A. E. Harper

Effect of leucine—Isoleucine antagonism on the amino acid pattern of plasma and tissues of the rat

Abstract The leucine, isoleucine, and valine patterns of blood plasma, intestinal contents, intestinal wall, liver, and skeletal muscle of rats were determined at various time intervals over a period of 12 days, during which the animals consumed ad libitum a diet containing enough leucine to cause a leucine—isoleucine and valine antagonism. Two control groups were used for comparison: one was fed a basal diet with no added leucine; the other was fed the high leucine diet supplemented with sufficient isoleucine and valine to alleviate the antagonism. The concentrations of isoleucine and valine in blood plasma from rats fed excess leucine were consistently lower than the respective concentrations in plasma from either of the control groups. Leucine concentration in plasma was greatly elevated when the dietary intake of leucine was high. The pattern for these three amino acids in muscle was similar to that in plasma. The pattern in liver was different from that in muscle or plasma and resembled that in intestinal wall and contents. It therefore appears that feeding a high leucine diet to rats results in normal isoleucine and valine concentrations in the intestine and liver, but markedly reduces concentrations of these amino acids in the skeletal muscle and systemic blood plasma.

The influence of thyroxine and thiouracil on rats fed excess tyrosine.

Summary Addition of thiouracil to a high tyrosine diet alleviated signs of tyrosine toxicity in the rat, whereas, daily injections of thyroxine aggravated them. Plasma tyrosine concentration and liver tyrosine transaminase activity were high in rats fed a high tyrosine diet; thyroxine administration increased them further, but depressed slightly the activity of liver p-hydroxyphenylpyruvate hydroxylase.