GLUTAMATE DEHYDROGENASE (GLDH) is an enzyme which catalyzes the interconversion of glutamate and a-ketoglutarate. Elevated blood serum GLDH levels indicate liver damage; thus, GLDH plays an important role in the diagnosis of liver disease, especially in combination with aminotransferases. Transgenic plants expressing microbial GLDHs are improved in tolerance to herbicide, water deficit, and pathogen infections. This non-radioactive, colorimetric GLDH assay is based on the reduction of the tetrazolium salt MTT in a NADH-coupled enzymatic reaction to a reduced form of MTT which exhibits an absorption maximum at 565 nm. The increase in absorbance at 565 nm is directly proportional to the enzyme activity.