Molecular cloning and expression of human carnitine octanoyltransferase: Evidence for its role in the peroxisomal beta-oxidation of branched-chain fatty acids

To study the putative role of human carnitine octanoyltransferase (COT) in the beta-oxidation of branched-chain fatty acids, we identified and cloned the cDNA encoding human COT and expressed it in the yeast Saccharomyces cer evisiae. Enzyme activity measurements showed that COT efficiently converts one of the end products of the peroxisomal beta-oxidation of pristanic acid , 4,8-dimethylnonanoyl-CoA, to its corresponding carnitine ester. Productio n of the carnitine ester of this branched/medium-chain acyl-CoA within the peroxisome is required for its transport to the mitochondrion where further beta-oxidation occurs. In contrast, 4,8-dimethylnonanoyl-CoA is not a subs trate for carnitine acetyltransferase, another acyltransferase localized in peroxisomes, which catalyzes the formation of carnitine esters of the othe r products of pristanic acid beta-oxidation, namely acetyl-CoA and propiony l-CoA. Our results shed new light on the function of COT in fatty acid meta bolism and point to a crucial role of COT in the beta-oxidation of branched -chain fatty acids. (C) 1999 Academic Press.