Selective Inhibition by Clofibric Acid

shown to actively utilize branched chaini amino acids and to produce alanine. The coniversioni of [1-"C]leucine to 14CO2 or to the [1-14C]keto-acid of leucine (a-keto-isocaproate) was linear for at least 2 h of incubation; the production of "CO2 from [1-'4C]leucine was saturable with a Km = 6.3 mM and a maximiium oxidation rate (Vmax) = 31 nmiol/mg protein per 120 min. Clofibric acid selectively inhibited the oxidation of [1-_4C]leucine (K, = 0.85 mM) and [U-'4C]isoleucine, but had no effect on the oxidation of [U-'4C]glutamate, -alanine, -lactate, or -palmitate. The inhibition of [1-14C]leucine oxidation by clofibrate was also observed in the rat quarter-diaphragm preparation. Clofibrate primarily inhibited the productioin of 14CO2 and had relatively little effect on the production of [1-14C]keto-acid of leucine. A physiological coincentration3.0 g/100 ml-of albumin, which actively binds clofibric acid, inhibited but did not abolish the effects of a 2-mM concentration ofclofibric acid on leucine oxidation. Clofibrate treatment stimulated the net consumption of pyruvate, and inhibited the net production of alanine. The drug also increased the cytosolic NADH/ NAD+ ratio as reflected by an increase in the lactate/ pyruvate ratio, in association with a decrease in cell Dr. Pardridge was the recipienit of Clinical Investigator Award AM-00409. Received for publication 19 October 197.9 and in revised fornm 7 March 1980. 88 aspartate levels. The changes in pyruvate metabolism and cell redox state induced by the drug were delayed compared with the nearly immediate inhibition of leucine oxidation. These studies suggest that clofibric acid, in concenitrations that approximate high therapeutic levels of the drug, selectively inhibits branched chain amino acid oxidation, possibly at the level of the branched chaiin keto-acid dehydrogeinase.