Introduction
Maple syrup urine disease (MSUD; OMIM 248600)
is an autosomal recessive metabolic disorder caused
by defective activity of the branched-chain a-keto-acid
dehydrogenase (BCKD) complex which catalyses the
catabolism of the branched-chain amino acids (BCAA):
leucine, isoleucine, and valine (Zhang et al 1991). The
BCKD is a multimeric complex comprising three catalytic
components: a branched-chain a-keto-acid decarboxylase
(E1), a dihydrolipoyl transacylase (E2), and a