Localization and Function of External and Internal Carnitine Acetyltransferases in Mitochondria of Rat Liver and Pig Kidney

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Abstract
Subfractionation of rat liver mitochondria by the “digitonin method” revealed that carnitine acetyltransferase is present in two different compartments. The major part of the mitochondrial activity of this enzyme is found to be located in the inner mitochondrial compartment which contains also the soluble enzymes of the citric acid cycle, fatty acid oxidation and amino metabolism. At least 25% of the total mitochondrial activity of carnitine acetyltransferase can be assigned to the outer mitochondrial compartment. The localization of this “external” carnitine acetyltransferase corresponds thus to that of mitochondrial adenylate kinase. Carnitine dependent oxidation of acetyl‐CoA was studied in pig kidney mitochondria after treatment with low digitonin concentrations. It was found that oxidation of acetyl‐CoA in the presence of carnitine depends on the amount of carnitine acetyltransferase present in the outer mitochondrial compartment. Extraction of external carnitine acetyltransferase decreases the oxidation rate of acetyl‐CoA + carnitine. This may be partly restored by re‐addition of extracted external carnitine acetyltransferase. Relatively high digitonin concentrations (0.75 mg/10 mg mitochondrial protein) render inner mitochondrial membrane permeable to acetyl‐CoA. It is concluded that the inner membrane is a barrier to acetyl‐CoA and that transport of acetyl‐CoA across this membrane is mediated by the function of external and internal carnitine acetyltransferase.