Catalyzes the first reaction in the catabolism of the essential branched chain amino acids leucine, isoleucine, and valine (PubMed:8702755, PubMed:25653144, PubMed:17050531). May also function as a transporter of branched chain alpha-keto acids (By similarity). 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. Ubiquitous. The disease is caused by variants affecting the gene represented in this entry. A patient with hypervalinemia and hyperleucine-isoleucinemia was identified as compound heterozygote for Gln-170 (inherited from his father) and Lys-264 (inherited from his mother), both variants reduced the catalytic activity of the enzyme. After treatment with vitamin B6, a precursor of pyridoxal 5'-phosphate, a BCAT2 cofactor, the blood levels of branched chain amino acids, especially valine, were decreased and brain lesions were improved. Belongs to the class-IV pyridoxal-phosphate-dependent aminotransferase family.