Inhibition of naphthohydroquinone autoxidation by DT-diaphorase (NAD(P)H:[quinone acceptor] oxidoreductase)

It has been reported that little redox cycling occurs during the reduction of 2-methyl-1,4-naphthoquinone by DT-diaphorase, suggesting that the reduction product, 2-methyl-1,4-naphthohydroquinone, does not readily undergo autoxidation. In the present study, however, it has been shown that DT-diaphorase, by virtue of its ability to re-reduce the naphthoquinone formed in the oxidation reaction, decreases the rate of autoxidation of 2-methyl-1,4- naphthohydroquinone. Therefore, the low rate of redox cycling observed does not reflect an intrinsic stability of the hydroquinone but inhibition of its autoxidation by the enzyme. Redox cycling of 2,3-dimethyl-, 2,3-dimethoxy- and 2-methoxy-1,4-naphthoquinone, and the autoxidation of their respective hydroquinones, were similarly inhibited by diaphorase. The concentration of the enzyme required for inhibition varied widely among the different compounds, and this was related to the autoxidation rate of the hydroquinone and the rate at which the corresponding quinone was reduced by diaphorase. The behaviour of 2-hydroxy-1,4-naphthoquinone was exceptional in that the rate of redox cycling increased with increasing levels of diaphorase and no inhibition of the autoxidation of the hydroquinone derived from this substance could be demonstrated, even at very high enzyme concentrations. The results of the present experiments indicate that the relative stability of naphthohydroquinones cannot be judged on the basis of studies involving reduction of the quinone by DT-diaphorase and suggest that current concepts on the role of this enzyme in the detoxification of quinones may need revision.