Purification, characterization, DNA sequence and cloning of a pimeloyl-CoA synthetase from Pseudomonas mendocina 35

A pimeloyl-CoA synthetase from Pseudomonas mendocina 35 was purified and characterized, the DNA sequence determined, and the gene cloned into Escherichia coli to yield an active enzyme. The purified enzyme had a pH optimum of approximately 8.0, Km values of 0.49 mM for pimelic acid, 0.18 mM for CoA and 0.72 mM for ATP, a subunit Mr of approximately 80000 as determined by SDS/PAGE, and was found to be a tetramer by gel-filtration chromatography. The specific activity of the purified enzyme was 77.3 units/mg of protein. The enzyme was not absolutely specific for pimelic acid. The relative activity for adipic acid (C6) was 72% and for azaleic acid (C9) was 18% of that for pimelic acid (C7). The N-terminal amino acid was blocked to amino acid sequencing, but controlled proteolysis resulted in three peptide fragments for which amino acid sequences were obtained. An oligonucleotide gene probe corresponding to one of the amino acid sequences was synthesized and used to isolate the gene (pauA, pimelic acid-utilizing A) coding for pimeloyl-CoA synthetase. The pauA gene, which codes for a protein with a theoretical Mr of 74643, was then sequenced. The deduced amino acid sequence of the enzyme showed similarity to hypothetical proteins from Archaeoglobus fulgidus, Methanococcus jannaschii, Pyrococcus horikoshii, E. coli and Streptomyces coelicolor, and some limited similarity to microbial succinyl-CoA synthetases. The similarity with the protein from A. fulgidus was especially strong, thus indicating a function for this unidentified protein. The pauA gene was cloned into E. coli, where it was expressed and resulted in an active enzyme.

Development and initial evaluation of a novel method for assessing tissue-specific plasma free fatty acid utilization in vivo using (R)-2-bromopalmitate tracer

We describe a method for assessing tissue-specific plasma free fatty acid (FFA) utilization in vivo using a non-beta-oxidizable FFA analog, [9,10-3H]-(R)-2-bromopalmitate (3H-R-BrP). Ideally 3H-R-BrP would be transported in plasma, taken up by tissues and activated by the enzyme acyl-CoA synthetase (ACS) like native FFA, but then 3H-labeled metabolites would be trapped. In vitro we found that 2-bromopalmitate and palmitate compete equivalently for the same ligand binding sites on albumin and intestinal fatty acid binding protein, and activation by ACS was stereoselective for the R-isomer. In vivo, oxidative and non-oxidative FFA metabolism was assessed in anesthetized Wistar rats by infusing, over 4 min, a mixture of 3H-R-BrP and [U-14C] palmitate (14C-palmitate). Indices of total FFA utilization (R*f) and incorporation into storage products (Rfs') were defined, based on tissue concentrations of 3H and 14C, respectively, 16 min after the start of tracer infusion. R*f, but not Rfs', was substantially increased in contracting (sciatic nerve stimulated) hindlimb muscles compared with contralateral non-contracting muscles. The contraction-induced increases in R*f were completely prevented by blockade of beta-oxidation with etomoxir. These results verify that 3H-R-BrP traces local total FFA utilization, including oxidative and non-oxidative metabolism. Separate estimates of the rates of loss of 3H activity indicated effective 3H metabolite retention in most tissues over a 16-min period, but appeared less effective in liver and heart. In conclusion, simultaneous use of 3H-R-BrP and [14C]palmitate tracers provides a new useful tool for in vivo studies of tissue-specific FFA transport, utilization and metabolic fate, especially in skeletal muscle and adipose tissue.

Effect of chlorophenoxyacetic acid herbicides on glycine conjugation of benzoic acid

