Assay of γ-glutamyltransferase with amino acid dehydrogenases from Bacillus stearothermophilus as auxiliary enzymes

A spectrophotometric assay for serum γ-glutamyltransferase (γ-GT, EC 2.3.2.2) based on the increasing absorbance at 340 nm due to NADH formation is described. Using γ-glutamyl dipeptides as the donor substrate, amino acids formed by the γ-glutamyl transfer from the donor substrate to the acceptor substrate glycylglycine are stoichiometrically converted by the corresponding amino acid dehydrogenases from Bacillus stearothermophilus to produce their keto acids and NADH. γ-Glutamylalanine was found to be the most specific and sensitive donor substrate. The method can be mechanized, is free of interference and correlates well with conventional methods.