Regulation of Volatile Fatty Acid Uptake by Mitochondrial Acyl CoA Synthetases of Bovine Liver

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Abstract
Mitochondria of bovine liver contain acyl CoA synthetases necessary for the uptake of propionate, butyrate, and valerta; acetate is bound only weakly. Purification of these enzymes separated a distinct propionyl CoA synthetase highly specific for propionate and acrylate and a butyrate-activating fraction with broad substrate specificity for short and medium chain fatty acids. Evidence from kinetic studies and sucrose density centrifugation suggested that this latter reaction was composed of 2 enzymes, a butyryl CoA synthetase and a valeryl CoA synthetase. The apparent MW of the propionyl, butyryl and valeryl CoA synthetases were 72,000, 67,000 and 65,000. The Km of propionyl CoA synthetase for propionate, ATP and CoA were 1.3 .times. 10-3 M, 1.3 .times. 10-3 M and 6.3 .times. 10-4 M. Enzyme activity is regulated by the concentration of proprionate in portal blood. Relative to propionyl, butyryl or valeryl CoA synthetases little acetyl CoA synthetase could be demonstrated. In ruminants, hepatic metabolism is such that use of acetate as an energy source is minimum. This ensures that an alternative energy source to glucose, as acetate units, will reach the extrahepatic tissues. Separation of a distinct propionyl CoA synthetase regulated by the concentration of propionate in portal blood is signiicant because a primary role of ruminant liver is to synthesize glucose from ruminally derived propionate.

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