Effects of ciprofibrate and 2-[5-(4-chlorophenyl)pentyl]oxirane-2-carboxylate (POCA) on the distribution of carnitine and CoA and their acyl-esters and on enzyme activities in rats. Relation between hepatic carnitine concentration and carnitine acetyltransferase activity

The tissue-specific metabolic effects of the PPARα agonist ciprofibrate in insulin-resistant male individuals: a double-blind, randomized, placebo-controlled crossover study

OBJECTIVE

Insulin resistance is characterized by ectopic fat accumulation leading to cardiac diastolic dysfunction and nonalcoholic fatty liver disease. The objective of this study was to determine whether treatment with the peroxisome proliferator-activated receptor-α (PPARα) agonist ciprofibrate has direct effects on cardiac and hepatic metabolism and can improve insulin sensitivity and cardiac function in insulin-resistant volunteers.

METHODS

Ten insulin-resistant male volunteers received 100 mg/d of ciprofibrate and placebo for 5 weeks in a randomized double-blind crossover study. Insulin-stimulated metabolic rate of glucose (MRgluc) was measured using dynamic 18 F-fluorodeoxyglucose-positron emission tomography (18 F-FDG-PET). Additionally, cardiac function, whole-body insulin sensitivity, intrahepatic lipid content, skeletal muscle gene expression, 24-hour blood pressure, and substrate metabolism were measured.

RESULTS

Whole-body insulin sensitivity, energy metabolism, and body composition were unchanged after ciprofibrate treatment. Ciprofibrate treatment decreased insulin-stimulated hepatic MRgluc and increased hepatic lipid content. Myocardial net MRgluc tended to decrease after ciprofibrate treatment, but ciprofibrate treatment had no effect on cardiac function and cardiac energy status. In addition, no changes in PPAR-related gene expression in muscle were found.

CONCLUSIONS

Ciprofibrate treatment increased hepatic lipid accumulation and lowered MRgluc, without affecting whole-body insulin sensitivity. Furthermore, parameters of cardiac function or cardiac energy status were not altered upon ciprofibrate treatment.

Sites of superoxide and hydrogen peroxide production by muscle mitochondria assessed ex vivo under conditions mimicking rest and exercise

The sites and rates of mitochondrial production of superoxide and H2O2 in vivo are not yet defined. At least 10 different mitochondrial sites can generate these species. Each site has a different maximum capacity (e.g. the outer quinol site in complex III (site IIIQo) has a very high capacity in rat skeletal muscle mitochondria, whereas the flavin site in complex I (site IF) has a very low capacity). The maximum capacities can greatly exceed the actual rates observed in the absence of electron transport chain inhibitors, so maximum capacities are a poor guide to actual rates. Here, we use new approaches to measure the rates at which different mitochondrial sites produce superoxide/H2O2 using isolated muscle mitochondria incubated in media mimicking the cytoplasmic substrate and effector mix of skeletal muscle during rest and exercise. We find that four or five sites dominate during rest in this ex vivo system. Remarkably, the quinol site in complex I (site IQ) and the flavin site in complex II (site IIF) each account for about a quarter of the total measured rate of H2O2 production. Site IF, site IIIQo, and perhaps site EF in the β-oxidation pathway account for most of the remainder. Under conditions mimicking mild and intense aerobic exercise, total production is much less, and the low capacity site IF dominates. These results give novel insights into which mitochondrial sites may produce superoxide/H2O2 in vivo.

Localization of mitochondrial carnitine/acylcarnitine translocase in sensory neurons from rat dorsal root ganglia

The carnitine/acylcarnitine transporter is a transport system whose function is essential for the mitochondrial β-oxidation of fatty acids. Here, the presence of carnitine/acylcarnitine carrier (CACT) in nervous tissue and its sub-cellular localization in dorsal root ganglia (DRG) neurons have been investigated. Western blot analysis using a polyclonal anti-CACT antibody produced in our laboratory revealed the presence of CACT in all the nervous tissue extracts analyzed. Confocal microscopy experiments performed on fixed and permeabilized DRG neurons co-stained with the anti-CACT antibody and the mitochondrial marker MitoTracker Red clearly showed a mitochondrial localization for the carnitine/acylcarnitine transporter. The transport activity of CACT from DRG extracts reconstituted into liposomes was about 50 % in respect to liver extracts. The experimental data here reported represent the first direct evidence of the expression of the carnitine/acylcarnitine transporter in sensory neurons, thus supporting the existence of the β-oxidation pathway in these cells.

