosteric-activator site. In the present work, we investigated the structural features of various sterols as substrates and/or activators in vitro. The results show that without cholesterol, the plant sterol sitosterol is a poor substrate for ACAT. In the presence of cholesterol, ACAT1-mediated esterification of sitosterol is highly activated while ACAT2-mediated esterification of sitosterol is only moderately activated. For ACAT1, we show that the stereochemistry of the 3-hydroxy group at steroid ring A is a critical structural feature for a sterol to serve as a substrate, but less critical for activation. Additionally, enantiomeric cholesterol, which has the same biophysical properties as cholesterol in membranes, fails to activate ACAT1. Thus ACAT1 activation by cholesterol is the result of stereo-specific interactions between cholesterol and ACAT1, and is not related to the biophysical properties of phospholipid membranes. To demonstrate the relevance of the ACAT1 allosteric model in intact cells, we showed that sitosterol esterification in human macrophages is activated upon cholesterol loading. We further show that the activation is not due to an increase in ACAT1 protein content, but is partly due to an increase in the cholesterol content in the endoplasmic reticulum where ACAT1 resides. Together, our results support the existence of a distinct sterol-activator site in addition to the sterol-substrate site of ACAT1 and demonstrate the applicability of the ACAT1 allosteric model in intact cells.

Fatty acyl-CoA elongation in Blatella germanica integumental microsomes

Insect cuticular hydrocarbons are synthesized de novo in integumental tissue through the concerted action of fatty acid synthases (FASs), fatty acyl-CoA elongases, a reductase, and a decarboxylase to produce hydrocarbons and CO2. Elongation of fatty acyl-CoAs to very long chain fatty acids was studied in the integumental microsomes of the German cockroach, Blatella germanica. Incubation of [1-14C]palmitoyl-CoA, malonyl-CoA, and NADPH resulted in the production of 18-CoA with minor amounts of C20, C22, C24, C30, and C32 labeled acyl-CoA moieties. Similar experiments with [1-14C]stearoyl-CoA rendered C20-CoA as the major product, and lesser amounts of C22 and C24-CoAs were also detected. After solubilization of the microsomal FAS, kinetic parameters were determined radiochemically or by measuring NADPH consumption. The reaction velocity was linear for up to 3 min incubation time, and with a protein concentration up to 0.025 microg/microl. The effect of the chain length on the reaction velocity was compared for palmitoyl-CoA, stearoyl-CoA, and eicosanoyl-CoA. The optimal substrate concentration was 10 microM for C16-CoA, between 8 and 12 microM for C18-CoA, and close to 3 microM for C20-CoA. In vivo hydrocarbon biosynthesis was inhibited from 55.5 to 72.5% in the presence of 1 mM trichloroacetic acid, a known inhibitor of elongation reactions.

Mutant acyl-coenzyme A:cholesterol acyltransferase 1 devoid of cysteine residues remains catalytically active

Acyl-coenzyme A:cholesterol acyltransferase (ACAT) plays important roles in cellular cholesterol homeostasis and in the early stages of atherosclerosis. ACAT1 is an integral membrane protein with multiple transmembrane domains. Human ACAT1 contains nine cysteine residues; its activity is severely inhibited by various thiol-specific modification reagents including p-chloromercuribenzene sulfonic acid, suggesting that certain cysteine residue(s) might be near or at the active site. We constructed various ACAT1 mutants that contained either single cysteine to alanine substitution at various positions, contained a reduced number of cysteines, or contained no cysteine at all. Each of these mutants retained 20% or more of the wild-type ACAT activity. Therefore, cysteine is not essential for ACAT catalysis. For the cysteine-free enzyme, its basic kinetic properties and intracellular localization in Chinese hamster ovary cells were shown to be very similar to those of the wild-type enzyme. The availability of the cysteine-free ACAT1 will facilitate future ACAT structure function studies. Additional studies show that Cys467 is one of the major target sites that leads to p-chloromercuribenzene sulfonic acid-mediated ACAT1 inactivation, suggesting that Cys467 may be near the ACAT active site(s).

Synergistic transcriptional activation of human Acyl-coenzyme A: cholesterol acyltransterase-1 gene by interferon-gamma and all-trans-retinoic acid THP-1 cells

Acyl-coenzyme A:cholesterol acyltransferase (ACAT) is an intracellular enzyme involved in cellular cholesterol homeostasis and in atherosclerotic foam cell formation. Human ACAT-1 gene contains two promoters (P1 and P7), each located in a different chromosome (1 and 7) (Li, B. L., Li, X. L., Duan, Z. J., Lee, O., Lin, S., Ma, Z. M., Chang, C. C., Yang, X. Y., Park, J. P., Mohandas, T. K., Noll, W., Chan, L., and Chang, T. Y. (1999) J. Biol Chem. 274, 11060-11071). Interferon-gamma (IFN-gamma), a cytokine that exerts many pro-atherosclerotic effects in vivo, causes up-regulation of ACAT-1 mRNA in human blood monocyte-derived macrophages and macrophage-like cells but not in other cell types. To examine the molecular nature of this observation, we identified within the ACAT-1 P1 promoter a 159-base pair core region. This region contains 4 Sp1 elements and an IFN-gamma activated sequence (GAS) that overlaps with the second Sp1 element. In the monocytic cell line THP-1 cell, the combination of IFN-gamma and all-trans-retinoic acid (a known differentiation agent) enhances the ACAT-1 P1 promoter but not the P7 promoter. Additional experiments showed that all-trans-retinoic acid causes large induction of the transcription factor STAT1, while IFN-gamma causes activation of STAT1 such that it binds to the GAS/Sp1 site in the ACAT-1 P1 promoter. Our work provides a molecular mechanism to account for the effect of IFN-gamma in causing transcriptional activation of ACAT-1 in macrophage-like cells.

