Reactivity of nicotinamide-adenine dinucleotide dimer in plant phenol oxidase systems

The enzymie system in mung bean seedlings which earlier workers characterized as a diaphorase capable of oxidizing the 1,2- and 1,6-dihydropyridine isomers of NADH is, in fact, a phenol oxidase. The NAD species whose oxidation was observed is actually the dimeric 1-electron product obtained in the electrolytic reduction of NAD⁺ solutions. The unidentified “cofactor” required along with the enzyme for the oxidation of the dimer is probably a naturally occurring phenol. Similar activity is found in a variety of plants, including other types of beans as well as corn, cotton, and wheat. The dimer is also reactive with respect to a commercial mushroom phenol oxidase preparation. It cannot be stated whether the NAD dimer is in any sense a natural reactant in such systems, but the supposedly unusual mung bean activity is clarified.