Carnitine--metabolism and functions

Carnitine was detected at the beginning of this century, but it was nearly forgotten among biochemists until its importance in fatty acid metabolism was established 50 years later. In the last 30 years, interest in the metabolism and functions of carnitine has steadily increased. Carnitine is synthesized in most eucaryotic organisms, although a few insects (and most likely some newborn animals) require it as a nutritional factor (vitamin BT). Carnitine biosynthesis is initiated by methylation of lysine. The trimethyllysine formed is subsequently converted to butyrobetaine in all tissues; the butyrobetaine is finally hydroxylated to carnitine in the liver and, in some animals, in the kidneys (see Fig. 1). It is released from these tissues and is then actively taken up by all other tissues. The turnover of carnitine in the body is slow, and the regulation of its synthesis is still incompletely understood. Microorganisms (e.g., in the intestine) can metabolize carnitine to trimethylamine, dehydrocarnitine (beta-keto-gamma-trimethylaminobutyric acid), betaine, and possibly to trimethylaminoacetone. In some insects carnitine can be converted to methylcholine, presumably with trimethylaminoacetone as an intermediate (see Fig. 3). In mammals the unphysiological isomer (+) carnitine is converted to trimethylaminoacetone. The natural isomer (-)carnitine is excreted unchanged in the urine, and it is still uncertain if it is degraded in mammalian tissues at all (Fig. 2). The only firmly established function of carnitine is its function as a carrier of activated fatty acids and activated acetate across the inner mitochondrial membrane. Two acyl-CoA:carnitine acyltransferases with overlapping chain-length specificities have been isolated: one acetyltransferase taking part in the transport of acetyl and short-chain acyl groups and one palmitoyltransferase taking part in the transport of long-chain acyl groups. An additional octanoyltransferase has been isolated from liver peroxisomes. Although a carnitine translocase that allows carnitine and acylcarnitine to penetrate the inner mitochondrial membrane has been deduced from functional studies (see Fig. 5), this translocase has not been isolated as a protein separate from the acyltransferases. Carnitine acetyltransferase and carnitine octanoyltransferase are also found in the peroxisomes. In these organelles the enzymes may be important in the transfer of acyl groups, which are produced by the peroxisomal beta-oxidation enzymes, to the mitochondria for oxidation in the citric acid cycle. The carnitine-dependent transport of activated fatty acids across the mitochondrial membrane is a regulated process. Malonyl-CoA inh

Reduced levels of peroxisomal enzymes in the kidney of the genetically obese (ob/ob) mouse. Contrast with liver

Kidney post-nuclear supernatants from genetically lean and obese mice were subjected to subcellular fractionation by dual centrifugation through sucrose gradients in B XIV zonal rotors. Considerable purification of peroxisomes was achieved which allowed the demonstration of acyl-CoA beta-oxidation enzymes and carnitine acyltransferases in these organelles. Comparison of kidney peroxisome-enriched fractions from obese and lean mice indicated a likely relative depression in beta-oxidation enzymes in the obese animal. Measurement of catalase, acyl-CoA oxidase and carnitine octanoyltransferase in whole homogenate of liver and kidney of obese and lean mice revealed significantly reduced levels (to approximately 2/3) of these peroxisomal enzymes in the kidney of ob/ob mice. In contrast the specific activity of catalase and acyl-CoA oxidase was significantly raised in the liver of obese mice.

Effects of pantethine and its metabolites on fatty acid oxidation in rat liver mitochondria

The mechanism of the activating effect of pantethine [D-bis-(N-pantothenyl-beta-aminoethyl)disulfide] on fatty acid oxidation was investigated in rat liver mitochondria. Pantethine, pantetheine and 4'-phosphopantetheine activated three steps of fatty acid oxidation, i.e., acyl-CoA synthetase, carnitine, acyltransferase and intramitochondrial oxidation, to various extents. Although their effects may have been partly due to CoASH derived from them, they also had specific effects.