Differential effects of acyl-CoA binding protein on enzymatic and non-enzymatic thioacylation of protein and peptide substrates

Both enzymatic and autocatalytic mechanisms have been proposed to account for protein thioacylation (commonly known as palmitoylation). Acyl-CoA binding proteins (ACBP) strongly suppress non-enzymatic thioacylation of cysteinyl-containing peptides by long-chain acyl-CoAs. At physiological concentrations of ACBP, acyl-CoAs, and membrane lipids, the rate of spontaneous acylation is expected to be too slow to contribute significantly to thioacylation of signaling proteins in mammalian cells (Leventis et al., Biochemistry 36 (1997) 5546-5553). Here we characterized the effects of ACBP on enzymatic thioacylation. A protein S-acyltransferase activity previously characterized using G-protein alpha-subunits as a substrate (Dunphy et al., J. Biol. Chem., 271 (1996) 7154-7159), was capable of thioacylating short lipid-modified cysteinyl-containing peptides. The minimum requirements for substrate recognition were a free cysteine thiol adjacent to a hydrophobic lipid anchor, either myristate or farnesyl isoprenoid. PAT activity displayed specificity for the acyl donor, efficiently utilizing long-chain acyl-CoAs, but not free fatty acid or S-palmitoyl-N-acetylcysteamine. ACBP only modestly inhibited enzymatic thioacylation of a myristoylated peptide or G-protein alpha-subunits under conditions where non-enzymatic thioacylation was reduced to background. Thus, protein S-acyltransferase remains active in the presence of physiological concentrations of ACBP and acyl-CoA in vitro and is likely to represent the predominant mechanism of thioacylation in vivo.

Evidence for triacylglycerol synthesis in the lumen of microsomes via a lipolysis-esterification pathway involving carnitine acyltransferases

In this study a pathway for the synthesis of triacylglycerol (TAG) within the lumen of the endoplasmic reticulum has been identified, using microsomes that had been preconditioned by depleting their endogenous substrates and then fusing them with biotinylated phosphatidylserine liposomes containing CoASH and Mg(2+). Incubating these fused microsomes with tri[(3)H] oleoylglycerol and [(14)C]oleoyl-CoA yielded microsome-associated triacylglycerol, which resisted extensive washing and had a [(3)H]:[(14)C] ratio close to 2:1. The data suggest that the precursor tri[(3)H]oleoylglycerol was hydrolyzed by microsomal lipase to membrane-bound di[(3)H]oleoylglycerol and subsequently re-esterified with luminal [(14)C]oleoyl-CoA. The accumulation of TAG within the microsomes, even when overt diacylglycerol acyltransferase (DGAT I) was inactive, is consistent with the existence of a latent diacylglycerol acyltransferase (DGAT II) within the microsomal lumen. Moreover, because luminal synthesis of TAG was carnitine-dependent and markedly reduced by glybenclamide, a potent carnitine acyltransferase inhibitor, microsomal carnitine acyltransferase appears to be essential for trafficking the [(14)C]oleoyl-CoA into the microsomal lumen for subsequent incorporation into newly synthesized TAG. This study thus provides the first direct demonstration of an enzymatic process leading to the synthesis of luminal triacylglycerol, which is a major component of very low density lipoproteins.

Recombinant acyl-CoA:cholesterol acyltransferase-1 (ACAT-1) purified to essential homogeneity utilizes cholesterol in mixed micelles or in vesicles in a highly cooperative manner

Acyl-coenzyme A:cholesterol acyltransferase (ACAT) is an integral membrane protein located in the endoplasmic reticulum. It catalyzes the formation of cholesteryl esters from cholesterol and long-chain fatty acyl coenzyme A. The first gene encoding the enzyme, designated as ACAT-1, was identified in 1993 through an expression cloning approach. We isolated a Chinese hamster ovary cell line that stably expresses the recombinant human ACAT-1 protein bearing an N-terminal hexahistidine tag. We purified this enzyme approximately 7000-fold from crude cell extracts by first solubilizing the cell membranes with the zwitterionic detergent 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate, then proceeding with an ACAT-1 monoclonal antibody affinity column and an immobilized metal affinity column. The final preparation is enzymologically active and migrates as a single band at 54 kDa on SDS-polyacrylamide gel electrophoresis. Pure ACAT-1 dispersed in mixed micelles containing sodium taurocholate, phosphatidylcholine, and cholesterol remains catalytically active. The cholesterol substrate saturation curves of the enzyme assayed either in mixed micelles or in reconstituted vesicles are both highly sigmoidal. The oleoyl-coenzyme A substrate saturation curves of the enzyme assayed under the same conditions are both hyperbolic. These results support the hypothesis that ACAT is an allosteric enzyme regulated by cholesterol.

