On the mechanism ofl-alloisoleucine formation: Studies on a healthy subject and in fibroblasts from normals and patients with maple syrup urine disease

Summary l-Alloisoleucine formation froml-isoleucine was studiedin vitro andin vivo. When a healthy subject was loaded withl-isoleucine, plasma levels ofl-isoleucine and 3-methyl-2-oxopentanoate (KMV), as well asl-alloisoleucine, increased. Peak values were reached successively and were in the orderl-isoleucine > > KMV > >l-alloisoleucine. Metabolic clearance ofl-isoleucine and KMV was rapid; clearance ofl-alloisoleucine was considerably delayed. When human skin fibroblast cultures were challenged withl-isoleucine, KMV accumulated at a gradually decreased rate, whereasl-alloisoleucine accumulated at a gradually accelerated rate. KMV andl-alloisoleucine formation were related and depended on thel-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 fibroblastsin vitro as opposed to the striking differences between healthy subjects and MSUD patientsin vivo are discussed with respect to the significance of physiological mechanisms participating in the formation and degradation ofl-alloisoleucine in man.