1. 2,4-Dichlorophenoxyacetic acid (2,4-D) and 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) (0.1-0.5 mmol/kg i.p.) delayed the disappearance of injected benzoate from blood and diminished the urinary excretion of the formed benzoylglycine, but elevated the blood levels of benzoylglycine in rat, suggesting that these herbicides interfere with both the formation and the renal transport of benzoylglycine. 2. Inhibition of the renal excretion of benzoylglycine by 2,4-D or 2,4,5-T (0.5 mmol/kg i.p.) was directly demonstrated in rat injected with benzoylglycine. 3. Inhibition of benzoylglycine formation from benzoic acid by 2,4-D or 2,4,5-T (0.5 mmol/kg i.p.) was directly demonstrated in renal pedicles-ligated rats injected with benzoate. 4. Neither 2,4-D nor 2,4,5-T influenced the hepatic concentrations of ATP, coenzyme A (CoA) or glycine; therefore, it is unlikely that they inhibit glycine conjugation of benzoic acid by diminishing the availability of co-substrates. 5. Although the chlorophenoxyacetic acids did not appear to be a substrate for the mitochondrial acyl-CoA synthetases, both 2,4-D and 2,4,5-T diminished the activity of benzoyl-CoA synthetase (but not that of benzoyl-CoA:glycine N-acyltransferase) in solubilized hepatic mitochondria. These findings suggest that 2,4-D and 2,4,5-T impair benzoylglycine formation in rat by inhibiting benzoyl-CoA synthetase.

Heterogeneous long chain acyl-CoA synthetases control distribution of individual fatty acids in newly-formed glycerolipids of neuronal cells undergoing neurite outgrowth

Using PC12 cells undergoing neurite outgrowth, we studied the activation of various fatty acids, of different chain lengths and degrees of saturation, by long chain acyl-CoA synthetases (LCASs). Cells treated with nerve growth factor (NGF) were labeled with [3H]glycerol, [3H]oleic acid (OA) or [3H]arachidonic acid (AA) in the presence of other unlabeled fatty acids of endogenous or exogenous origin. Triacsin C (4.8 microM), an inhibitor of acyl-CoA synthetase, decreased the incorporation of exogenous [3H]OA into glycerolipids by 30-90%, and increased by about 60% the accumulation of free [3H]OA in the cells. However it did not affect the incorporation of endogenous fatty acids nor of exogenous [3H]AA into phospholipids, suggesting that LCASs which activate exogenous AA and at least some endogenous fatty acids are relatively insensitive to this drug. Activities of the LCAS that is specific for AA (ACS), or of the non-specific LCAS which activates OA and other fatty acids (OCS), were much higher in microsomal and cytoplasmic fractions than in mitochondria or nuclei. The Vmax and Km values of ACS and OCS in microsomes were 12 and 0.7 nmol/min/mg protein and 70 and 37 microM, respectively; and in cytoplasm, 6 and 0.6 nmol/min/mg protein and 38 and 60 microM, respectively. Triacsin C (2-33 microM) did not affect ACS activity in microsomal or cytoplasmal fractions, but inhibited OCS activities dose-dependently and competitively: IC50 and apparent Ki values were 13.5 microM and 14 microM in microsomes, and 3.8 microM and 4 microM in cytoplasm. NGF stimulated the activities of the LCASs, and, consistently, the incorporation of the various fatty acids into glycerolipids. These data indicate that LCASs are heterogeneous with respect to their intracellular locations, substrate specificities, kinetic characteristics and sensitivities to triacsin C; and that this heterogeneity affects the extents to which individual fatty acids are utilized to form glycerolipids.

Cresol metabolism by the sulfate-reducing bacterium Desulfotomaculum sp. strain Groll

The metabolism of cresols under sulfate-reducing conditions was investigated in Desulfotomaculum sp. strain Groll. This strain grows on a variety of aromatic compounds, including para- and meta- but not ortho-cresol. Degradation of p-cresol proceeded by oxidation reactions of the methyl group to yield p-hydroxybenzoate, which was then dehydroxylated to benzoate. The aromatic intermediates expected for this pathway, p-hydroxybenzyl alcohol, p-hydroxybenzaldehyde, p-hydroxybenzoate, and benzoate, were readily metabolized by strain Groll. Utilization of these intermediates generally preceded and inhibited the degradation of p-cresol. p-Hydroxybenzoate and benzoate were detected in culture fluid as metabolites of p-cresol. p-Hydroxybenzaldehyde and p-hydroxybenzoate were detected in cultures degrading p-hydroxybenzyl alcohol. Enzyme activities responsible for utilization of p- and m-cresol, induced by growth on the respective cresol, were detected in cell-free extracts of strain Groll. The compounds detected in culture fluids and the enzyme activities detected in cell-free extracts indicate that the pathways for the degradation of p- and m-cresol converge on benzoate, followed by metabolism to benzoyl-coenzyme A (CoA). Strain Groll can utilize both cresol isomers under sulfate-reducing conditions by similar reactions, but the enzyme activities catalyzing these transformations of the two isomers appear distinct.