Identification and quantitation of the fatty acids composing the CoA ester pool of bovine retina, heart, and liver

Several proteins found in retinal photoreceptor cells (guanylate cyclase activating protein, protein kinase A, recoverin, and transducin) are N-terminally modified with the fatty acids 12:0, 14:0, 14:1n-9, and 14:2n-6, whereas similar proteins in other tissues contain only 14:0. It has been hypothesized that the acyl-CoA pool of the retina contains amounts of 12:0, 14:1n-9, and 14:2n-6 elevated over 14:0, in comparison to other tissues, and this accounts for the specificity of N-terminal fatty acylation. To test this hypothesis, we performed fatty acid analysis on total acyl-CoAs purified from bovine retina (light-adapted), heart, and liver. We also examined the N- and S-linked fatty acid composition of the total protein pools from these tissues. Acyl-CoAs were prepared from heart, liver, and retina and separated by high performance liquid chromatography (HPLC). Identities of peaks were based on HPLC of standard 12:0, 14:0, 14:1n-9, and 14:2n-6 CoAs. Total protein was subjected to base hydrolysis followed by acidic methanolysis to release S- and N-linked fatty acids, respectively, and fatty acid phenacyl esters were prepared for HPLC analysis. Retina had levels of 12:0 (2.7 +/- 2.1%), 14:1n-9 (2.9 +/- 2.2%), and 14:2n-6 (1.6 +/- 0.7%) CoAs below that of 14:0 CoA (7.0 +/- 1.8%). Likewise, heart levels of 14:2n-6 CoA (3.7 +/- 0.1%) were near and 12:0 (2.6 +/- 0. 6%) and 14:1n-9 (0.7 +/- 0.3%) CoAs were below that of 14:0 CoA (3.8 +/- 1.0%). Liver had levels of 12:0 (16.1 +/- 5.7%) and 14:2n-6 (8.1 +/- 1.2%) CoAs above and 14:1n-9 CoA (1.2 +/- 0.6%) below that of 14:0 CoA (5.9 +/- 0.8%). Fatty acid analysis of total protein showed that all tissues contained S-linked 16:0, 18:0, and 18:1n-9. Retina proteins contained N-linked 14:0, 14:1n-9, and 14:2n-6, whereas heart and liver had only 14:0. Our findings do not support the hypothesis that the CoA ester pool of the retina is enriched with 12:0, 14:1n-9, and 14:2n-6 over 14:0, in comparison to other tissues. This suggests that alternative models must be considered for the regulation of N-terminal fatty acylation of proteins in photoreceptor cells.

A protein tyrosine kinase associated with the ATP-dependent inactivation of adipose diacylglycerol acyltransferase

The activity that has been previously reported to reversibly inactivate adipose glycerolphosphate acyltransferase (GPAT) and diacylglycerol acyltransferase (DGAT) in vitro in the presence of ATP is shown here to be partially purified from adipose tissue with an apparent molecular weight of 68 kDa. The activity responsible for inactivating DGAT is associated with a kinase activity as determined by phosphate incorporation both into microsomal proteins and into a synthetic tyrosine-containing peptide as substrate for protein tyrosine kinase. Two microsomal polypeptides of 53 and 69 kDa are major substrates of this kinase. Both DGAT inactivating and kinase activities assayed from the purified sample have been found to be insensitive to the Ser/Thr kinase inhibitor H-7 while being sensitive to genistein and tyrphostin-25. A crude protein phosphatase preparation from liver was capable of reversing the effects of both activities. The purified sample was also shown to inactivate GPAT in the presence of ATP. These results suggest that a protein tyrosine kinase, in concert with a protein tyrosine phosphatase, may regulate the activities of DGAT and GPAT by a phosphorylation-dephosphorylation mechanism.

Synthesis of sn-1,2-diacylglycerols by monoacylglycerol acyltransferase from Manduca sexta fat body

The pathway for the synthesis of sn-1,2-diacylglycerol stimulated by the action of adipokinetic hormone (AKH) in the insect fat body is unknown. Previous results from this laboratory suggested that the hydrolysis of stored triacylglycerol to sn-2-monoacylglycerol followed by the stereospecific acylation of sn-2-monoacylglycerol catalyzed by a monoacylglycerol-acyltransferase (MGAT) could be the major route of AKH-stimulated sn-1,2-diacylglycerol synthesis. Thus, MGAT might represent a key enzyme of this pathway. In this study we characterized the MGAT activity from the Manduca sexta fat body. The activity, which was assayed by acylation of 2-monoolein using radioactive labeled palmitoyl-CoA, was found to be primarily a microsomal enzyme. The products of the acylation of 2-monoolein were 1,2-diacylglycerol (40-50%), 1,3-diacylglycerol (20-30%), and triacylglycerol (30-40%). The presence of triacylglycerol as a product revealed the presence of diacylglycerol-acyltransferase activity in the fat body microsomes. The pH optimum of MGAT activity was 7.0, and the dependence of the activity on the concentration of 2-monoolein showed saturation kinetics. An endogenous MGAT activity, which represented 20% of the maximal activity observed with added substrate, was detected. Optimal concentrations of palmitoyl-CoA ranged between 0.10-0.20 mM. The specific activity of MGAT, measured under optimal conditions, was about 0.6 nmol DG formed/min-mg protein. MGAT activity was greatest with 2-monoolein, and lower activity was observed when a saturated 2-monoacylglycerol was employed. The activity observed with sn-1-monoacylglycerol was lower than that observed with sn-2-monoacylglycerol. AKH did not stimulate MGAT activity, suggesting that either the enzyme is not under hormonal regulation or the monoacylglycerol pathway is not involved in the AKH-stimulated production of sn-1,2-diacylglycerol in the M. sexta fat body.

