3-Acetylpyridine-adenine dinucleotide, reduced form (AcPdADH) is able to act as an alternative reductant in glutamate-synthase-catalysed glutamate synthesis. In the AcPdADH-dependent reaction, kcat and Km values for the other substrates are fourfold lower than those for the NADH-dependent reaction, and Km for AcPdADH is about 3 microM. AcPdADH acts as a competitive inhibitor with respect to NADH in NADH-dependent glutamate synthesis, with a Ki of 1 microM. Glutamate synthase catalyses NADH-dependent reduction of AcPdAD+. This appears to proceed by a substituted-enzyme (ping-pong) mechanism, with competitive substrate inhibition by NADH at high levels. The Km values for this reaction are 1.4 microM for NADH and 14 microM for AcPdAD+ and kcat is 51 s-1; Ki for NADH is about 10 microM. The latter findings suggest that NADH is capable of reducing the enzyme molecule in the absence of other substrates and that a reduced form of the enzyme can exist in the absence of bound NADH.