The prpE gene of Salmonella typhimurium LT2 encodes propionyl-CoA synthetase

Biochemical and genetic evidence is presented to demonstrate that the prpE gene of Salmonella typhimurium encodes propionyl-CoA synthetase, an enzyme required for the catabolism of propionate in this bacterium. While prpE mutants used propionate as carbon and energy source, prpE mutants that lacked acetyl-CoA synthetase (encoded by acs) did not, indicating that Acs can compensate for the lack of PrpE in prpE mutants. Cell-free extracts enriched for PrpE catalysed the formation of propionyl-CoA in a propionate-, ATP-, Mg2+- and HS-CoA dependent manner. Acetate substituted for propionate in the reaction at 48% the rate of propionate; butyrate was not a substrate for PrpE. The propionyl-CoA synthetase activity of PrpE was specific for ATP. GTP, ITP, CTP and TTP were not used as substrates by the enzyme. UV-visible spectrophotometry, HPLC and MS data demonstrated that propionyl-CoA was the product of the reaction catalysed by PrpE.

Human very long-chain acyl-CoA synthetase and two human homologs: initial characterization and relationship to fatty acid transport protein

Several human genes with a high degree of homology to rat very long-chain acyl-CoA synthetase (rVLCS) and mouse fatty acid transport protein (mFATP) were identified. Full-length cDNA clones were obtained for three genes, and predicted amino acid sequences were generated. Initial characterization indicated that one gene was most likely hVLCS, the human ortholog of rVLCS. The other two (hVLCS-H1 and hVLCS-H2) were more closely related to rVLCS than to mFATP. Phylogenetic analysis of amino acid sequences confirmed that hVLCS-H1 and hVLCS-H2 were evolutionarily closer to VLCSs than FATPs. Alignment of predicted amino acid sequences of human, rat and mouse VLCSs and FATPs revealed the existence of two highly conserved motifs. While one motif is also present in long-chain acyl-CoA synthetases, the other serves to distinguish the VLCS/FATP family from the long-chain synthetase family. Elucidation of the biochemical functions of all VLCS/FATP family members should provide new insights into cellular fatty acid metabolism.

Localization of adipocyte long-chain fatty acyl-CoA synthetase at the plasma membrane

Long-chain fatty acyl-CoA synthetase (FACS) catalyzes esterification of long-chain fatty acids (LCFAs) with coenzyme A (CoA), the first step in fatty acid metabolism. FACS has been shown to play a role in LCFA import into bacteria and implicated to function in mammalian cell LCFA import. In the present study, we demonstrate that FACS overexpression in fibroblasts increases LCFA uptake, and overexpression of both FACS and the fatty acid transport protein (FATP) have synergistic effects on LCFA uptake. To explore how FACS contributes to LCFA import, we examined the subcellular location of this enzyme in 3T3-L1 adipocytes which natively express this protein and which efficiently take up LCFAs. We demonstrate for the first time that FACS is an integral membrane protein. Subcellular fractionation of adipocytes by differential density centrifugation reveals immunoreactive and enzymatically active FACS in several membrane fractions, including the plasma membrane. Immunofluorescence studies on adipocyte plasma membrane lawns confirm that FACS resides at the plasma membrane of adipocytes, where it co-distributes with FATP. Taken together, our data support a model in which imported LCFAs are immediately esterified at the plasma membrane upon uptake, and in which FATP and FACS function coordinately to facilitate LCFA movement across the plasma membrane of mammalian cells.

Human very-long-chain acyl-CoA synthetase: cloning, topography, and relevance to branched-chain fatty acid metabolism

Very-long-chain acyl-CoA synthetases (VLCS) activate very-long-chain fatty acids (VLCFA) containing 22 or more carbons to their CoA derivatives. We cloned the human ortholog (hVLCS) of the gene encoding the rat liver enzyme (rVLCS). Both hVLCS and rVLCS contain 620 amino acids, are expressed primarily in liver and kidney, and have a potential peroxisome targeting signal 1 (-LKL) at their carboxy termini. When expressed in COS-1 cells, hVLCS activated the VLCFA lignoceric acid (C24:0), a long-chain fatty acid (C16:0), and two branched-chain fatty acids, phytanic acid and pristanic acid. Immunofluorescence and immunoblot studies localized hVLCS to both peroxisomes and endoplasmic reticulum. In peroxisomes of HepG2 cells, hVLCS was topographically oriented facing the matrix and not the cytoplasm. This orientation, coupled with the observation that hVLCS activates branched-chain fatty acids, suggests that hVLCS could play a role in the intraperoxisomal reactivation of pristanic acid produced via alpha-oxidation of phytanic acid.