Hypocholesterolemic activity of a novel inhibitor of cholesterol absorption, SCH 48461

The amount of cholesterol that circulates in the plasma as lipoproteins can be affected by the balance of cholesterol metabolism within and between the intestines and liver. In the present report, we describe a novel hypocholesterolemic agent and document its pharmacological effects in animal models of hypercholesterolemia. The oral administration of (3R,4S)-1,4-bis-(4-methoxyphenyl)-3-(3-phenylpropyl)-2-azetidinone (SCH 48461) reduced plasma cholesterol concentrations in cholesterol-fed hamsters, rats and rhesus monkeys with ED50s of 1, 2 and 0.2 mg/kg per day, respectively, SCH 48461 was also highly effective in reducing hepatic cholesteryl ester accumulation in cholesterol-fed hamsters and rats after 7 days of treatment. In one 3 week study, rhesus monkeys were fed a 0.25% cholesterol/22% saturated fat diet with or without SCH 48461. At the end of the 3 week period the control group's VLDL + LDL-cholesterol increased to 180 Mg/dl from a baseline of approximately 65 mg/dl while plasma apolipoprotein B levels had doubled. Animals treated daily with 1 mg/kg SCH 48461 maintained their baseline levels of VLDL + LDL-cholesterol, HDL-cholesterol, and plasma apolipoproteins B and A-I. After 3 weeks the diets of the two groups were switched. Within 1 week SCH 48461 (1 mg/kg per day) rapidly reversed the elevated VLDL + LDL-cholesterol levels of the previous control group to near baseline values. SCH 48461 exerted its hypocholesterolemic effect through the inhibition of cholesterol absorption. A dose of 10 mg/kg per day inhibited cholesterol absorption in cholesterol-fed hamsters by 68% while a similar reduction was achieved in chow-fed monkeys with 3 mg/kg per day. This latter dose inhibited cholesterol absorption in cholesterol-fed monkeys by 95%. Treatment of cholesterol-fed monkeys with 10 mg/kg per day SCH 48461 significantly increased fecal neutral sterol excretion (52 vs. 32 mg/kg) but had no effect on acidic sterol excretion. Using a 2-h absorption model in cholesterol-fed hamsters, SCH 48461 caused a 46% inhibition of unesterified [14C]cholesterol accumulation in the intestinal wall and a 90% inhibition of cholesteryl ester formation at a dose of 10 mg/kg. Similar data were observed when the plasma radioactivity was assessed, indicating inhibition of both free (61%) and esterified (85%) cholesterol appearance. In contrast, CI-976, a potent acyl-CoA:cholesterol acyltransferase (ACAT) inhibitor, did not affect the uptake of free cholesterol into the intestines while inhibiting cholesterol esterification (98% inhibition).(ABSTRACT TRUNCATED AT 400 WORDS)

Translocation of both lysosomal LDL-derived cholesterol and plasma membrane cholesterol to the endoplasmic reticulum for esterification may require common cellular factors involved in cholesterol egress from the acidic compartments (lysosomes/endosomes)

Using a stable cell line 25-RA derived from wild-type Chinese hamster ovary (CHO) cells as the parental cell, this laboratory previously reported the isolation and characterization of CHO cell mutants (cholesterol-trafficking or CT) defective in transporting LDL-derived cholesterol out of the acidic compartment(s) (lysosomes/endosomes) to the endoplasmic reticulum (ER) for esterification. In this report, we show that the CT mutation can be complemented by fusion with human cells; however, attempts to complement the CT defect through DNA transfection have resulted in a collection of stable cell lines designated as ST cells. Under cholesterol starvation condition, the ST cells exhibit an elevated rate of cholesterol ester biosynthesis (by 3- to 5-fold) compared to both the parental CHO cells and the CT cells. The phenotypes of the ST cells are stable. ST cells are thus new cell lines arisen from the CT cells. When the plasma membranes of the parental, CT, and ST cells are labelled with [3H]cholesterol, ST cells show rates of [3H]cholesterol esterification much higher than that observed in CT cells but lower than that observed in the parental CHO cells. This result shows that translocation of plasma membrane cholesterol to the ER for esterification is defective in the CT cells. This result also suggests that ST cells acquire increased cholesterol trafficking activity between the lysosome and the ER without mixing the plasma membrane cholesterol pool. The characteristics of CT cells and ST cells reported here suggest that translocation of both lysosomal LDL-derived cholesterol and plasma membrane cholesterol to the ER for esterification may require common cellular factors involved in cholesterol egress from the acidic compartment(s) (lysosomes/endosomes).

