Valine metabolism in vivo: effects of high dietary levels of leucine and isoleucine.

The short-term effects of feeding rats high levels of L-leucine or L-isoleucine on valine metabolism in vivo have been investigated. Consumption of a low-protein diet containing an additional 5% of leucine resulted in depression within one hour of the plasma concentrations of isoleucine, valine, alpha-keto-beta-methylvalerate, and alpha-ketoisovalerate. Concurrently with these changes in blood branched-chain amino acids and branched-chain ketoacids was a rapid increase (51%) in whole-body L-[1-14C]-valine oxidation. Studies with intragastrically administered leucine solutions indicated that the depressions in blood concentrations of valine occurred over the same time period as the stimulation in valine oxidation. In contrast, consumption of a low-protein diet containing an additional 5% of isoleucine had no significant effect on the plasma concentrations of leucine, valine, and alpha-ketoisocaproate; a significant (P less than 0.01) depression in the plasma concentration of alpha-ketoisovalerate was observed three hours after the diet containing excess isoleucine had been consumed. In contrast to the results obtained with excess leucine, consumption of excess isoleucine had no significant effect on the rate of valine oxidation in vivo. As part of an effort to explain the leucine-induced depletion of plasma valine and stimulation of valine oxidation, liver and muscle branched-chain aminotransferase and liver branched-chain ketoacid dehydrogenase activities were measured. Consumption of excess leucine had no significant effect on either muscle or liver aminotransferase activities, but was associated with a greater than two-fold increase in hepatic dehydrogenase activity.(ABSTRACT TRUNCATED AT 250 WORDS)