Increased peroxisomal fatty acid beta-oxidation and enhanced expression of peroxisome proliferator-activated receptor-alpha in diabetic rat liver

To determine whether the increased fatty acid beta-oxidation in the peroxisomes of diabetic rat liver is mediated by a common peroxisome proliferation mechanism, we measured the activation of long-chain (LC) and very long chain (VLC) fatty acids catalyzed by palmitoyl CoA ligase (PAL) and lignoceryl CoA ligase and oxidation of LC (palmitic acid) and VLC (lignoceric acid) fatty acids by isotopic methods. Immunoblot analysis of acyl-CoA oxidase (ACO), and Northern blot analysis of peroxisome proliferator-activated receptor (PPAR-alpha), ACO, and PAL were also performed. The PAL activity increased in peroxisomes and mitochondria from the liver of diabetic rats by 2.6-fold and 2.1 -fold, respectively. The lignoceroyl-CoA ligase activity increased by 2.6-fold in diabetic peroxisomes. Palmitic acid oxidation increased in the diabetic peroxisomes and mitochondria by 2.5-fold and 2.7-fold, respectively, while lignoceric acid oxidation increased by 2.0-fold in the peroxisomes. Immunoreactive ACO protein increased by 2-fold in the diabetic group. The mRNA levels for PPAR-alpha, ACO and PAL increased 2.9-, 2.8- and 1.6-fold, respectively, in the diabetic group. These results suggest that the increased supply of fatty acids to liver in diabetic state stimulates the expression of PPAR-alpha and its target genes responsible for the metabolism of fatty acids.

Acyl-CoA synthetase activity in liver microsomes from calcium-deficient rats

A study on the kinetic properties of the nonspecific acyl-coenzyme A (CoA) synthetase activity in liver microsomal vesicles from both normal and calcium-deficient Wistar rats was carried out. After a 65-d treatment, the calcium-deficient diet reflected a 75% increase in the synthetase activity with respect to control animals. The apparent Vm was significantly enhanced, while the Km remained unchanged. We also provided experimental evidence about various fatty acids of different carbon length and unsaturation which depressed the biosynthesis of palmitoyl-CoA following different behaviors in control or calcium-deprived liver microsomes. In addition, we studied in detail the inhibition reflected by stearic, alpha-linolenic, or arachidonic acids, in the biosynthesis of palmitoyl-CoA in microsomal suspensions either from control or hypocalcemic rats. In control microsomes, stearic acid produced a pure competitive effect, while the other fatty acids followed a mixed-type inhibition. The competitive effect of stearic acid was not observed in calcium-deprived microsomes. At the same time, a mixed-type inhibition produced by either alpha-linolenic or arachidonic acid was diminished in deprived microsomes due to an increase in the noncompetitive component (alphaKi). These changes observed in apparent kinetic constants (Km, Vm, Ki, and alphaKi), as determined by Lineweaver-Burks and Dixon plots, were attributed to the important alterations in the physicochemical properties of the endoplasmic reticulum membranes induced by the calcium-deficient diet. The solubilization of the enzyme activity from both types of microsomes demonstrated that the kinetic behavior of the enzyme depends on the microenvironment in the membrane, and that the calcium ion plays a crucial role in determining the alterations observed.

Acyl-CoA synthetase catalyzes the synthesis of diadenosine hexaphosphate (Ap6A)

The synthesis of diadenosine hexaphosphate (Ap6A), a potent vasoconstrictor, is catalyzed by acyl-CoA synthetase from Pseudomonas fragi. In a first step AMP is transferred from ATP to tetrapolyphosphate (P4) originating adenosine pentaphosphate (p5A) which, subsequently, is the acceptor of another AMP moiety from ATP generating diadenosine hexaphosphate (Ap6A). Diadenosine pentaphosphate (Ap5A) and diadenosine tetraphosphate (Ap4A) were also synthesized in the course of the reaction. In view of the variety of biological effects described for these compounds the potential capacity of synthesis of diadenosine polyphosphates by the mammalian acyl-CoA synthetases may be relevant.