Localization of carnitine acyltransferases and acyl-CoA beta-oxidation enzymes in small intestinal microperoxisomes (peroxisomes) of normal and clofibrate treated mice

Dietary clofibrate for 21 days induced a rise in the specific activities of crotonase, acyl-CoA oxidase and carnitine acetyltransferase in a crude particulate fraction from mouse small intestinal mucosa. Subcellular fractionation of post-nuclear supernatant prepared from mucosal homogenates of normal and clofibrate treated animals allowed substantial separation of peroxisomes from contaminating organelles. Analysis of fractions demonstrated that intestinal peroxisomes contain acyl-CoA oxidase, crotonase, beta-hydroxyacyl-CoA dehydrogenase and carnitine acyltransferase activities. It is concluded that intestinal peroxisomes are equipped to engage in fatty acid oxidation.

Influence of valproic acid on hepatic carbohydrate and lipid metabolism

Valproic acid (dipropylacetic acid), an antiepileptic agent known to be hepatotoxic in some patients, caused inhibition of lactate gluconeogenesis, fatty acid oxidation, and fatty acid synthesis by isolated hepatocytes. The latter process was the most sensitive to valproic acid, 50% inhibition occurring at ca. 125 microM with cells from meal-fed female rats. The medium-chain acyl-CoA ester fraction was increased whereas coenzyme A (CoA), acetyl-CoA, and the long chain acyl-CoA fractions were decreased by valproic acid. The increase in the medium chain acyl-CoA fraction was found by high-pressure liquid chromatography to be due to the accumulation of valproyl-CoA plus an apparent CoAester metabolite of valproyl-CoA. Salicylate inhibited valproyl-CoA formation and partially protected against valproic acid inhibition of hepatic metabolic processes. Octanoate had a similar protective effect, suggesting that activation of valproic acid in the mitosol is required for its inhibitory effects. It is proposed that either valproyl-CoA itself or the sequestration of CoA causes inhibition of metabolic processes. Valproyl-CoA formation also appears to explain valproic acid inhibition of gluconeogenesis by isolated kidney tubules. No evidence was found for the accumulation of valproyl-CoA in brain tissue, suggesting that the effects of valproic acid in the central nervous system are independent of the formation of this metabolite.

Detection of acyl-CoA beta-oxidation enzymes in peroxisomes (microperoxisomes) of mouse heart

Homogenate of mouse heart was analyzed by using rate-dependent banding followed by density-dependent banding in sucrose density gradients held in zonal rotors. This protocol allowed substantial separation of myocardial microperoxisomes from other organelles. Particulate acyl-CoA oxidase activity was localized in microperoxisomes, while beta-hydroxyacyl-CoA dehydrogenase, carnitine acetyltransferase, carnitine octanoyltransferase and probably enoyl-CoA hydratase were partially localized in these organelles with most of the activity residing in the mitochondria. The specific activity of acyl-CoA oxidase in the peak microperoxisome fraction was at least 1/3 to 1/4 of that in highly purified renal or hepatic peroxisomes from mouse.

Carnitine octanoyltransferase of mouse liver peroxisomes: properties and effect of hypolipidemic drugs

Carnitine octanoyltransferase (COT) in 500g supernatant fluids from mouse liver has a specific activity at least twice that of carnitine acetyltransferase (CAT) or carnitine palmitoyltransferase (CPT). When mice are fed diets containing the lipid-lowering drugs, clofibrate or nafenopin, the specific activity of COT increases 4- and 11-fold, respectively. Liver homogenates from mice fed a control diet, and diets containing clofibrate, nafenopin, or Wy-14,643 were fractionated by sucrose gradient centrifugation, and the subcellular distribution of carnitine acyltransferases was determined. In the controls, peroxisomes contained about 70% of the total COT. The specific activity of COT in the peroxisomal peak was 12-fold greater than either CAT or CPT, and 20-fold greater than the COT activity in the mitochondrial fraction. Treatment with hypolipidemic drugs increased the specific activity of peroxisomal COT 2- to 3-fold and CAT 6- to 12-fold, while mitochondrial COT increased 5- to 11-fold and CAT 19- to 54-fold. COT was purified to homogeneity from livers of mice treated with Wy-14,643. It had an apparent Mr of 60,000 by Sephadex G-100 and sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis, and a maximum activity for octanoyl-CoA with acetyl-CoA and palmitoyl-CoA having activities of 2 and 10%, respectively, when 100 microM acyl-CoA substrates were used. The Km's for 1-carnitine, octanoyl-CoA, palmitoyl-CoA, and acetyl-CoA were 130, 15, 69, and 155 microM, respectively, in the forward direction; and in the reverse direction were 110, 100, 104, and 783 microM for CoASH, octanoylcarnitine, palmitoylcarnitine, and acetylcarnitine, respectively. With Vmax conditions, acetyl-CoA and palmitoyl-CoA had activities of 8 and 26% of the activity for octanoyl-CoA, and acetylcarnitine and palmitoylcarnitine had activities of 7 and 22%, respectively, of the activity for octanoylcarnitine. It is concluded that COT is a separate enzyme present in large amounts in the matrix of mouse liver peroxisomes, with kinetic properties that greatly favor medium-chain acylcarnitine formation.

