Biochemical effects of the hypoglycaemic compound pent 4 enoic acid and related non hypoglycaemic fatty acids. Effects of their coenzyme A esters on enzymes of fatty acid oxidation

Abstract

Pent 4 enoyl CoA and its metabolites penta 2,4 dienoyl CoA and acryloyl CoA, as well as n pentanoyl CoA, cyclopropanecarbonyl CoA and cyclobutanecarbonyl CoA, were examined as substrates or inhibitors of purified enzymes of β oxidation in an investigation to locate the site of inhibition of fatty acid oxidation by pent 4 enoate. The reactions of various acyl CoA derivatives with L carnitine and of various acyl L carnitine derivatives with CoA, catalysed by carnitine acetyltransferase, were investigated and V(max) and K(m) values were determined. Pent 4 enoyl CoA and n pentanoyl CoA were good substrates, whereas cyclobutanecarbonyl CoA, cyclopropanecarbonyl CoA and acryloyl CoA reacted more slowly. A very slow rate with penta 2,4 dienoyl CoA was detected. Pent 4 enoyl L carnitine, n pentanoyl L carnitine and cyclobutanecarbonyl L carnitine were good substrates and cyclopropanecarbonyl L carnitine reacted more slowly. Pent 4 enoyl CoA and n pentanoyl CoA were substrates for butyryl CoA dehydrogenase and for octanoyl CoA dehydrogenase, and both compounds were equally effective competitive inhibitors of these enzymes with butyryl CoA or palmitoyl CoA respectively as substrates. V(max), K(m) and K(i) values were determined. None of the acyl CoA derivatives inhibited enoyl CoA hydratase or 3 hydroxybutyryl CoA dehydrogenase. Penta 2,4 dienoyl CoA was a substrate for enoyl CoA hydratase when the reaction was coupled to that catalysed by 3 hydroxybutyryl CoA dehydrogenase. In a reconstituted sequence with purified enzymes crotonoyl CoA was largely converted into acetyl CoA, and pent 2 enoyl CoA into acetyl CoA and propionyl CoA. Penta 2,4 dienoyl CoA was slowly converted into acetyl CoA and acryloyl CoA. Penta 2,4 dienoyl CoA, a unique metabolite of pent 4 enoate, was the only compound that specifically inhibited an enzyme of the β oxidation sequence, 3 oxoacyl CoA thiolase. The formation of penta 2,4 dienoyl CoA could explain the strong inhibition of fatty acid oxidation in intact mitochondria by pent 4 enoate.