UTILIZATION OF VOLATILE FATTY ACIDS IN RUMINANTS .3. COMPARISON OF MITOCHONDRIAL ACYL COENZYME A SYNTHETASE ACTIVITY AND SUBSTRATE SPECIFICITY IN DIFFERENT TISSUES

Studies were undertaken to establish the important rate-limiting steps controlling acetate, propionate, and butyrate metabolism in ruminants. As an initial approach to this problem, the activity of acetyl, of propionyl, and of butyryl coenzyme A synthetase in mitochondria from various ruminant tissues was studied. Mitochondria were frozen and thawed three times to solubilize the enzymes. The acyl coenzyme A synthetase preparation was partially purified and its substrate specificity was studied. The results show that the activity of the enzyme active on acetate is high in highly aerobic tissues such as heart and mammary gland and low in liver, in brain, in aorta, in skeletal muscle, and in rumen muscle. In heart, in mammary gland, in kidney, and in testes, the enzyme preparations were all about equally active on acetate and on propionate. In liver the enzyme preparation was equally active on propionate and on butyrate and least active on acetate. The enzyme from rumen epithelium was most active on butyrate, less active on propionate, and least active on acetate. Lung differs from all the other tissues in that the enzyme is very active on propionate and shows equal but lower activity for acetate and for butyrate. Marginal enzyme activity was found in skeletal muscle, in rumen muscle, in brain, and in aorta. The results offer an explanation for the preferential utilization of acetate by extrahepatic tissues, propionate by the liver, and butyrate by rumen epithelium. Butyrate, if present in peripheral blood, has a pharmacological effect on the pancreas. The ruminant has a unique mechanism for preventing butyrate appearance in peripheral blood, i.e., a high butyryl coenzyme synthetase activity in rumen epithelium ensures rapid activation of butyrate for subsequent oxidation to β-hydroxybutyrate by the epithelium. If some butyrate escapes metabolism by the rumen epithelium, it is readily removed from portal blood by the liver where a high butyryl coenzyme A synthetase activity ensures rapid activation and subsequent metabolism to ketone bodies. Substrate specificity studies show that in addition to the three wellknown acyl coenzyme A synthetases active on straight-chain fatty acids (acidiligase (adenosine monophosphate), EC 6.2.1.1, 6.2.1.2, and 6.2.1.3) there are at least three other acyl coenzyme A synthetases with distinct substrate specificities. The acyl coenzyme A synthetase from sheep liver is specific for C3 to C7 straight-chain fatty acids. The acyl coenzyme A synthetase from sheep kidney is specific for C2 to C7 straightchain fatty acids. Sheep lung has an acyl coenzyme A synthetase specific for propionate. This work along with earlier studies by others would suggest that there are at least six acyl coenzyme A synthetases in animal tissues with varying substrate specificities. © 1969, American Chemical Society. All rights reserved.