Vasopressin V1a and V1b Receptors: From Molecules to Physiological Systems

The neurohypophysial hormone arginine vasopressin (AVP) is essential for a wide range of physiological functions, including water reabsorption, cardiovascular homeostasis, hormone secretion, and social behavior. These and other actions of AVP are mediated by at least three distinct receptor subtypes: V1a, V1b, and V2. Although the antidiuretic action of AVP and V2 receptor in renal distal tubules and collecting ducts is relatively well understood, recent years have seen an increasing understanding of the physiological roles of V1a and V1b receptors. The V1a receptor is originally found in the vascular smooth muscle and the V1b receptor in the anterior pituitary. Deletion of V1a or V1b receptor genes in mice revealed that the contributions of these receptors extend far beyond cardiovascular or hormone-secreting functions. Together with extensively developed pharmacological tools, genetically altered rodent models have advanced the understanding of a variety of AVP systems. Our report reviews the findings in this important field by covering a wide range of research, from the molecular physiology of V1a and V1b receptors to studies on whole animals, including gene knockout/knockdown studies.

Carnitine insufficiency caused by aging and overnutrition compromises mitochondrial performance and metabolic control

In addition to its essential role in permitting mitochondrial import and oxidation of long chain fatty acids, carnitine also functions as an acyl group acceptor that facilitates mitochondrial export of excess carbons in the form of acylcarnitines. Recent evidence suggests carnitine requirements increase under conditions of sustained metabolic stress. Accordingly, we hypothesized that carnitine insufficiency might contribute to mitochondrial dysfunction and obesity-related impairments in glucose tolerance. Consistent with this prediction whole body carnitine diminution was identified as a common feature of insulin-resistant states such as advanced age, genetic diabetes, and diet-induced obesity. In rodents fed a lifelong (12 month) high fat diet, compromised carnitine status corresponded with increased skeletal muscle accumulation of acylcarnitine esters and diminished hepatic expression of carnitine biosynthetic genes. Diminished carnitine reserves in muscle of obese rats was accompanied by marked perturbations in mitochondrial fuel metabolism, including low rates of complete fatty acid oxidation, elevated incomplete beta-oxidation, and impaired substrate switching from fatty acid to pyruvate. These mitochondrial abnormalities were reversed by 8 weeks of oral carnitine supplementation, in concert with increased tissue efflux and urinary excretion of acetylcarnitine and improvement of whole body glucose tolerance. Acetylcarnitine is produced by the mitochondrial matrix enzyme, carnitine acetyltransferase (CrAT). A role for this enzyme in combating glucose intolerance was further supported by the finding that CrAT overexpression in primary human skeletal myocytes increased glucose uptake and attenuated lipid-induced suppression of glucose oxidation. These results implicate carnitine insufficiency and reduced CrAT activity as reversible components of the metabolic syndrome.

Metabolic profiling of PPARalpha-/- mice reveals defects in carnitine and amino acid homeostasis that are partially reversed by oral carnitine supplementation

Peroxisome proliferator-activated receptor-alpha (PPARalpha) is a master transcriptional regulator of beta-oxidation and a prominent target of hypolipidemic drugs. To gain deeper insights into the systemic consequences of impaired fat catabolism, we used quantitative, mass spectrometry-based metabolic profiling to investigate the fed-to-fasted transition in PPARalpha(+/+) and PPARalpha(-/-) mice. Compared to PPARalpha(+/+) animals, acylcarnitine profiles of PPARalpha(-/-) mice revealed 2- to 4-fold accumulation of long-chain species in the plasma, whereas short-chain species were reduced by as much as 69% in plasma, liver, and skeletal muscle. These results reflect a metabolic bottleneck downstream of carnitine palmitoyltransferase-1, a mitochondrial enzyme that catalyzes the first step in beta-oxidation. Organic and amino acid profiles of starved PPARalpha(-/-) mice suggested compromised citric acid cycle flux, enhanced urea cycle activity, and increased amino acid catabolism. PPARalpha(-/-) mice had 40-50% lower plasma and tissue levels of free carnitine, corresponding with diminished hepatic expression of genes involved in carnitine biosynthesis and transport. One week of oral carnitine supplementation conferred partial metabolic recovery in the PPARalpha(-/-) mice. In summary, comprehensive metabolic profiling revealed novel biomarkers of defective fat oxidation, while also highlighting the potential value of supplemental carnitine as a therapy and diagnostic tool for metabolic disorders.