Unusual mechanism of hydrocarbon formation in the housefly: cytochrome P450 converts aldehyde to the sex pheromone component (Z)-9-tricosene and CO2

An unusual mechanism for hydrocarbon biosynthesis is proposed from work examining the formation of (Z)-9-tricosene (Z9-23:Hy), the major sex pheromone component of the female housefly, Musca domestica. Incubation of (Z)-15-[1-14C]- and (Z)-15-[15,16-3H2]tetracosenoic acid (24:1 fatty acid) with microsomes from houseflies gave equal amounts of [3H]Z9-23:Hy and 14CO2. The formation of CO2 and not CO, as reported for hydrocarbon formation in plants, animals, and microorganisms [Dennis, M. & Kolattukudy, P. E. (1992) Proc. Natl. Acad. Sci. USA 89, 5306-5310], was verified by trapping agents and by radio-GLC analysis. Incubation of (Z)-15-[15,16-3H2]tetracosenoyl-CoA with microsomal preparations in the presence of NADPH and O2 gave almost equal amounts of (Z)-15-3H2]tetrasosenal (24:1 aldehyde) and Z9-23:Hy. Addition of increasing amounts of hydroxylamine (aldehyde trapping agent) caused a decrease in hydrocarbon formation with a concomitant increase in oxime (aldehyde derivative) formation. The 24:1 aldehyde was efficiently converted to (Z)-9-tricosene only in the presence of both NADPH and O2. Bubbling carbon monoxide (20:80 CO/O2) or including an antibody against housefly cytochrome P450 reductase inhibited the formation Z9-23:Hy from 24:1 aldehyde. These data demonstrate an unusual mechanism for hydrocarbon formation in insects in which the acyl-CoA is reduced to the corresponding aldehyde and then carbon-1 is removed as CO2. The requirement for NADPH and O2 and the inhibition by CO and the antibody to cytochrome P450 reductase strongly implicate the participation of a cytochrome P450 in this reaction.

Demonstration of a direct effect on hepatic acyl CoA: cholesterol acyl transferase (ACAT) activity by an orally administered enzyme inhibitor in the hamster

Orally active inhibitors of acyl CoA:cholesterol acyl transferase (ACAT), such as Lederle CL277082 (LE), are known to reduce plasma and hepatic cholesteryl ester levels, although the mechanisms are not well understood. Several groups have reported the inhibition of cholesterol absorption upon oral ACAT inhibitor administration. In this study, we used 7-day dietary and drug treatments of hamsters to examine the possible effects of LE on hepatic ACAT. ACAT assays were performed using liver homogenates in the absence and presence of a saturating level of exogenously added cholesterol. LE (100 mg/kg/day) treatment of chow or 0.5% cholesterol-fed animals caused reductions in ACAT activity without additional cholesterol as compared with non-treated animals. When a saturating level of cholesterol was added to the assays, reductions in ACAT activity upon LE treatment of chow- or cholesterol-fed animals were also observed. Treatment of cholesterol-fed animals with cholestyramine in the diet reduced ACAT activity in the absence of added cholesterol. However, ACAT activities similar to those of non-treated animals were observed at a saturating level of cholesterol. This latter effect demonstrates that inhibition of cholesterol absorption reduces cholesterol delivery to the liver but does not reduce cholesterol esterifying capacity since cholestyramine is not absorbed and has no direct effect on the liver. The decreased ACAT activity in homogenates from LE-treated animals could also be mimicked in a dose-dependent manner by the addition of exogenous LE to liver homogenates from non-treated animals. These results indicate that hepatic ACAT activity is regulated by the availability of free cholesterol, and that orally administered LE has a direct effect on hepatic ACAT activity in the liver. In addition, the data are consistent with LE activity in the liver as being responsible, in part, for the reduced hepatic and plasma cholesteryl esters in treated animals.

Photolabeling of soybean microsomal membrane proteins with photoreactive acyl-CoA analogs

Synthesis of 32P-labeled 12-azidooleoyl-CoA and 125I-labeled 12-[(azidosalicyl)amino]dodecanoyl-CoA (ASD-CoA) was achieved. The synthesized radioactive, photoreactive reagents were tested as photoaffinity labels for acyl-CoA:lysophosphatidylcholine acyltransferase from the microsomal membranes of developing soybean cotyledons. When a mixture of microsomal membranes and the azidooleoyl-CoA or ASD-CoA were incubated in the dark, the analogs were recognized as substrate and competitive inhibitor, respectively. The enzyme preferentially utilizes unsaturated acyl-CoAs rather than saturated acyl-CoAs. Incubation of microsomal membranes with acyl-CoA analogs and immediately followed by photolysis resulted in an irreversible inhibition of lysophosphatidylcholine acyltransferase activity. Analysis of photolyzed microsomal membranes by SDS/PAGE and autoradiography revealed that azidooleoyl-CoA preferentially labeled eight acyl-CoA binding proteins, but ASD-CoA labeled only three polypeptides with molecular masses of 110, 90 and 32 kDa that are commonly labeled by both the analogs. Oleoyl-CoA and dodecanoyl-CoA protect the enzyme against photoinactivation by azidooleoyl-CoA and ASD-CoA, respectively. The protection was profound in 110-kDa polypeptide indicating that this protein could be lysophosphatidylcholine acyltransferase. These results demonstrate that the photoaffinity of acyl-CoA analogs makes them potential probes to identify and characterize lipid biosynthetic enzymes.

