A mitochondrial carnitine acylcarnitine translocase system.

Acetylation of added (-)carnitine by heart mitochondira coupled to the oxidation of pyruvate in the presence of malonate was inhibited, apparently competitively, by long chain acyl(+)carnitines although the activity of carnitine acetyltransferase (EC 2.3.1.7) itself was not affected. Mitochondria have been found to possess a translocase system that allows the transport of carnitine and acylcarnitines by exchange diffusion, and interaction with this transport appears to be the cause of long chain acyl(+)carnitine inhibition. These c-nclusions are based on the following observations: (a) exposure of intact, but not of denatured or disrupted, mitochondria to [14-C]carnitine efflux was saturable at low lwvels of carnitine or acylcarnitines, showed temperature dependence, and was more rapid with acyl(-)carnitines than with acyl(+)carnitines--such stereospecificity was not noticeable with free carnitine; (c) long chain acyl-(+)carnitines inhibited the carnitine-carnitine exchange and higher concentrations of carnitine decreased this inhibition; (d) direct estimations showed the presence of endogenous (-)carnitine in mitochondria that effluxed by freezing and thawing of mitochondria; (e) the amount of total endogenous (-)carnitine present was not affected by prior exposure of mitochondria to (-)carnitine or acetyl(-)carnitine. These results indicate that the carnitine-dependent translocation of acyl groups across mitochondrial inner membrane involves the participation of a carnitine acylcarnitine translocase system.