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.