Metabolism of orally administered branched-chain alpha-keto acids.

The changes in serum branched-chain alpha-keto acid (BCKA) and plasma amino acid concentrations, in response to a therapeutic oral dose of an essential amino acid/keto acid mixture, were studied in fasting healthy adults. Of the branched-chain amino acids (BCAA), only the plasma leucine concentration rose significantly despite increases in al three serum BCKA concentrations. The plasma valine concentration tended to rise, but plasma isoleucine concentrations fell. When KMVA (keto-isoleucine) alone was given, there followed an increase in plasma isoleucine concentration and a fall in valine and leucine. Similarly, when KIVA (keto-valine) was given, plasma valine rose and leucine and isoleucine fell. These results suggest some transamination of the keto acid with amino groups of the other BCAA. KICA (keto-leucine), however, produced larger falls in plasma valine and isoleucine than was expected from the rise in leucine. In addition, KICA caused significant, insulin-independent reductions in plasma threonine, serine, cystine, methionine, tyrosine, phenylalanine, and alanine. We conclude that although orally administered BCKA's will increase the BCAA supply, their value may not simply relate to the supply of essential amino acids for protein synthesis but to a direct effect of KICA on protein metabolism.