Carnitine is associated with fatty acid metabolism in plants

The finding of acylcarnitines alongside free carnitine in Arabidopsis thaliana and other plant species, using tandem mass spectrometry coupled to liquid chromatography shows a link between carnitine and plant fatty acid metabolism. Moreover the occurrence of both medium- and long-chain acylcarnitines suggests that carnitine is connected to diverse fatty acid metabolic pathways in plant tissues. The carnitine and acylcarnitine contents in plant tissues are respectively a hundred and a thousand times lower than in animal tissues, and acylcarnitines represent less than 2% of the total carnitine pool whereas this percentage reaches 30% in animal tissues. These results suggest that carnitine plays a lesser role in lipid metabolism in plants than it does in animals.

Chemical knockout of pantothenate kinase reveals the metabolic and genetic program responsible for hepatic coenzyme A homeostasis

Coenzyme A (CoA) is the major acyl group carrier in intermediary metabolism. Hopantenate (HoPan), a competitive inhibitor of the pantothenate kinases, was used to chemically antagonize CoA biosynthesis. HoPan dramatically reduced liver CoA and mice developed severe hypoglycemia. Insulin was reduced, glucagon and corticosterone were elevated, and fasting accelerated hypoglycemia. Metabolic profiling revealed a large increase in acylcarnitines, illustrating the role of carnitine in buffering acyl groups to maintain the nonesterified CoASH level. HoPan triggered significant changes in hepatic gene expression that substantially increased the thioesterases, which liberate CoASH from acyl-CoA, and increased pyruvate dehydrogenase kinase 1, which prevents the conversion of CoASH to acetyl-CoA. These results identify the metabolic rearrangements that maintain the CoASH pool which is critical to mitochondrial functions, including gluconeogenesis, fatty acid oxidation, and the tricarboxylic acid and urea cycles.

Feedback regulation of murine pantothenate kinase 3 by coenzyme A and coenzyme A thioesters

Pantothenate kinase catalyzes a key regulatory step in coenzyme A biosynthesis, and there are four mammalian genes that encode isoforms of this enzyme. Pantothenate kinase isoform PanK3 is highly related to the previously characterized PanK1beta isoform (79% identical, 91% similar), and these two almost identical proteins are expressed most highly in the same tissues. PanK1beta and PanK3 had very similar molecular sizes, oligomeric form, cytoplasmic cellular location, and kinetic constants for ATP and pantothenate. However, these two PanK isoforms possessed distinct regulatory properties. PanK3 was significantly more sensitive to feedback regulation by acetyl-CoA (IC50 = 1 microm) than PanK1beta (IC50 = 10 microm), and PanK3 was stringently regulated by long-chain acyl-CoA (IC50 = 2 microm), whereas PanK1beta was not. Domain swapping experiments localized the difference in the two proteins to a 48-amino-acid domain, where they are the most divergent. Consistent with these more stringent regulatory properties, metabolic labeling experiments showed that coenzyme A (CoA) levels in cells overexpressing PanK3 were lower than in cells overexpressing an equivalent amount of PanK1beta. Thus, the distinct regulatory properties exhibited by the family of the pantothenate kinases allowed the rate of CoA biosynthesis to be controlled by regulatory signals from CoA thioesters involved in different branches of intermediary metabolism.

