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. Brain, low expression in ovary and placenta, but not found in liver, kidney, and skeletal muscle. The N-terminus is blocked. Belongs to the class-IV pyridoxal-phosphate-dependent aminotransferase family.