Glutamate dehydrogenase catalyzes the reduction of a Schiff base (delta 1-pyrroline-2-carboxylic acid) by NADPH.

alpha-Iminoglutarate has long been postulated as an obligatory intermediate in the glutamate dehydrogenase catalyzed reaction, but direct proof of its participation is lacking. We report here the glutamate dehydrogenase catalyzed reduction of delta 1-pyrroline-2-carboxylic acid (a cyclic-alpha-imino acid) to proline (an alpha-amino acid). The catalysis occurs at the normal catalytic site of the enzyme. The imine and the enzyme-NADPH complex are the active oxidant and reductant, respectively. The latter is about 500 times more reactive than NADPH itself. These findings provide direct evidence that the glutamate dehydrogenase catalyzed reaction does indeed proceed by way of an enzyme-bound form of an alpha-iminocarboxylic acid.