Non-enzymatic reduction of azo dyes by NADH

Nicotinamide adenine dinucleotide (NADH) reduces a variety of azo dyes by four electrons to generate the corresponding aromatic amines. This reduction is pH-dependent and increases with decreasing pH. Reduction of 4-(4′-sulfophenylazo)-phenol and 2-(4′-sulfophenylazo)-phenol, specifically substituted with methyl, methoxy, halo, and nitro groups, was examined to determine the susceptibility of azo dyes to reduction by NADH. Except for the nitro-substituted azo dyes, all other azo dyes were reduced. Possible mechanisms of reduction are proposed. The implications of our findings to microbial degradation and mammalian metabolism of azo dyes are discussed.