(2R,3R)-2,3-dihydroxy-3-methylpentanoate + NADP(+) = (S)-2-ethyl-2-hydroxy-3-oxobutanoate + NADPH + H(+)
(2r,3r)-2,3-dihydroxy-3-methylpentanoate + nadp(+) = (s)-2-ethyl-2-hydroxy-3-oxobutanoate + nadph + h(+)