Glutamate dehydrogenase (GDH) is an important branch-point enzyme between carbon and nitrogen metabolism. GDH catalyzes the reversible NAD (P)+-linked oxidative deamination of glutamate into ?- ketoglutarate and ammonia. This colorimetric assay is based on GDH catalyzed oxidation of glutamate, in which the formed NADH reduces a terazolium salt MTT. The intensity of the product color, which exhibits an absorbance maximum at 570 nm, is proportional to the GDH activity in the sample.