PPARalpha controls the intracellular coenzyme A concentration via regulation of PANK1alpha gene expression

Pantothenate kinase (PanK) is thought to catalyze the first rate-limiting step in CoA biosynthesis. The full-length cDNA encoding the human PanK1alpha protein was isolated, and the complete human PANK1 gene structure was determined. Bezafibrate (BF), a hypolipidemic drug and a peroxisome proliferator activator receptor-alpha (PPARalpha) agonist, specifically increased hPANK1alpha mRNA expression in human hepatoblastoma (HepG2) cells as a function of time and dose of the drug, compared with hPANK1beta, hPANK2, and hPANK3, which did not significantly increase. Four putative PPARalpha response elements were identified in the PANKIalpha promoter, and BF stimulated hPANK1alpha promoter activity but did not alter the mRNA half-life. Increased hPANK1alpha mRNA resulted in higher hPanK1 protein, localized in the cytoplasm, and elevated PanK enzyme activity. The enhanced hPANK1alpha gene expression translated into increased activity of the CoA biosynthetic pathway and established a higher steady-state CoA level in HepG2 cells. These data are consistent with a key role for PanK1alpha in the control of cellular CoA content and point to the PPARalpha transcription factor as a major factor governing hepatic CoA levels by specific modulation of PANK1alpha gene expression.

In vivo mechanistic studies on the metabolic activation of 2-phenylpropionic acid in rat

Two alternative metabolic pathways, acyl glucuronidation and acyl-CoA formation, are implicated in the generation of reactive acylating metabolites of carboxylic acids. Here, we describe studies that determine the relative importance of these two pathways in the metabolic activation of a model substrate, 2-phenylpropionic acid (2-PPA), in vivo in rats. Male Sprague-Dawley rats were pretreated with and without (-)-borneol (320 mg/kg i.p.), an inhibitor of acyl glucuronidation, or trimethylacetic acid (TMA, 500 mg/kg i.p.), an inhibitor of acyl-CoA formation, before receiving 2-PPA (racemic, 130 mg/kg). After administration of 2-PPA, livers were collected over a 2-h period and analyzed for 2-PPA acyl glucuronidation and 2-PPA-CoA formation by high-performance liquid chromatography. Covalent binding was measured by scintillation counting of washed liver protein precipitates. Results showed that pretreatment with TMA led to a 49% decrease in covalent binding of 2-PPA to liver proteins, when a 64% decrease in the exposure of 2-PPA-CoA was observed. Conversely, 95% inhibition of acyl glucuronidation by (-)-borneol, led to a 23% decrease in covalent binding to protein. These results suggest that metabolic activation by 2-PPA-CoA formation contributes to covalent adduct formation to protein in vivo to a greater extent than metabolic activation by acyl glucuronidation for this model substrate.

The murine pantothenate kinase (Pank1) gene encodes two differentially regulated pantothenate kinase isozymes

Pantothenate kinase (PanK) is a rate-determining enzyme in coenzyme A (CoA) biosynthesis. The mouse murine pantothenate kinase (Pank1) gene consists of seven introns and eight exons and is located on chromosome 19 (19C2-3). Two biochemically distinct PanK1 protein isoforms, PanK1 alpha and PanK1 beta, are encoded by the Pank1 gene. Both proteins have the same 363 amino acid catalytic domain encoded by exons 2 through 7. The PanK1 beta transcript begins with exon 1 beta and translates into a ten-residue amino terminus plus the catalytic domain. The PanK1 alpha transcript initiates at an alternate upstream site at exon 1 alpha which is spliced with exon 2, excluding exon 1 beta. Exon 1 alpha encodes a 184-residue regulatory domain at the amino terminus of the PanK1 alpha protein that confers feedback inhibition by free CoA and long-chain acyl-CoA, and increases the regulation of PanK enzyme activity by acetyl-CoA and malonyl-CoA. Differential expression of the PanK1 alpha and PanK1 beta transcripts would alter the amount of CoA produced in cells as a function of the ratio of free CoA to acetyl-CoA, a reflection of the metabolic status of the tissue.

