A method for branched-chain amino acid aminotransferase activity in microgram and nanogram tissue samples

A method for branched-chain amino acid aminotransferase is described which is based on running the reaction in the reverse of the usual direction with glutamate and alpha-ketoisocaproate as substrates. The alpha-ketoglutarate generated is reduced with glutamate dehydrogenase and NADH. For sensitivity in the nanomole range, the NAD+ generated is measured directly by converting to the highly fluorescent strong alkali product. For smaller samples, down to the 0.2- to 2-pmol range, the NAD+ is amplified by enzymatic cycling.