On the mechanism of L-alloisoleucine formation: studies on a healthy subject and in fibroblasts from normals and patients with maple syrup urine disease.

L-Alloisoleucine formation from L-isoleucine was studied in vitro and in vivo. When a healthy subject was loaded with L-isoleucine, plasma levels of L-isoleucine and 3-methyl-2-oxopentanoate (KMV), as well as L-alloisoleucine, increased. Peak values were reached successively and were in the order L-isoleucine much greater than KMV much greater than L-alloisoleucine. Metabolic clearance of L-isoleucine and KMV was rapid; clearance of L-alloisoleucine was considerably delayed. When human skin fibroblast cultures were challenged with L-isoleucine, KMV accumulated at a gradually decreased rate, whereas L-alloisoleucine accumulated at a gradually accelerated rate. KMV and L-alloisoleucine formation were related and depended on the L-isoleucine concentration applied. In cell lines derived from MSUD patients (classical form), metabolite formation was only about 2-fold higher than in control strains. The relatively small difference between normal and MSUD fibroblasts in vitro as opposed to the striking differences between healthy subjects and MSUD patients in vivo are discussed with respect to the significance of physiological mechanisms participating in the formation and degradation of L-alloisoleucine in man.