Photoaffinity labeling of acyl-CoA oxidase with 12-azidooleoyl-CoA and 12-[(4-azidosalicyl)amino]dodecanoyl-CoA

Synthesis of 32P-labeled CoA of high specific activity was achieved using partially purified dephospho-CoA kinase (EC 2.7.1.24) from pig liver with [gamma-32P]ATP as donor and dephospho-CoA as acceptor. A photoaffinity dodecanoic acid analog, 12-[(4-azidosalicyl)amino]dodecanoic acid was synthesized, as were its CoA derivative (ASD-CoA) and the CoA derivative of 12-azidooleic acid. The CoA derivatives were synthesized from azido fatty acid analogs by acyl-CoA synthetase. The synthesized photolabile reagents were tested as photoaffinity labels for acyl-CoA oxidase (EC 1.3.99.3) from Arthrobacter species. When a mixture of oxidase and the acyl-CoA analogs were incubated in the absence of ultraviolet light, the analogs were recognized as substrate. Acyl-CoA oxidase was incubated in the presence of acyl-CoA analogs and immediately photolyzed, which resulted in irreversible inhibition. Oleoyl-CoA and dodecanoyl-CoA protect the enzyme from photoactivated inhibition by 12-azidooleoyl-CoA and ASD-CoA, respectively. Analysis of photolyzed enzyme preparations by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and autoradiography revealed that both analogs preferentially labeled a 54,000 molecular weight protein. These results demonstrate that the photoaffinity acyl-CoA analogs have potential application as probes to identify and characterize lipid biosynthetic enzymes and to identify the active site of these proteins.

A high-performance liquid chromatography-based radiometric assay for acyl-CoA:alcohol transacylase from jojoba

Acyl-CoA:alcohol transacylase catalyzes the final step in the biosynthesis of storage liquid wax esters from acyl-CoA fatty acids and fatty alcohols in a limited number of microbes, algae, and Simmondsia chinensis Link (jojoba). An improved and automated method of enzyme assay for this catalyst from cotyledons of jojoba is described. The assay method uses reversed-phase C18 high performance liquid chromatography (HPLC) to separate the labeled C30:1 liquid wax product, [14C]-dodecanyl-octadecenoate, from the unreacted substrate, [14C]octadecenoyl-CoA (oleyl-CoA), and other components produced from enzymes present in the crude homogenate of jojoba cotyledons, including [14C]-octadecenoic acid (oleic acid) and [14C]octadecenol (oleyol). Methods are also described for microscale chemical synthesis in one vessel of 14C-radiolabeled substrates and products for the transacylase. These labeled reagents are required to confirm the HPLC separations of reaction products. The radioactive components are quantitated using an on-line flow-through scintillation detector enabling sensitive and precise analysis of the reaction products.

Interactions of acyl-coenzyme A with phosphatidylcholine bilayers and serum albumin

Interactions of oleoyl- and octanoyl-coenzyme A (CoA) with phosphatidylcholine (PC) vesicles and bovine serum albumin (BSA) were investigated by NMR spectroscopy. Binding of acyl-CoA to small unilamellar PC vesicles and to BSA was detected by changes in 13C and 31P chemical shifts relative to the chemical shifts for aqueous acyl-CoA. When oleoyl-CoA (less than or equal to 15 mol %) was added to preformed vesicles, the 13C thioester signal (200.1 ppm) was upfield from the signal for micellar oleoyl-CoA (201.7 ppm), suggesting decreased H-bonding (partial dehydration) at the carbonyl group upon binding to the bilayer. When vesicles were prepared by cosonication of oleoyl-CoA and PC, a second peak (199.8 ppm) was seen. The major peak at 200.1 ppm broadened and shifted after addition of Dy(NO3)3 and was not seen after addition of BSA, while the peak at 199.8 ppm was unaffected by either perturbation. Thus, oleoyl-CoA in each bilayer leaflet was distinguished, and transbilayer movement was shown to be slow (t 1/2 greater than or equal to hours). PC vesicles remained intact with less than or equal to 15 mol % oleoyl-CoA, while higher oleoyl-CoA proportions produced mixed micelles. In contrast, 13C spectra revealed rapid exchange (ms) of octanoyl-CoA between the aqueous phase and PC vesicles and a low affinity for the bilayer. Thus, the binding affinity of acyl-CoA for PC bilayers is dependent on the acyl chain length. Oleoyl-CoA in the presence of BSA (1 mol/mol) gave rise to three carbonyl signals at 197.2-203.6 ppm. With 2-5 mol of oleoyl-CoA/BSA, 1-2 additional signals were observed. None of the signals corresponded to unbound oleoyl-CoA. Addition of [13C]carboxyl-enriched oleic acid to oleoyl-CoA/BSA mixtures revealed simultaneous binding of oleic acid and oleoyl-CoA to BSA, with some perturbation of binding interactions. Thus, BSA contains multiple binding sites for oleoyl-CoA and can bind fatty acid and acyl-CoA simultaneously.