Pantothenate kinase regulation of the intracellular concentration of coenzyme A

Pantothenate kinase (PanK) is the key regulatory enzyme in the CoA biosynthetic pathway in bacteria and is thought to play a similar role in mammalian cells. We examined this hypothesis by identifying and characterizing two murine cDNAs that encoded PanK. The two cDNAs were predicted to arise from alternate splicing of the same gene to yield different mRNAs that encode two isoforms (mPanK1alpha and mPanK1beta) with distinct amino termini. The predicted protein sequence of mPanK1 was not related to bacterial PanK but exhibited significant similarity to Aspergillus nidulans PanK. mPanK1alpha was most highly expressed in heart and kidney, whereas mPanK1beta mRNA was detected primarily in liver and kidney. Pantothenate was the most abundant pathway component (42.8%) in normal cells providing clear evidence that pantothenate phosphorylation was a rate-controlling step in CoA biosynthesis. Enhanced mPanK1beta expression eliminated the intracellular pantothenate pool and triggered a 13-fold increase in intracellular CoA content. mPanK1beta activity in vitro was stimulated by CoA and strongly inhibited by acetyl-CoA illustrating that differential modulation of mPanK1beta activity by pathway end products also contributed to the management of CoA levels. These data support the concept that the expression and/or activity of PanK is a determining factor in the physiological regulation of the intracellular CoA concentration.

The role of the carnitine system in peroxisomal fatty acid oxidation

Peroxisomes are small, subcellular organelles that play a major role in lipid metabolism. Inherited disorders of peroxisomal structure and metabolism can result from defective assembly, missing protein import transporters, or individual enzyme deficiencies. Molecular studies helped by the range of disorders have now elucidated many of the pathways, including the paths of alpha-oxidation for phytanic acid and beta-oxidation for very-long-chain and branched-chain fatty acids and for bile acid synthesis. The mechanism of the transfer of substrates, intermediates, and products across the membrane is poorly understood. The carnitine system, known to transport activated acyl groups between localized coenzyme A pools, is presented. The evidence for the involvement of carnitine in the transfer of activated acyl groups to and from the peroxisomes is reviewed.

Alteration of protein kinase C isoform-specific expression during rat hepatocarcinogenesis after exposure to the peroxisome proliferator WY-14,643

The role of protein kinase C (PKC) isoforms in mediating peroxisome proliferator chemical- (PPC) induced hepatocarcinogenesis was examined. After an acute gavage exposure to WY-14,643 (WY) membrane-bound PKCdelta and cytosolic PKCbeta decreased, whereas the expression of the other isoforms was not altered. After a 13-week chronic exposure, membrane-bound PKCbeta, delta and zeta levels decreased. In WY-induced hepatocellular adenomas, PKCalpha was increased, and PKCbeta was further decreased in membrane fractions. These results, taken together with previous studies, indicate that alterations in PKCalpha, beta and delta isoforms, which regulate mitogenesis, could play important roles in perpetuating the high cell proliferative rate in PPC-induced hepatocellular adenomas.

Spontaneous development of fatty liver in ferrets in a toxicology study

Ferrets were maintained for 12 months on different diets (A, meat and biscuit; B, all meat; C, meat and fish; D, high fibre) to ascertain the cause of spontaneous development of fatty liver. High hepatic triglyceride contents resulted on diets B = C > D; whereas ferrets on diet A (control) showed no accumulation of lipid in liver. Serum triglyceride and total cholesterol were unchanged by diet. These ferrets (F0 generation) were mated with ferrets on the same diet and the offspring (F1 generation), maintained on the same diets as the parents, were killed at 12 months and the livers studied similarly. Histology showed that hepatic lipid accumulation in the F1 generation was identical with that in the same dietary groups of the F0 generation; liver glutathione (GSH) reductase and thiobarbituric acid-reacting substances (an index of lipid peroxidation) were increased in ferrets maintained on diets B, C and D, liver GSH concentration and GSH peroxidase activities were unchanged. Other ferrets fed a high-fat diet (diet A plus 20% w/w beef suet) for 18 days exhibited hepatic lipid accumulation and decreased hepatic cyanide-insensitive palmitoyl CoA oxidation (-30%), but hepatic lauric acid hydroxylation and carnitine acyl transferase activities were unchanged. These data indicate that ferrets on high-fat diets show no increased rates of liver fatty acid oxidation, as seen in rats, but instead accumulate triglyceride in the liver with some degree of lipid peroxidation.