The effect of malonyl-CoA on overt and latent carnitine acyltransferase activities in rat liver and adipocyte mitochondria

1. Carnitine palmitoyltransferase and carnitine octanoyltransferase activities were measured in mitochondria at various acyl-CoA concentrations before and after sonication, thus permitting assessment of both overt and latent activities. 2. Overt carnitine palmitoyltransferase in liver and adipocyte mitochondria and overt carnitine octanoyltransferase in liver mitochondria were inhibited by malonyl-CoA. None of the latent activities were affected by this metabolite. 3. 5,5'-Dithiobis-(2-nitrobenzoic acid) stimulated latent hepatic carnitine palmitoyltransferase at low [palmitoyl-CoA]. 4. Starvation (24 h) decreased overt carnitine palmitoyltransferase activity in adipocyte mitochondria, but did not alter the sensitivity of this activity to malonyl-CoA.

Hepatic mitochondrial fatty acid oxidation during the perinatal period in the rat

1. The activity of hepatic mitochondrial carnitine acyltransferase I increases rapidly after birth, is high during the suckling period and falls after weaning. In contrast, carnitine acyltransferase II and acyl-CpA dehydrogenase exhibit few developmental changes. 2. These and previous studies indicate that outer mitochondrial membrane acyl-CoA synthetase and inner membrane carnitine acyltransferase I increase in activity after birth much more rapidly than to any other enzymes of fatty acid oxidation. 3. Studies of the 18 hr after caesarian delivery indicate that whereas the major increase in the activity of acyl-CoA synthetase occurs within 3 hr of birth the change in carnitine acyltransferase I activity is less rapid. 4. Prolonged pregnancy, starvation of the mother or feeding the mother a high polyunsaturated fat content diet resulted in increased activities of acyl-CoA synthetase and carnitine acyltransferase I in the fetal liver.

Possible functions of short-chain and medium-chain carnitine acyltransferases

Several mammalian tissue contain water-soluble, branched chain acylcarnitines and other short-chain aliphatic acylcarnitines and also contain a broad spectrum of short-chain and medium-chain carnitine acyltransferase (CAT) activities. Although carnitine can stimulate the oxidation of branched chain alpha-ketoacids, it has not been established that carnitine is required for the oxidation of the alpha-ketoacids in the matrix of mitochondria. Rather it probably acts as a reversible sink for acyl residues, thereby generating CoASH, which can be used to maintain normal metabolism; thus carnitine would have a facilitative rather than an obligatory role. Microsomes and peroxisomes contain medium- and short-chain CATs. This occurrence is short- and medium-chain CATs in peroxisomes is consistent with carnitine's being involved in shuttling the chain-shortened products of beta-oxidation out of peroxisomes. Human urine contains a spectrum of short-chain acylcarnitines and data are presented that show a large amount of propionylcarnitine in the urine of the individual with a metabolic problem. The cumulative data are consistent with the conclusion that carnitine has multiple roles in mammalian metabolism, including the shuttling of beta-oxidation chain-shortened products out of peroxisomes in liver, the modulation of the acyl-CoA/CoASH ratio in mammalian cells, and the translocation of acetyl units for selective synthesis in a yeast.