Epidermal growth factor regulates fatty acid uptake and metabolism in Caco-2 cells

Epidermal growth factor (EGF) has been reported to stimulate carbohydrate, amino acid, and electrolyte transport in the small intestine, but its effects on lipid transport are poorly documented. This study aimed to investigate EGF effects on fatty acid uptake and esterification in a human enterocyte cell line (Caco-2). EGF inhibited cell uptake of [14C]palmitate and markedly reduced its incorporation into triglycerides. In contrast, the incorporation in phospholipids was enhanced. To elucidate the mechanisms involved, key steps of lipid synthesis were investigated. The amount of intestinal fatty acid-binding protein (I-FABP), which is thought to be important for fatty acid absorption, and the activity of diacylglycerol acyltransferase (DGAT), an enzyme at the branch point of diacylglycerol utilization, were reduced. EGF effects on DGAT and on palmitate esterification occurred at 2-10 ng/ml, whereas effects on I-FABP and palmitate uptake occurred only at 10 ng/ml. This suggests that EGF inhibited palmitate uptake by reducing the I-FABP level and shifted its utilization from triglycerides to phospholipids by inhibiting DGAT. This increase in phospholipid synthesis might play a role in the restoration of enterocyte absorption function after intestinal mucosa injury.

The Saccharomyces cerevisiae FAT1 gene encodes an acyl-CoA synthetase that is required for maintenance of very long chain fatty acid levels

The Saccharomyces cerevisiae FAT1 gene appears to encode an acyl-CoA synthetase that is involved in the regulation of very long chain (C20-C26) fatty acids. Fat1p, has homology to a rat peroxisomal very long chain fatty acyl-CoA synthetase. Very long chain acyl-CoA synthetase activity is reduced in strains containing a disrupted FAT1 gene and is increased when FAT1 is expressed in insect cells under control of a baculovirus promoter. Fat1p accounts for approximately 90% of the C24-specific acyl-CoA synthetase activity in glucose-grown cells and approximately 66% of the total activity in cells grown under peroxisomal induction conditions. Localization of functional Fat1p:green fluorescent protein gene fusions and subcellular fractionation of C24 acyl-CoA synthetase activities indicate that the majority of Fat1p is located in internal cellular locations. Disruption of the FAT1 gene results in the accumulation of very long chain fatty acids in the sphingolipid and phospholipid fractions. This includes a 10-fold increase in C24 acids and a 6-fold increase in C22 acids. These abnormal accumulations are further increased by perturbation of very long chain fatty acid synthesis. Overexpression of Elo2p, a component of the fatty acid elongation system, in fat1Delta cells causes C20-C26 levels to rise to approximately 20% of the total fatty acids. These data suggest that Fat1p is involved in the maintenance of cellular very long chain fatty acid levels, apparently by facilitating beta-oxidation of excess intermediate length (C20-C24) species. Although fat1Delta cells were reported to grow poorly in oleic acid-supplemented medium when fatty acid synthase activity is inactivated by cerulenin, fatty acid import is not significantly affected in cells containing disrupted alleles of FAT1 and FAS2 (a subunit of fatty acid synthase). These results suggest that the primary cause of the growth-defective phenotype is a failure to metabolize the incorporated fatty acid rather than a defect in fatty acid transport. Certain fatty acyl-CoA synthetase activities, however, do appear to be essential for bulk fatty acid transport in Saccharomyces. Simultaneous disruption of FAA1 and FAA4, which encode long chain (C14-C18) fatty acyl-CoA synthetases, effectively blocks the import of long chain saturated and unsaturated fatty acids.

Adrenoleukodystrophy protein enhances association of very long-chain acyl-coenzyme A synthetase with the peroxisome

OBJECTIVE

To clarify the function of adrenoleukodystrophy protein (ALDP) using our ALDP-deficient mice established by gene targeting.

BACKGROUND

X-linked adrenoleukodystrophy (ALD) is characterized biochemically by the accumulation of very long-chain fatty acids (VLCFA) in tissues and body fluids, and is caused by impairment of peroxisomal beta-oxidation. In ALD, very long-chain acyl-coenzyme A synthetase (VLACS), which is necessary for peroxisomal beta-oxidation, does not function.

METHODS

The ALDP-deficient mice and C57BL/6J mice were used. VLACS or ALDP were transiently expressed by lipofection in murine fibroblasts, and VLCFA beta-oxidation was assayed. Liver peroxisomes were purified by sequential centrifugations and a Nycodenz gradient centrifugation. The peroxisomal localization of VLACS was compared between the mutant and control mice using a Western blot analysis.

