L-allo-Isoleucine

L-alloisoleucine is a branched chain amino acid and is a stereoisomer of L-isoleucine. It is a common constituent of human plasma (albeit at low levels) . L-alloisoleucine is produced as a byproduct of isoleucine transamination. Plasma L-alloisoleucine which is derived from L-isoleucine in vivo can be used for the diagnosis of maple syrup urine disease (MSUD) a genetic disorder. Indeed plasma L-alloisoleucine levels above 5 μmol/L is the most specific and most sensitive diagnostic marker for all forms of MSUD.