Whole liver and isolated liver mitochondria are able to release free acetate, especially under conditions of increased fatty acid oxidation. In the present paper it is shown that rat liver contains acetyl-CoA deacylase (EC 3.1.2.1) activity (0.72,umol/min per g wet wt.of liver at 30°C and 0.5 mM-acetyl-CoA). At 0.5 mM-acetyl-CoA 73 % of total enzyme activity was found in the mitochondria, 8 % in the lysosomal fraction and 19% in the postmicrosomal supernatant. Mitochondrial subfractionation shows that mitochondrial acetyl-CoA deacylase activity is restricted to the matrix space. Mitochondrial acetylCoA deacylase showed almost no activity with either butyryl-or hexanoyl-CoA. AcetylCoA hydrolase activity from purified rat liver lysosomes exhibited a very low affinity for acetyl-CoA (apparent Km >15 mm compared with an apparent Km value of 0.5 mm for the mitochondrial enzyme) and reacted at about the same rate with acetyl-, n-butyryland hexanoyl-CoA. We could not confirm the findings of Costa & Snoswell [(1975) Biochem. J. 152, 167-172] according to which mitochondrial acetyl-CoA deacylase was considered to be an artifact resulting from the combined actions of acetyl-CoA-Lcarnitine acetyltransferase (EC 2.3.1.7) and acetylcarnitine hydrolase. The results are in line with the concept that free acetate released by the liver under physiological conditions stems from the intramitochondrial deacylation of acetyl-CoA.

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