Carnitine and carnitine palmitoyltransferase in fatty acid oxidation and ketosis

Carnitine is an essential factor in long-chain fatty acid oxidation. Carnitine acts as a carrier of fatty acyl groups from the cytoplasm to the mitochondrion. Long-chain acyl-CoA derivatives do not penetrate the mitochondrial inner membrane. Carnitine palmitoyltransferase A (CPT-A), located on the external surface of the inner membrane, catalyzes the conversion of cytoplasmic long-chain acyl-CoA and carnitine into acylcarnitine. The acylcarnitine is reconverted to intramitochondrial acyl-CoA by the action of carnitine palmitoyltransferase B located in the inner membrane. Now, the acyl-CoA is available for beta-oxidation in the matrix. An inner membrane carnitine-acylcarnitine translocase exchanges carnitine and acylcarnitine across the inner membrane but its role is long-chain acyl transfer has not been established. The tissue concentration of carnitine is important; liver carnitine is correlated with the rate of hepatic ketoacid production. In liver, malonyl-CoA, an intermediate in fatty acid synthesis, is proposed to regulate the activity of CPT-A. Studies using various purified transferases have not provided an answer to the question of whether the two activities expressed in mitochondria are separate enzymes. The absence of only CPT-A activity in isolated skeletal muscle mitochondria obtained from a patient with a lipid-storage myopathy suggests two separate activities.

The sidedness of carnitine acetyltransferase and carnitine octanoyltransferase of rat liver endoplasmic reticulum

The location of carnitine acetyltransferase and carnitine octanoyltransferase on the inner and outer surfaces of rat liver microsomes was investigated. Latency of mannose-6-phosphate phosphate showed that the microsomes were 90-94% sealed. All of the octanoyltransferase is associated with the cytosolic face, while the acetyltransferase is distributed between the cytosolic face (68-73%) and the lumen face (27-32%) of the endoplasmic reticulum membrane. Small amounts of trypsin inhibit the carnitine octanoyltransferase equally in either sealed or permeable microsomes but the acetyltransferase of sealed microsomes is stimulated. Large amounts of trypsin inhibit all transferase activities by about 60%, expect for acetyltransferase of sealed microsomes. Other studies show that 0.1% Triton X-100 partially inhibits carnitine octanoyltransferase of microsomes but does not inhibit the acetyltransferase or any of the mitochondrial carnitine acyltransferase.

Carnitine acyltransferase activities in rat liver and heart measured with palmitoyl-CoA and octanoyl-CoA. Latency, effects of K+, bivalent metal ions and malonyl-CoA

1. Liver carnitine acyltransferase activities with palmitoyl-CoA and octanoyl-CoA as substrates and heart carnitine palmitoyltransferase were measured as overt activities in whole mitochondria or in mitochondria disrupted by sonication or detergent treatment. All measurements were made in sucrose/KCl-based media of 300 mosmol/litre. 2. In liver mitochondria, acyltransferase measured with octanoyl-CoA, like carnitine palmitoyltransferase, was found to have latent and overt activities. 3. Liver acyltransferase activities measured with octanoyl-CoA and palmitoyl-CoA differed in their response to changes in [K+], Triton X-100 treatment and, in particular, in their response to Mg2+. Mg2+ stimulated activity with octanoyl-CoA, but inhibited carnitine palmitoyltransferase. 4. The effects of K+ and Mg2+ on liver overt carnitine palmitoyltransferase activity were abolished by Triton X-100 treatment. 5. Heart overt carnitine palmitoyltransferase activity differed from the corresponding activity in liver in that it was more sensitive to changes in [K+] and was stimulated by Mg2+. Heart had less latent carnitine palmitoyltransferase activity than did liver. 6. Overt carnitine palmitoyltransferase in heart mitochondria was extremely sensitive to inhibition by malonyl-CoA. Triton X-100 abolished the effect of low concentrations of malonyl-CoA on this activity. 7. The inhibitory effect of malonyl-CoA on heart carnitine palmitoyltransferase could be overcome by increasing the concentration of palmitoyl-CoA.

