LOCALIZATION AND SOLUBILIZATION OF A RAT-LIVER MICROSOMAL CARNITINE ACETYLTRANSFERASE

Carnitine acetyltransferase (EC 2.3.1.7) activity had been previously shown to occur in peroxisomes, mitochondria and a membranous fraction of rat and pig hepatocytes. When components of this 3rd subcellular fraction (plasma membranes, components of the Golgi apparatus and microsomes) were further separated, carnitine acetyltransferase fractionated with the microsomes. Microsomes isolated by 3 different methods (isopycnic sucrose density zonal centrifugation, high-speed differential centrifugation, and aggregation with Ca2+ followed by low-speed differential centrifugation) all contained carnitine acetyltransferase activity. The lability of carnitine acetyltransferase in microsomes isolated by different methods and in different isolation media is reported. When total microsomes were subfractionated into rough and smooth components, carnitine acetyltransferase activity was found to the same extent in both and was tightly associated with the microsomal membrane. The microsomal enzyme was rapidly inactivated in 0.25 M sucrose or 0.1 M phosphate, but was stable for at least 2 wk in 0.4 M KCl. Extensive treatment with high ionic strength salt solutions, 1% Trition X-100, or a combination of the 2 was used to solubilize microsomal carnitine acetyltransferase activity. Carnitine octanoyltransferase activity was also found in the microsomal fractions isolated by 3 different methods, but no carnitine palmitoyltransferase was detected in the microsomal fractions. Microsomal carnitine acetyl- and octanoyltransferases could be involved in the transfer of acyl groups across the microsomal membrane, thereby providing a source of acetyl and other acyl CoA''s at sites of acetylation reactions and synthesis.