Role of leucine as a precursor of glutamine alpha-amino nitrogen in vivo in humans.

To assess the role of leucine as a precursor of glutamine alpha-amino N in vivo, five healthy men received primed continuous intravenous infusions of L-[15N]leucine, L-[15N]alanine, and L-[2-15N]glutamine in the postabsorptive state on three separate days and over 7, 4, and 4 h, respectively. A steady state in isotopic enrichment was observed in every amino acid during the last hour of each isotope infusion and was used to calculate the rates of amino acid appearance (Ra) and interconversion. Leucine, alanine, and glutamine Ra values were 137 +/- 18, 234 +/- 42, and 320 +/- 18 (SD) mumol.kg-1.h-1, respectively. Rates of N transfer from leucine to glutamine and alanine were 29 +/- 9 and 29 +/- 11 mumol.kg-1.h-1, respectively; these rates represented 21 +/- 8 and 21 +/- 12%, respectively, of leucine's N and 9 +/- 3 and 13 +/- 4% of glutamine's alpha-amino N and alanine N, respectively. These results suggest that the endogenous branched-chain amino acids released by protein breakdown are major precursors of both glutamine and alanine de novo synthesis in postabsorptive humans.