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

L-Alloisoleucine (2S, 3R), a diastereomer of L-isoleucine (2S, 3S), is a no rmal constituent of human plasma. Considerable amounts accumulate in maple syrup urine disease, in which the branched-chain 2-oxo acid dehydrogenase s tep is impaired. The mechanism of L-alloisoleucine formation, however, is u nclear. We addressed this issue by performing oral L-[1-C-13]isoleucine loa ding (38 mu mol/kg body wt, 50% 1-C-13) in overnight-fasted healthy subject s (n = 4) and measuring the 3-h kinetics of C-13-label incorporation into L -isoleucine plasma metabolites. Compared with L-isoleucine, the time course of C-13-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, respec tively. The kinetics of C-13-enrichment in S- and R-3-methyl-2-oxopentanoat e enantiomorphs were similar and linearly correlated (p much less than 0.00 1). In L-alloisoleucine, however, C-13-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 fu rther suggest that alloisoleucine is primarily formed via re-transamination of 3-methyl-2-oxopenanoate in vivo.