RESULTS

Impairment of VLCFA beta-oxidation in ALDP-deficient fibroblasts was not corrected by the additional expression of VLACS alone but was by the coexpression of VLACS and ALDP. Although the tissue-specific expression of VLACS was similar in ALDP-deficient and normal mice, peroxisomal VLACS was clearly lower in ALDP-deficient than in normal mice.

CONCLUSIONS

ALDP plays a role in the peroxisomal localization of VLACS, and VLACS does not function unless localized in the peroxisome.

Regulation of putative fatty acid transporters and Acyl-CoA synthetase in liver and adipose tissue in ob/ob mice

The hyperlipidemia associated with obesity and type 2 diabetes is caused by an increase in hepatic triglyceride synthesis and secretion that is secondary to an increase in de novo lipogenesis, a decrease in fatty acid (FA) oxidation, and an increase in the flux of peripherally derived FA to the liver. The uptake of FA across the plasma membrane may be mediated by three distinct proteins--FA translocase (FAT), plasma membrane FA binding protein (FABP-pm), and FA transport protein (FATP)--that have recently been characterized. Acyl-CoA synthetase (ACS) enhances the uptake of FAs by catalyzing their activation to acyl-CoA esters for subsequent use in anabolic or catabolic pathways. In this study, we examine the mRNA levels of FAT, FABP-pm, FATP, and ACS in the liver and adipose tissue of genetically obese (ob/ob) mice and their control littermates. FAT mRNA levels were 15-fold higher in liver and 60-80% higher in adipose tissue of ob/ob mice. FABP-pm mRNA levels were twofold higher in liver and 50% higher in adipose tissue of ob/ob mice. FATP mRNA levels were not increased in liver or adipose tissue. ACS mRNA levels were higher in adipose tissue but remained unchanged in liver. However, the distribution of ACS activity associated with mitochondria and microsomes in liver was altered in ob/ob mice. In control littermates, 61% of ACS activity was associated with mitochondria and 39% with microsomes, whereas in ob/ob mice 34% of ACS activity was associated with mitochondria and 66% with microsomes; this distribution would make more FA available for esterification, rather than oxidation, in ob/ob mouse liver. Taken together, our results suggest that the upregulation of FAT and FABP-pm mRNAs may increase the uptake of FA in adipose tissue and liver in ob/ob mice, which, coupled with an increase in microsomal ACS activity in liver, will enhance the esterification of FA and support the increased triglyceride synthesis and VLDL production that characterizes obesity and type 2 diabetes.

The modified beta-ketoadipate pathway in Rhodococcus rhodochrous N75: enzymology of 3-methylmuconolactone metabolism

Rhodococcus rhodochrous N75 is able to metabolize 4-methylcatechol via a modified beta-ketoadipate pathway. This organism has been shown to activate 3-methylmuconolactone by the addition of coenzyme A (CoA) prior to hydrolysis of the butenolide ring. A lactone-CoA synthetase is induced by growth of R. rhodochrous N75 on p-toluate as a sole source of carbon. The enzyme has been purified 221-fold by ammonium sulfate fractionation, hydrophobic chromatography, gel filtration, and anion-exchange chromatography. The enzyme, termed 3-methylmuconolactone-CoA synthetase, has a pH optimum of 8.0, a native Mr of 128,000, and a subunit Mr of 62,000, suggesting that the enzyme is homodimeric. The enzyme is very specific for its 3-methylmuconolactone substrate and displays little or no activity with other monoene and diene lactone analogues. Equimolar amounts of these lactone analogues brought about less than 30% (most brought about less than 15%) inhibition of the CoA synthetase reaction with its natural substrate.

New insights in peroxisomal beta-oxidation. Implications for human peroxisomal disorders

