Formation of L-alloisoleucine in vivo: an L-[13C]isoleucine study in man.

L-Alloisoleucine (2 S, 3 R), a diastereomer of L-isoleucine (2 S, 3 S), is a normal constituent of human plasma. Considerable amounts accumulate in maple syrup urine disease, in which the branched-chain 2-oxo acid dehydrogenase step is impaired. The mechanism of L-alloisoleucine formation, however, is unclear. We addressed this issue by performing oral L-[1-13C]isoleucine loading (38 μmol/kg body wt, 50% 1-13C) in overnight-fasted healthy subjects (n = 4) and measuring the 3-h kinetics of 13C-label incorporation into L-isoleucine plasma metabolites. Compared with L-isoleucine, the time course of 13C-enrichment in the related 2-oxo acid, S-3-methyl-2-oxopentanoate, was only slightly delayed. Peak values, amounting to 18 ± 4 and 17 ± 3 mol percent excess, respectively, were reached within 35 and 45 min, respectively. The kinetics of 13C-enrichment in S- and R-3-methyl-2-oxopentanoate enantiomorphs were similar and linearly correlated (p 0.001). In L-alloisoleucine, however, 13C-label accumulated only gradually and in minor amounts. Our results indicate that R-3-methyl-2-oxopentanoate is an immediate and inevitable byproduct of L-isoleucine transamination and further suggest that alloisoleucine is primarily formed via retransamination of 3-methyl-2-oxopenanoate in vivo.