Formation of L-alloisoleucine in vivo: an L-[13C]isoleucine study in man.
L-alloisoleucine (2S, 3R), a diastereomer of L-isoleucine (2S, 3S), 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 micromol/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.
Schadewaldt
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P
Diabetes Forschungsinstitut, Heinrich-Heine-Universität Düsseldorf, Germany.
Bodner-Leidecker
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A
Hammen
H W
HW
Wendel
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U
D000328:Adult; D002247:Carbon Isotopes; D005260:Female; D006801:Humans; D007532:Isoleucine / Q000096:biosynthesis*; D008297:Male; D008956:Models, Chemical; D012016:Reference Values
D002247:Carbon Isotopes; D007532:Isoleucine