Reductive biotransformations of organic compounds by cells or enzymes of yeast

Saccharomyces cerevisiae catalyses the asymmetric reductive biotransformation of a variety of compounds containing a carbonyl group or carbon-carbon double bond. Oxidoreductases participating in these reactions which have commercial potential in biotransformation processes are likely to have relatively broad substrate specificity. Important carbonyl reductases falling into this category include YADH- and yeast NADP-dependent beta-ketoester reductases. The enoyl reductase component of the FAS complex may have a role in asymmetric yeast reduction of carbon-carbon double bonds of unnatural substrates. Other nicotinamide-requiring oxidoreductases of yeast are also surveyed to rationalize observed biotransformations of whole yeast cells in terms of specific enzymes. Genetic and protein engineering may enable enzymes to be tailored to accept new substrates. A greater understanding of the enzymes and reactions involved will facilitate further optimization and exploitation of these catalytic systems in industrial processes.

12-[(5-iodo-4-azido-2-hydroxybenzoyl)amino]dodecanoic acid: biological recognition by cholesterol esterase and acyl-CoA:cholesterol O-acyltransferase

Potential probes of protein cholesterol and fatty acid binding sites, namely, 12-[(5-iodo-4-azido-2-hydroxybenzoyl)amino]dodecanoate (IFA) and its coenzyme A (IFA:CoA) and cholesteryl (IFA:CEA) esters, were synthesized. These radioactive, photoreactive lipid analogues were recognized as substrates and inhibitors of acyl-CoA:cholesterol O-acyltransferase (ACAT) and cholesterol esterase, neutral lipid binding enzymes which are key elements in the regulation of cellular cholesterol metabolism. In the dark, IFA reversibly inhibited cholesteryl [14C]oleate hydrolysis by purified bovine pancreatic cholesterol esterase with an apparent Ki of 150 microM. Cholesterol esterase inhibition by IFA became irreversible after photolysis with UV light and oleic acid (1 mM) provided 50% protection against inactivation. Incubation of homogeneous bovine pancreatic cholesterol esterase with IFA:CEA resulted in its hydrolysis to IFA and cholesterol, indicating recognition of IFA:CEA as a substrate by cholesterol esterase. The coenzyme A ester, IFA:CoA, was a reversible inhibitor of microsomal ACAT activity under dark conditions (apparent Ki = 20 microM), and photolysis resulted in irreversible inhibition of enzyme activity with 87% efficiency. IFA:CoA was also recognized as a substrate by both liver and aortic microsomal ACATs, with resultant synthesis of 125IFA:CEA. IFA and its derivatives, IFA:CEA and IFA:CoA, are thus inhibitors and substrates for cholesterol esterase and ACAT. Biological recognition of these photoaffinity lipid analogues will facilitate the identification and structural analysis of hitherto uncharacterized protein lipid binding sites.

A simple enzymatic method for the preparation of radiolabeled erucoyl-CoA and other long-chain fatty acyl-CoAs and their characterization by mass spectrometry

A simple two-step method for the biosynthesis of radiolabeled erucoyl-coenzyme A of high specific activity and other long-chain fatty acyl-coenzyme A (acyl-CoA) thioesters is reported. 1-14C-labeled erucic and oleic acids, as well as unlabeled ricinoleic and nervonic acids, were incubated at 35 degrees C with coenzyme A in the presence of ATP, MgCl2, and acyl-CoA synthetase (EC 6.2.1.3) from Pseudomonas spp. to yield the corresponding CoA thioesters. Following incubation, each thioester was purified by rapid passage through a disposable reverse-phase C18 extraction column. The overall yields were greater than 90% and the purities greater than 95%, based on the distribution of radioactivity, and chromatographic and spectral properties. Fast ion bombardment-mass spectrometry was employed to confirm the structures of the various acyl-CoAs.

Isolation and characterization of Chinese hamster ovary cell mutants defective in intracellular low density lipoprotein-cholesterol trafficking

This paper reports the isolation and characterization of Chinese hamster ovary cell mutants defective in low density lipoprotein (LDL)-cholesterol trafficking. The parental cell line was 25-RA, which possesses LDL receptors and various cholesterogenic enzyme activities that are partially resistant to down regulation by exogenous sterols (Chang, T. Y., and J. S. Limanek. 1980. J. Biol. Chem. 255:7787-7795). Because these cells accumulate a large amount of intracellular cholesteryl ester when grown in medium containing 10% fetal calf serum, mutagenized populations of 25-RA cells were grown in the presence of a specific inhibitor of acyl-coenzyme A: cholesterol acyltransferase (ACAT), which depleted their cholesteryl ester stores. Without this cholesterol ester storage, 99% of 25-RA cells die after 5-d growth in cholesterol starvation medium, while the mutant cells, which accumulate free cholesterol intracellularly, survived. In two mutant clones chosen for characterization, activation of cholesteryl ester synthesis by LDL was markedly reduced in the mutant cells compared with 25-RA cells. This lack of activation of cholesterol ester synthesis in the mutant cells could not be explained by defective uptake and/or processing of LDL or by a decreased amount of ACAT, as determined by in vitro enzyme activity. Mutant cells grown in the presence of LDL contain numerous cytosolic particles that stain intensely with the fluorescent compound acridine orange, suggesting that they are acidic. The particles are also stained with filipin, a cholesterol-specific fluorescent dye. Indirect immunofluorescence with a monoclonal antibody specific for a lysosomal/endosomal fraction revealed a staining pattern that colocalized with the filipin signal. The mutant phenotype was recessive. The available evidence indicates that the mutant cells can take up and process LDL normally, but the hydrolyzed cholesterol accumulates in an acidic compartment, probably the lysosomes, where it can not be transported to its normal intracellular destinations.

