the blood BCAA levels are normal [1]. The overall incidence of MSUD in the general population is 1:185,000 [1], and in certain population groups, such as Mennonites of Pennsylvania, the incidence is estimated to be as high as 1:176 [2].
The BCKDH complex, the deficient enzyme in MSUD, consists of three catalytic proteins, a decarboxylase (E1), a dihydrolipoyl transacylase (E2), and a dihydrolipoyl dehydrogenase (E3). The E1 component is a heterotetramer composed of two α and two β subunits [3]. The E1 and E2 components are specific to BCKDH, whereas E3 is also used by pyruvate and α-ketoglutarate dehydrogenase complexes [3] and the glycine cleavag