In mammals including man, peroxisomes play a pivotal role in the breakdown of various carboxylates via beta-oxidation. Physiological substrates include very long chain fatty acids (e.g. lignoceric acid), medium and long chain dicarboxylic acids, certain polyunsaturated fatty acids, 2-methylbranched isoprenoid-derived fatty acids (e.g. pristanic acid), prostanoids (prostaglandins, leukotrienes thromboxanes), and bile acid intermediates (di- and trihydroxycoprostanic acid). Substrate spectrum and specificity studies of the four different beta-oxidation steps in rat and man indicate that these carboxylates, in contrast to previous belief, are degraded by separate systems composed of different enzymes. Bile acid intermediates are degraded in hepatic peroxisomes via 2-methylacyl-CoA racemase, trihydroxycoprostanoyl-CoA oxidase (in rat) or branched acyl-CoA oxidase (in man), D-specific multifunctional protein 2 (MFP 2) and sterol carrier protein X/thiolase. beta-oxidation of pristanic acid can occur in all tissues and relies on the action of 2-methylacyl-CoA racemase (for the 2R-isomer), pristanoyl-CoA oxidase (in rat) or branched chain acyl-CoA oxidase (in man), D-specific multifunctional protein 2 (MFP 2) and sterol carrier protein X/thiolase. The enzymes catalyzing the breakdown of straight chain fatty acids are palmitoyl-CoA oxidase, L-specific multifunctional protein 1 (MFP 1) and the dimeric thiolase. These enzymes are present in all tissues and are identical to those initially characterized in hepatic peroxisomes. Due to the presence of peroxisome targeting signals in all the above mentioned proteins, they are localised in the cytosolic or absent (due to proteolysis) in tissues of patients with a generalized peroxisome deficiency (e.g. Zellweger syndrome). In addition to these lethal inherited disorders that are caused by defects in the biogenesis of peroxisomes, a growing number of patients with peroxisomal beta-oxidation deficiencies have been described. The implications of the presence of separate beta-oxidation systems for the latter disorders is quite profound and calls, in many cases, for a reevaluation of the diagnosis of such patients.

Cholesterol-lowering effects of NTE-122, a novel acyl-CoA:cholesterol acyltransferase (ACAT) inhibitor, on cholesterol diet-fed rats and rabbits

Pharmacological characterization of NTE-122 (trans-1,4-bis[[1-cyclohexyl-3-(4-dimethylamino phenyl)ureido]methyl]cyclohexane), a novel acyl-CoA:cholesterol acyltransferase (ACAT) inhibitor, was performed with both in vitro and in vivo assay systems. NTE-122 inhibited microsomal ACAT activities of various tissues (liver of rabbit and rat, small intestine of rabbit and rat, and aorta of rabbit) and cultured cells (HepG2 and CaCo-2), with IC50 values from 1.2 to 9.6 nM. The inhibition mode of NTE-122 was competitive for HepG2 ACAT. NTE-122 had no effect on other lipid metabolizing enzymes, such as 3-hydroxy-3-methylglutaryl-CoA reductase, acyl-CoA synthetase, cholesterol esterase, lecithin:cholesterol acyltransferase, acyl-CoA:sn-glycerol-3-phosphate acyltransferase and cholesterol 7alpha-hydroxylase up to 10 microM. When NTE-122 was administered to the cholesterol diet-fed rats, serum and liver cholesterol levels were markedly reduced with an ED50 of 0.12 and 0.44 mg/kg/day, respectively. In the cholesterol diet-fed rabbits, NTE-122 significantly lowered plasma and liver cholesterol levels at more than 2 mg/kg/day. These results indicate that NTE-122 is a potent, selective and competitive inhibitor of ACAT, making it a worth while therapeutic agent for hypercholesterolemia and atherosclerosis.

Localization of nervonic acid beta-oxidation in human and rodent peroxisomes: impaired oxidation in Zellweger syndrome and X-linked adrenoleukodystrophy

Studies with purified subcellular organelles from rat liver indicate that nervonic acid (C24:1) is beta-oxidized preferentially in peroxisomes. Lack of effect by etomoxir, inhibitor of mitochondrial beta-oxidation, on beta-oxidation of lignoceric acid (C24:0), a peroxisomal function, and that of nervonic acid (24:1) compared to the inhibition of palmitic acid (16:0) oxidation, a mitochondrial function, supports the conclusion that nervonic acid is oxidized in peroxisomes. Moreover, the oxidation of nervonic and lignoceric acids was deficient in fibroblasts from patients with defects in peroxisomal beta-oxidation [Zellweger syndrome (ZS) and X-linked adrenoleukodystrophy (X-ALD)]. Similar to lignoceric acid, the activation and beta-oxidation of nervonic acid was deficient in peroxisomes isolated from X-ALD fibroblasts. Transfection of X-ALD fibroblasts with human cDNA encoding for ALDP (X-ALD gene product) restored the oxidation of both nervonic and lignoceric acids, demonstrating that the same molecular defect may be responsible for the abnormality in the oxidation of nervonic as well as lignoceric acid. Moreover, immunoprecipitation of activities for acyl-CoA ligase for both lignoceric acid and nervonic acid indicate that saturated and monoenoic very long chain (VLC) fatty acids may be activated by the same enzyme. These results clearly demonstrate that similar to saturated VLC fatty acids (e.g., lignoceric acid), VLC monounsaturated fatty acids (e.g., nervonic acid) are oxidized preferentially in peroxisomes and that this activity is impaired in X-ALD. In view of the fact that the oxidation of unsaturated VLC fatty acids is defective in X-ALD patients, the efficacy of dietary monoene therapy, "Lorenzo's oil," in X-ALD needs to be evaluated.

