REGULATION OF THE LONG-CHAIN CARNITINE ACYLTRANSFERASES

Long-chain carnitine acyltransferases are a family of enzymes found in mitochondria, peroxisomes, and endoplasmic reticulum that catalyze th e exchange of carnitine for coenzyme A in the fatty acyl-CoA. Conversi on of the fatty acyl-CoA to fatty acylcarnitine renders the fatty acid more permeable to the various cellular membranes. The mitochondrial c arnitine palmitoyltransferases are considered important in the regulat ion of mitochondrial beta-oxidation of long-chain fatty acids. However , palmitoylcarnitine produced by peroxisomal carnitine octanoyltransfe rase or by microsomal carnitine palmitoyltransferase is not different from that produced by the mitochondrial enzyme. Therefore, for there t o be control of fatty acid oxidation by the long-chain carnitine acylt ransferases, there would have to be some mechanism to coordinately reg ulate these varied enzymes. The first system of regulation involves in hibition by malonyl-CoA, an intermediate in the synthesis of fatty aci ds. Malonyl-CoA inhibits long-chain carnitine acyltransferase activity by all three enzymes at similar concentrations in the physiological r ange. In addition, the mitochondrial and peroxisomal enzymes are known to be regulated at the level of mRNA transcription by a number of sha red factors. Although the microsomal enzyme is less well studied, ther e does, indeed, appear to be a pattern of coordinate regulation for th is system.