Carnitine Palmityltransferase

Abstract

Abstract Carnitine palmityltransferase in rat liver was investigated by three different assay methods. Intracellular distribution studies showed that carnitine palmityltransferase was an exclusively mitochondrial enzyme. Digitonin fractionation of rat liver mitochondria demonstrated that carnitine palmityltransferase had a dual localization within the mitochondria. The enzymatic activity catalyzing the formation of palmityl-l-carnitine from palmityl-CoA and l-carnitine, designated carnitine palmityltransferase A, was loosely bound to the external surface of the inner membrane. Digitonin treatment released this enzymatic activity from mitochondria resulting in a preparation which had a severely limited ability for carnitine-dependent palmityl-CoA oxidation, but retained the ability to oxidize palmityl-l-carnitine. The other mitochondrial enzymatic activity, carnitine palmityltransferase B, catalyzed the formation of intramitochondrial palmityl-CoA from exogenous palmityl-l-carnitine and endogenous CoA. It was firmly bound to the inner membranes and not released by digitonin. An unusual biphasic CoA-saturation curve was observed for mitochondrial carnitine palmityltransferase. The possibility was explored that the biphasic CoA-saturation curve reflected different CoA affinities for the two carnitine palmityltransferases in mitochondria. CoA-saturation curves were done on fractions isolated following digitonin treatment of mitochondria which were enriched with either carnitine palmityltransferase A or B activity. In these crude fractions half-maximal activity occurred at CoA concentrations which differed by only about 0.5 mm CoA. Therefore, following fractionation, instead of a magnification of differences in CoA affinity, suggested by the biphasic curve, the observed values revealed no significant differences. The designations, carnitine palmityltransferase A and B, represent enzymatic activities in two different intramitochondrial compartments with different functional roles. Further evidence is needed to determine if these are isoenzymes. A scheme is proposed for the role of the two carnitine palmityltransferase activities in the oxidation of long chain fatty acids.