The effects of 3-hydroxy-3-methylglutaryl-CoA reductase inhibition on tissue levels of carnitine and carnitine acyltransferase activity in the rabbit

Recently, a new class of lipid lowering agents [3-hydroxy-3-methylglutaryl (HMG)-CoA reductase inhibitors] was introduced into clinical practice. The use of these agents could lead to a secondary deficiency in carnitine, which may manifest clinically as a myalgia/myositis-a side effect that is occasionally seen with this class of drugs. In the present study, we examined the effect of an HMG-CoA reductase inhibitor (lovastatin) on serum and tissue levels of carnitine and carnitine acyltransferase activities in the rabbit. Rabbits (n = 6) were fed chow containing lovastatin (30 mg/d) for 16 wk. Blood was collected and tissues (liver, heart, and skeletal muscle) harvested at sacrifice. Free and total carnitine were measured in serum and tissues by a radioenzymatic method. Carnitine acetyltransferase and carnitine palmitoyltransferase (CPT) activities were determined and expressed relative to DNA. Serum free (24.0 +/- 2.6 vs. 29.4 +/- 3.1 microM) and total (35.1 +/- 4.7 vs. 52.8 +/- 8.8 microM) carnitine levels increased significantly with 16 wk of treatment. This increase in total carnitine was mainly due to an increase in the levels of serum acylcarnitine (12.7 +/- 3.1 vs 26.5 +/- 5.7 microM). Tissue levels of total carnitine were significantly decreased by the treatment. Carnitine acetyltransferase was unaffected by the treatment, whereas there was a significant increase in the activity of CPT in the liver and heart.

Palmitoylcarnitine, a surface-active metabolite

Palmitoylcarnitine is a well-known intermediate in mitochondrial fatty acid oxidation. Less known are its properties as a surfactant, with a capacity to solubilize biological membranes similar to that of many synthetic detergents used in the biochemical laboratory. Some of the physico-chemical properties of palmitoylcarnitine may help to explain the need for coenzyme A-carnitine-coenzyme A acyl exchange during mitochondrial fatty acid import. The amphiphilic character of palmitoylcarnitine may also explain its proposed involvement in the pathogenesis of myocardial ischemia.

Saturable binding sites for the coenzyme A ester of nafenopin, a peroxisome proliferator, in rat liver cytosol

1. At least three different molecular weight binding sites exist in rat liver cytosol for nafenopin-CoA, the coenzyme A ester and metabolic product of the carcinogenic peroxisome proliferator nafenopin. No binding sites for the free drug were observed. 2. Polypeptides of 35-40 kDa molecular weight range where no acyl-CoA binding proteins have been previously described bind the highest proportion of nafenopin-CoA (60-70%). Binding is displaceable by the CoA esters of other peroxisome proliferators (ciprofibrate and tibric acid) and also by oleoyl-CoA but by palmitoyl-CoA. Direct binding studies show that 35-40-kDa polypeptides bind oleoyl-CoA but not oleic or palmitic acid, or palmitoyl-CoA. 3. Polypeptides of 10-14 and 65-70 kDa also bind nafenopin-CoA. However, in contrast with 35-40-kDa polypeptides they also bind oleic and palmitic acid as well as their correspondent acyl-CoA thioesters.

Effects of added l-carnitine, acetyl-CoA and CoA on peroxisomal beta-oxidation of [U-14C]hexadecanoate by isolated peroxisomal fractions

(1) During peroxisomal beta-oxidation of [U-14C]hexadecanoate, at concentrations higher than 100 microM, long-chain 3-oxoacyl-CoA-esters and 3-oxobutyryl-CoA accumulate. Only 3-oxobutyryl-CoA accumulates at a low concentration of [U-14C]hexadecanoate. Accumulation of long chain 3-oxoacyl-CoA esters is most extensive when the supply of CoA can be considered limiting for beta-oxidation. (2) Added acetyl-CoA was found to inhibit peroxisomal beta-oxidation. This inhibition was not significantly relieved by added L-carnitine and carnitine acetyltransferase (EC 2.3.17). (3) Added L-carnitine, at concentrations below 0.2 mM, was found to stimulate peroxisomal beta-oxidation of [U-14C]hexadecanoate by up to 20%, causing the conversion of acetyl-CoA into acetylcarnitine. Higher concentrations of L-carnitine were progressively inhibitory to beta-oxidation. This effect was specific for L-carnitine as both D-carnitine and aminocarnitine neither caused stimulation at low concentrations, nor inhibition at higher concentrations. Added L-carnitine caused accumulation of acylcarnitines of chain-lengths ranging from 4 to 16 carbon-atoms. The inhibition observed with higher concentrations of added L-carnitine is likely due to conversion of [U-14C]hexadecanoate into [U-14C]hexadecanoylcarnitine. (4) Low concentrations of added hexadecanoylcarnitine was shown to inhibit peroxisomal beta-oxidation by about 15%, while added acetylcarnitine did not inhibit at concentrations up to 100 microM. (5) These data are interpreted to indicate significant control being exerted on flux at the stage of thiolysis either directly by means of CoA availability, or indirectly by means of the rate of acetyl-CoA generation.