Metabolism of very long-chain monounsaturated fatty acids (22:1) and the adaptation to their presence in the diet

Unadapted rats and other animal species have a limited capacity to metabolize monounsaturated fatty acids with 22 carbons (22:1). Excess amounts in the diet of fats containing these fatty acids cause a transient accumulation (lipidosis) of triacylglycerol in the heart and other tissues but not in the liver, which seems to export the 22:1 fatty acids as very low density lipoproteins to the blood plasma. The acute lipidosis most probably is explained by a slow oxidation of 22:1 acyl-CoA by the mitochondrial acyl-CoA dehydrogenase combined with an inhibitory effect of this CoA ester on the oxidation of acyl-CoA esters of a more "normal" chain length. Other fatty acid metabolizing enzymes also show slow reaction rates with the 22:1 fatty acids. Upon continued feeding of diets with 22:1 fatty acids, an adaptation takes place and the lipidosis disappears. This adaptation coincides with the development of an increased capacity to chain-shorten the 22:1 fatty acids, especially in the liver, but also in the heart. The chain-shortening seems to be due to a partial beta-oxidation of the 22:1 fatty acids by the peroxisomal beta-oxidation enzyme system which shows an increased activity in adapted rats. In such rats, less 22:1 fatty acids circulate in the plasma very low density lipoproteins than in unadapted rats. The drug clofibrate (ethyl-p-chlorophenoxyisobutyrate) which induces increased activity of the peroxisomal beta-oxidation enzymes, provides partial protection against the lipidosis in unadapted animals. Hydrogenated fish oil (containing different 22:1 isomers and many fatty acids with trans double bonds) is more efficient as an inducer of the chain-shortening of erucic acid in the liver than is rapeseed oil, which contains only one 22:1 fatty acid isomer and no fatty acids with trans double bonds. The hydrogenated fish oil causes less lipidosis than does rapeseed oil when diets containing the same amount of 22:1 fatty acids are fed. It is suggested that CoA esters that are poorly oxidized by the mitochondria (e.g., esters of erucic acid, of some fatty acids with trans double bonds, and of clofibric acid) may trigger the adaptation process.-Bremer, J., and K. R. Norum. Metabolism of very long-chain monounsaturated fatty acids (22:1) and the adaptation to their presence in the diet.

Carnitine:acylcarnitine translocase of rat heart mitochondria. Competition for carnitine uptake by carnitine esters

The kinetic behavior of the carnitine:acylcarnitine translocase was studied in isolated rat heart mitochondria. The kinetic parameters, Km(apparent) and Vmax, for carnitine were determined by measuring the rates of influx of [14C]carnitine using two different methods to quench the exchange reaction. The range of the Km(app) was 0.38-1.50 mM and the Vmax was 0.20-0.34 nmol/mg . min by both methods. Carnitine esters of acetyl isobutyryl, and octanoyl groups were competitive with carnitine for uptake and Ki values for these esters were 1.1, 2.6, and 0.10 mM, respectively. The Km(app) for carnitine was increased in the presence of these carnitine esters, while the Vmax for carnitine was unchanged. Distribution of radiolabel from free [14C]carnitine into acetylcarnitine, isobutyrylcarnitine, and octanoylcarnitine during the incubations was examined by thin layer chromatography and was negligible. The Km values for carnitine and the Ki value for acetylcarnitine are within the concentration ranges of these compounds in the intact heart (Idell-Wenger, J. A., Grotyohann, L. W., and Neely, J. R. (1978) J. Biol. Chem. 253, 4310-4318). the Ki values for isobutyrylcarnitine and octanoylcarnitine may also be within their concentration ranges in vivo, but exact concentrations in heart muscle are not known. These data support the concept that carnitine esters of short (acetylcarnitine), branched (isobutyrylcarnitine), and medium (octanoylcarnitine) chain acyl groups compete with free carnitine for transport into the mitochondria under physiological conditions.