Carnitine:acylcarnitine translocase of rat heart mitochondria. Competition for carnitine uptake by carnitine esters.

The kinetic behavior of the carnitine:acylcarnitine translocase was studied in isolated rat heart mitochondria. The kinetic parameters, Km(apparent) and Vmax, for carnitine were determined by measuring the rates of influx of [14C]carnitine using two different methods to quench the exchange reaction. The range of the Km(app) was 0.38-1.50 mM and the Vmax was 0.20-0.34 nmol/mg . min by both methods. Carnitine esters of acetyl isobutyryl, and octanoyl groups were competitive with carnitine for uptake and Ki values for these esters were 1.1, 2.6, and 0.10 mM, respectively. The Km(app) for carnitine was increased in the presence of these carnitine esters, while the Vmax for carnitine was unchanged. Distribution of radiolabel from free [14C]carnitine into acetylcarnitine, isobutyrylcarnitine, and octanoylcarnitine during the incubations was examined by thin layer chromatography and was negligible. The Km values for carnitine and the Ki value for acetylcarnitine are within the concentration ranges of these compounds in the intact heart (Idell-Wenger, J. A., Grotyohann, L. W., and Neely, J. R. (1978) J. Biol. Chem. 253, 4310-4318). the Ki values for isobutyrylcarnitine and octanoylcarnitine may also be within their concentration ranges in vivo, but exact concentrations in heart muscle are not known. These data support the concept that carnitine esters of short (acetylcarnitine), branched (isobutyrylcarnitine), and medium (octanoylcarnitine) chain acyl groups compete with free carnitine for transport into the mitochondria under physiological conditions.