Catalyzes the first reaction in the catabolism of the essential branched chain amino acids leucine, isoleucine, and valine. 2-oxoglutarate + L-leucine = 4-methyl-2-oxopentanoate + L-glutamate 2-oxoglutarate + L-isoleucine = (S)-3-methyl-2-oxopentanoate + L-glutamate 2-oxoglutarate + L-valine = 3-methyl-2-oxobutanoate + L-glutamate Homodimer. Expressed in brain and kidney. Overexpressed in MYC-induced brain tumors, lymphomas, as well as in a teratocarcinoma cell line. Highly expressed at day 9 of embryogenesis. Expression decreases to moderate levels through day 13. In the developing embryo, expressed in the brain, somites and mesenophric tubules. Belongs to the class-IV pyridoxal-phosphate-dependent aminotransferase family.