Glutamate dehydrogenase catalyzes the anaplerotic reaction below (see here also):

Glutamate +   NADP+ (or NAD⁺) + H2O <=> -Ketoglutarate + NADPH (or NADH) +  NH4⁺

Bacteria growing with ammonia as their sole nitrogen source use this reaction as the primary route for nitrogen assimilation. In animal cells, the reversible reaction can function in either direction. The enzyme is allosterically regulated. ATP or GTP inhibits its action.