EFFECT OF DIETARY-PROTEIN ON THE LIVER CONTENT AND SUBUNIT COMPOSITION OF THE BRANCHED-CHAIN ALPHA-KETOACID DEHYDROGENASE COMPLEX

Levels of expression of two subunits of the liver branched-chain alpha -ketoacid dehydrogenase complex in response to extremes of dietary pro tein intake (50% versus 0% protein diet) were determined by quantitati ve immunoblotting. Dietary protein deficiency decreased the amount of E1 alpha protein to a greater extent than E2 protein. The ratio of E1 alpha to E2 was below 1 in the liver of animals starved for protein an d above 1 in the liver of animals fed the high-protein diet. Supplemen tation of the O% protein diet with 5% leucine (but not 5% valine) had the same effect as the 50% protein diet. The extremes of dietary prote in also resulted in a divergent pattern of expression of the mRNAs for the subunits of the complex. The E1 beta message showed the expected corollary of being greater in the liver of the high-protein-fed rats t han the no-protein-fed rats. In contrast, the E2 message was not affec ted by the two extremes of dietary protein and the E1 alpha message wa s greater in the liver of the no-protein-fed rats than the high-protei n-fed rats. Thus, coordinate regulation of gene expression of the subu nits of the complex does not occur in response to dietary protein. Pos t-transcriptional regulatory mechanisms most likely determine the amou nt of the complex and the ratio of its subunits. The decrease in E1 al pha/E2 protein ratio that occurs in dietary protein deficiency may inc rease sensitivity of the complex to phosphorylation-mediated inhibitio n by branched-chain alpha-ketoacid dehydrogenase kinase. (C) 1994 Acad emic Press, Inc.