The role of carnitine, the carnitine acyltransferases and the carnitine-exchange system [proceedings]

Molecular enzymology of carnitine transfer and transport

Carnitine (L-3-hydroxy-4-N-trimethylaminobutyric acid) forms esters with a wide range of acyl groups and functions to transport and excrete these groups. It is found in most cells at millimolar levels after uptake via the sodium-dependent carrier, OCTN2. The acylation state of the mobile carnitine pool is linked to that of the limited and compartmentalised coenzyme A pools by the action of the family of carnitine acyltransferases and the mitochondrial membrane transporter, CACT. The genes and sequences of the carriers and the acyltransferases are reviewed along with mutations that affect activity. After summarising the accepted enzymatic background, recent molecular studies on the carnitine acyltransferases are described to provide a picture of the role and function of these freely reversible enzymes. The kinetic and chemical mechanisms are also discussed in relation to the different inhibitors under study for their potential to control diseases of lipid metabolism.

Effect of carboxylic acid xenobiotics and their metabolites on the activity of carnitine acyltransferases

Salicylyl-CoA and benzoyl-CoA were good inhibitors of carnitine acetyltransferase (CAT), competing with acetyl-CoA with Ki values of 7.5 and 22 microM respectively in the forward direction and with CoA in the reverse reaction with similar Ki values. They were also competitive inhibitors of carnitine octanoyltransferase (Ki = 261 and 295 microM respectively), but were only weakly inhibitory to carnitine palmitoyltransferase. Inhibition of energy production by salicylate may result from the inhibition of CAT by salicylyl-CoA.

Aggregation of submitochondrial particles by heparin and its application to the study of carnitine transport

A novel technique for the separation of submitochondrial particles from the external medium, an essential procedure in transport studies, was devised. Very low concentrations of heparin (5-10 micrograms/ml) aggregate the particles and permit their rapid sedimentation in a micro-centrifuge. The transfer of activated fatty acids into mitochondria for oxidation depends on the exchange of matrix carnitine for external fatty-acylcarnitine. To study the matrix face of the carnitine/acylcarnitine translocase, inverted submitochondrial particles were prepared and loaded with L-[14C]carnitine. As found in intact mitochondria, the Km value for L-carnitine was 8 mM, that for palmitoyl-L-carnitine was two orders of magnitude lower, and 11-trimethylaminoundecanoyl-DL-carnitine was a competitive inhibitor. The properties of the carrier exposed to the outer and to the matrix sides of the mitochondrial inner membrane are thus similar.

Carnitine long-chain acyltransferase and oxidation of palmitate, palmitoyl coenzyme A and palmitoylcarnitine by pea mitochondria preparations

Palmitoylcarnitine was oxidised by pea mitochondria.L-carnitine was an essential addition for the oxidation of palmitate or palmitoylCoA. When palmitate was sole substrate, ATP and Mg(2+) were also essential additives for maximum oxidation. Additions of CoA inhibited the oxidation of palmitate. It was shown that CoA was acting as a competitive inhibitor of the carnitine-stimulated O2 uptake. It is suggested that palmitoylacarnitine and carnitine passed through the mitochondrial barrier with ease but palmitoylCoA and CoA did not. The presence of carnitine long-chain acyl (palmitoyl)transferase (EC 2.3.1.21) in pea-cotyledon mitochondria was shown. This enzyme may play a role in the transport of long-chain acyl groups through membrane barriers.

Computer simulation of metabolism in palmitate-perfused rat heart. I. Palmitate oxidation

A computer model of the fatty acid oxidation pathway in perfused rat heart was constructed. It includes uptake, activation, and beta-oxidation of fatty acids, triglyceride synthesis and hydrolysis, and carnitine-dependent transport of acyl groups across the mitochondrial membrane under pseudosteady state conditions. Fatty acid utilization may be limited by beta-oxidation in hypoxia or ischemia but probably not in aerobic conditions. Nonesterified fatty acids bound to proteins are found to be metabolically available. The model predicts that stearate, but not palmitate, can support the highest observed respiration rate for perfused rat heart without supplementation by other substrates. Fatty acids are preferentially oxidized rather than being stored as triglycerides because the cystosolic acyl CoA level is lower than the Km for triglyceride synthesis. It is suggested that feedback inhibition of triglyceride lipase regulates utilization of triglycerides as fuel in aerobic hearts.

Palmitate oxidation in human muscle: comparison to CPT and carnitine

The evaluating of palmitate oxidation in muscle tissue may be a useful screening test for detecting defects in fatty acid metabolism in human neuromuscular disease. If the test is to be useful, it is necessary to obtain data on a wide variety of muscle illnesses for comparative purposes. We report our experience with palmitate oxidation, muscle carnitine, and carnitine palmityl transferase (CPT) activity in 148 muscles biopsies from a variety of illnesses. The efficacy of using total protein, citrate synthase, and (1-14C) pyruvate oxidation as internal references was investigated. Palmitate oxidation was significantly less than normal (P less than or equal to 0.01) in Duchenne muscular dystrophy, congenital nonprogressive myopathy, congenital muscular dystrophy, malignant hyperpyrexia, and denervation, depending on the internal reference used. Muscle carnitine levels followed a similar pattern, however, CPT activity did not. The possibility of these findings being secondary to inactivity is discussed.

Hepatic lipid metabolism in domestic fowl as influenced by dietary cereal

The influence of dietary cereal on hepatic lipid metabolism was studied in adult Japanese quail and mature female chickens fed isoenergetic and isonitrogenous corn-soy and wheat-soy diets ad libitum. Hepatic lipid accumulation and rate of biosynthesis were significantly higher in birds fed the corn-soy diet. The differential response to the cereals was seen in ovipositing females but not in mature male quail. There were no significant concomitant changes in body composition, egg production, egg weight, body weight, feed, and energy intake. Results of sequential changes in hepatic lipid metabolism showed that reduction in liver fat was significant 2 to 3 weeks after feeding the wheat diet and was due to a significant reduction in rate of lipogenesis and the amount of triacylglycerols deposited. Plasma levels of free fatty acids and glucose were not significantly altered by the dietary regimes in laying hens. In Japanese quail fed the corn-soy diet, plasma glucose was elevated and there was evidence of impaired glucose tolerance. In vitro rate of oxidation of labeled palmitate was significantly higher in liver homogenates from birds fed the corn-soy diet. Addition of carnitine to the incubation mixtures resulted in an increase of equivalent magnitude for both treatments. Liver hemorrhages were observed in laying hens but not in Japanese quail. Addition of myo-inositol to the corn diet for Japanese quail failed to significantly alter hepatic lipid accumulation. These studies show that differences in liver lipid accumulation due to corn and wheat diets are related to changes in rate of lipogenesis.