Long-chain 3-hydroxyacyl-coenzyme A dehydrogenase deficiency--diagnosis, plasma carnitine fractions and management in a further patient

Long-chain 3-hydroxyacyl-coenzyme A dehydrogenase (LCHAD), the third enzyme of the mitochondrial beta-oxidation pathway, carries out the dehydrogenation of 3-hydroxyacyl-CoA compounds of 12-18 carbon length. To date only nine cases of LCHAD deficiency have been documented. We report a further patient who as a neonate developed non-specific gastrointestinal symptoms and at 5 months of age cardiomyopathy, recurrent hypoketotic hypoglycaemia and gross alterations of plasma carnitine fractions. Dietary management with medium chain triglycerides led rapidly to clinical improvement. There was a close correlation between the clinical condition, plasma carnitine fractions and cardiac function. At 2 years of age she is developing normally.

Hypolipidaemic drugs are activated to acyl-CoA esters in isolated rat hepatocytes. Detection of drug activation by human liver homogenates and by human platelets

The formation of acyl-CoA esters of the hypolipidaemic peroxisome proliferators clofibric acid, ciprofibrate and nafenopin was studied in isolated rat hepatocytes. The concentration of ciprofibroyl-CoA in the liver of ciprofibrate-treated rats was in the range of 10-30 microM. The three drugs formed acyl-CoA esters when incubated with isolated hepatocytes. Their formation was saturable and reached a plateau after 30 min incubation. Maximal intracellular concentrations of ciprofibroyl-CoA and clofibroyl-CoA (100 microM and 55 microM respectively) were attained at 0.5 mM of the free drugs in the incubation medium, whereas for nafenopin-CoA, the maximal intracellular concentration (9 microM) was reached at 1 mM-nafenopin. At low concentrations of the hypolipidaemic compounds in the incubation medium a significant proportion of the total intracellular drug was present as its acyl-CoA ester (25-35% for ciprofibrate). When isolated hepatocytes were incubated with a ciprofibrate concentration comparable with that observed in the blood of drug-treated rats (0.1 mM), ciprofibroyl-CoA attained an intracellular concentration similar to that previously observed in the liver of treated rats. The formation of ciprofibroyl-CoA by isolated rat hepatocytes was stimulated by the addition of carnitine and partially inhibited by the addition of palmitate. Further, it was shown that human liver homogenates synthesized ciprofibroyl-CoA at a rate similar to that observed for rat liver homogenates. Solubilized human platelets also formed ciprofibroyl-CoA, although at a rate two orders of magnitude lower than that of liver. The results support the view that acyl-CoA esters of hypolipidaemic peroxisome proliferators may be the pharmacologically active species of the drugs.

Substituted 2-oxiranecarboxylic acids: a new group of candidate hypoglycaemic drugs

Drugs to treat diabetes that can be taken orally have long been sought, although the successful management of insulin-dependent diabetes mellitus by simple chemotherapy may be an unachievable goal. The only drugs currently used for the treatment of non-insulin-dependent diabetes have limited effectiveness. In this article Peter Selby and Stanley Sherratt describe the development of a new group of candidate hypoglycaemic drugs, esters of substituted 2-oxiranecarboxylic acids, which merit full clinical evaluation. These drugs are hydrolysed to the free acids which are then converted to their coenzyme A esters in cells. The CoA esters inactivate carnitine palmitoyltransferase I in the outer mitochondrial membrane, thus preventing the excessive oxidation of long-chain fatty acids that occurs in diabetes. This causes a secondary decrease in hepatic gluconeogenesis and an increase in peripheral glucose utilization leading to improved glucose tolerance.