Role of hepatic fatty acid:coenzyme A ligases in the metabolism of xenobiotic carboxylic acids

1. Formation of acyl-coenzymes (Co)A occurs as an obligatory step in the metabolism of a variety of endogenous substrates, including fatty acids. The reaction is catalysed by ATP-dependent acid:CoA ligases (EC 6.2.1.1-2.1.3; AMP forming), classified on the basis of their ability to conjugate saturated fatty acids of differing chain lengths, short (C2-C4), medium (C4-C12) and long (C10-C22). The enzymes are located in various cell compartments (cytosol, smooth endoplasmic reticulum, mitochondria and peroxisomes) and exhibit wide tissue distribution, with highest activity associated with liver and adipose tissue. 2. Formation of acyl-CoA is not unique to endogenous substrates, but also occurs as an obligatory step in the metabolism of some xenobiotic carboxylic acids. The mitochondrial medium-chain CoA ligase is principally associated with metabolism via amino acid conjugation and activates substrates such as benzoic and salicylic acids. Although amino acid conjugation was previously considered an a priori route of metabolism for xenobiotic-CoA, it is now recognized that these highly reactive and potentially toxic intermediates function as alternative substrates in pathways of intermediary metabolism, particularly those associated with lipid biosyntheses. 3. In addition to a role in fatty acid metabolism, the hepatic microsomal and peroxisomal long-chain-CoA-ligases have been implicated in the formation of the acyl-CoA thioesters of a variety of hypolipidaemic and peroxisome proliferating agents (e.g. clofibric acid) and of the R(-)-enantiomers of the commonly used 2-arylpropionic acid non-steroidal anti-inflammatory drugs (e.g. ibuprofen). In vitro kinetic studies using rat hepatic microsomes and peroxisomes have alluded to the possibility of xenobiotic-CoA ligase multiplicity. Although cDNA encoding a long-chain ligase have been isolated from rat and human liver, there is currently no molecular evidence of multiple isoforms. The gene has been localized to chromosome 4 and homology searches have revealed a significant similarity with enzymes of the luciferase family. 4. Increasing recognition that formation of a CoA conjugate increases chemical reactivity of xenobiotic carboxylic acids has led to an awareness that the relative activity, substrate specificity and intracellular location of the xenobiotic-CoA ligases may explain differences in toxicity. 5. Continued characterization of the human xenobiotic-CoA ligases in terms of substrate/inhibitor profiles and regulation, will allow a greater understanding of the role of these enzymes in the metabolism of carboxylic acids.

A semiautomated enzymatic method for determination of nonesterified fatty acid concentration in milk and plasma

An enzymatic assay for the determination of nonesterified fatty acid concentrations in milk and plasma is described. The procedure is semiautomated for use with a plate luminometer or plate spectrophotometer and enables routine batch processing of large numbers of small samples (< or =5 microL). Following the activation of nonesterified fatty acids (NEFA) by acylCoA synthetase, the current assay utilizes UDP-glucose pyrophosphorylase to link inorganic pyrophosphate to the production of NADH through the reactions catalyzed by phosphoglucomutase and glucose-6-phosphate 1-dehydrogenase. With this assay sequence the formation of NADH from NEFA is complete within 50 min at 37 degrees C. Enzymatic spectrophotometric techniques were unsuitable for NEFA determination in human milk due to the opacity of the sample. The use of the NADH-luciferase system has overcome this problem, allowing the enzymatic determination of NEFA in human milk. Sample collection and treatment procedures for milk and plasma have been developed to prevent enzymatic lipolysis and to limit interference from enzymes present in milk. The recovery of palmitic acid added to milk and plasma samples was 94.9+/-2.9 and 100+/-4.5%, respectively. There was no difference (P = 0.13) in plasma NEFA concentrations determined by the current method and a commercially available enzymatic spectrophotometric technique (Wako NEFA-C kit). Plasma NEFA concentrations determined by gas chromatography were 28% higher compared to both the Wako NEFA-C kit and the current method.