Mechanism of biosynthesis of unsaturated fatty acids in Pseudomonas sp. strain E-3, a psychrotrophic bacterium

Biosynthesis of palmitic, palmitoleic, and cis-vaccenic acids in Pseudomonas sp. strain E-3 was investigated with in vitro and in vivo systems. [1-14C]palmitic acid was aerobically converted to palmitoleate and cis-vaccenate, and the radioactivities on their carboxyl carbons were 100 and 43%, respectively, of the total radioactivity in the fatty acids. Palmitoyl coenzyme A desaturase activity was found in the membrane fraction. [1-14C]stearic acid was converted to octadecenoate and C16 fatty acids. The octadecenoate contained oleate and cis-vaccenate, but only oleate was produced in the presence of cerulenin. [1-14C]lauric acid was aerobically converted to palmitate, palmitoleate, and cis-vaccenate. Under anaerobic conditions, palmitate (62%), palmitoleate (4%), and cis-vaccenate (34%) were produced from [1-14C]acetic acid, while they amounted to 48, 39, and 14%, respectively, under aerobic conditions. In these incorporation experiments, 3 to 19% of the added radioactivity was detected in released 14CO2, indicating that part of the added fatty acids were oxidatively decomposed. Partially purified fatty acid synthetase produced saturated and unsaturated fatty acids with chain lengths of C10 to C18. These results indicated that both aerobic and anaerobic mechanisms for the synthesis of unsaturated fatty acid are operating in this bacterium.

Isolation of Chinese hamster ovary cell lines expressing human acyl-coenzyme A/cholesterol acyltransferase activity

We have previously reported the isolation of Chinese hamster ovary cell mutants deficient in acylcoenzyme A/cholesterol acyltransferase (ACAT) activity (Cadigan, K. M., J. G. Heider, and T. Y. Chang. 1988, J. Biol. Chem. 263:274-282). We now describe a procedure for isolating cells from these mutants that have regained the ability to synthesize cholesterol esters. The protocol uses the fluorescent stain Nile red, which is specific for neutral lipids such as cholesterol ester. After ACAT mutant populations were subjected to chemical mutagenesis or transfected with human fibroblast whole genomic DNA, two revertants and one primary transformant were isolated by virtue of their higher fluorescent intensities using flow cytofluorimetry. Both the revertants and transformant have regained large amounts of intracellular cholesterol ester and ACAT activity. However, heat inactivation experiments revealed that the enzyme activity of the transformant had heat stability properties identical to that of human fibroblasts, while the ACAT activities of the revertants were similar to that of other Chinese hamster ovary cell lines. These results suggest that the molecular lesion in the ACAT mutants resides in the structural gene for the enzyme, and the transformant has corrected this defect by acquiring and stably expressing a human gene encoding the ACAT polypeptide. Secondary transformants were isolated by transfection of ACAT mutant cells with primary transformant genomic DNA. Genomic Southern analysis of the secondary transformants using a probe specific for human DNA revealed several distinct restriction fragments common to all the transformants which most likely comprise part or all of the human ACAT gene. The cell lines described here should facilitate the cloning of the gene encoding the human ACAT enzyme.

Chemical modification of acyl-CoA:cholesterol O-acyltransferase. 1. Identification of acyl-CoA:cholesterol O-acyltransferase subtypes by differential diethyl pyrocarbonate sensitivity

Acyl-CoA:cholesterol O-acyltransferase (EC 2.3.1.26) (ACAT) catalyzes the intracellular synthesis of cholesteryl esters from cholesterol and fatty acyl-CoA at neutral pH. Despite the probable pathophysiologic role of ACAT in vascular cholesteryl ester accumulation during atherogenesis, its mechanism of action and its regulation remain to be elucidated because the enzyme polypeptide has never been identified or purified. Present chemical modification results identify two distinct tissue types of ACAT, based on marked differences in reactivity of an active-site histidine residue toward diethyl pyrocarbonate (DEP) and acetic anhydride. The apparent Ki of the DEP-sensitive ACAT subtype, typified by aortic ACAT, was 40 microM, but the apparent Ki of the DEP-resistant ACAT subtype, typified by liver ACAT, was 1500 microM, indicating a 38-fold difference in sensitivity to DEP. Apparent Ki's of aortic and liver ACAT for inhibition by acetic anhydride were also discordant (less than 500 microM and greater than 5 mM, respectively). On the basis of the reversibility of inhibition by hydroxylamine, a neutral pKa for maximal modification, and acetic anhydride protection against DEP inactivation, DEP and acetic anhydride appear to modify a common histidine residue. Oleoyl-CoA provided partial protection against inactivation by DEP and acetic anhydride, suggesting that the modified histidine is at or near the active site of ACAT. Systematic investigation of ACAT activity from 14 different organs confirmed the existence of 2 subtypes of ACAT on the basis of their different reactivities toward DEP and acetic anhydride.(ABSTRACT TRUNCATED AT 250 WORDS)