Metabolism of amino acids in protein-calorie-deficient rats.

The overall oxidative degradation of leucine and phenylalanine, measured in vivo in rats fed on a 2%-casein diet for 8 weeks, is markedly decreased as compared with controls, whereas that of glutamate and alanine is apparently unaffected. The decrease in leucine degradation is due, at least in part, to a block before the formation of 3-methylbutyryl-CoA (isovaleryl-CoA) in the catabolic pathway. This phenomenon is accompanied by increased incorporation of [(14)C]leucine into liver proteins, decreased urinary excretion of leucine and increased urinary excretion of 4-methyl-2-oxopentanoate (alpha-oxoisocaproate) by protein-depleted animals. The results suggest the existence of adaptive mechanisms that function to conserve an indispensable carbon skeleton.