Of the four possible stereoisomers of isoleucine, only L-alloisoleucine and L-isoleucine were found by capillary gas chromatography in the plasma of two maple syrup urine disease (MSUD) patients, one with classical and one with variant MSUD. The relative plasma concentration ratios of L-alloisoleucine/L-isoleucine were 0.795 +/- 0.025 (+/- 95% confidence limits) and 0.637 +/- 0.016 in the classical- and variant-MSUD patients, respectively. The patients were also studied in the postabsorptive state with a 6-hr continuous infusion of L-[15N]leucine. In each patient plasma leucine 15N enrichment approximated plateau after 150 min, and there was rapid appearance of [15N]isoleucine and [15N]alloisoleucine which were identical at plateau, although in variant-MSUD patient [15N]alloisoleucine enrichment did not equal that of [15N]isoleucine until 240 min of infusion. These results offer direct in vivo evidence for the rapid equilibrium of plasma isoleucine and alloisoleucine through keto-enol tautomerization of alpha-keto-beta-methylvalerate.