BOTH INDUCTION AND ACTIVATION OF THE BRANCHED-CHAIN 2-OXO ACID DEHYDROGENASE COMPLEX IN PRIMARY-CULTURED RAT HEPATOCYTES BY CLOFIBRATE

Clofibrate administration to rats caused both the activation and induction of the branched-chain 2-oxo acid dehydrogenase complex in the liver; the former phenomenon occurred within the first 6 h after clofibrate administration whereas the latter occurred after 12 h. Essentially the same results were obtained with primary cultures of rat hepatocytes in the presence of 0.5 mM clofibrate, though about three-fourths of the enzyme complex in control cells (without clofibrate addition) was inactivated during a culture for 44 h, with little reduction of the enzyme amount. This was also confirmed by immunotitration analysis with antibodies raised against the purified decarboxylase and transacylase components of the enzyme complex. On the other hand, the activity of dihydrolipoamide dehydrogenase (a constituent of the complex) was little affected by clofibrate administration. The half lives of the decarboxylase and transacylase components in the primary cultures were estimated to be in the range of 22-26 h, and were unchanged in the presence of clofibrate, when determined with the use of cycloheximide and by a pulse-chase experiment. On the contrary, the rates of synthesis of these two enzyme components had increased to about 1.9-fold after 32 h cultivation in the presence of clofibrate. Thus, the increase in the synthesis of both the components resulted in induction of the complex.