DT-diaphorase-catalysed reduction of 1,4-naphthoquinone derivatives and glutathionyl-quinone conjugates. Effect of substituents on autoxidation rates

DT-diaphorase catalysed the reduction of 1,4-naphthoquinones with hydroxy, methyl, methoxy and glutathionyl substituents at the expense of reducing equivalents from NADPH. The initial rates of quinone reduction did not correlate with either the half-wave reduction potential (E1/2) value (determined by h.p.l.c. with electrochemical detection against an Ag/AgCl reference electrode) or the partition coefficient of the quinones. After their reduction by DT-diaphorase the 1,4-naphthoquinone derivatives autoxidized at distinct rates, the extent of which was influenced by the nature of the substituents. Thus for the 1,4-naphthoquinone series the following order of rate of autoxidation was found: 5-hydroxy-1,4-naphthoquinone greater than 3-glutathionyl-1,4-naphthoquinone greater than 5-hydroxy-3-glutathionyl-1,4-naphthoquinone greater than 1,4-naphthoquinone greater than 2-hydroxy-1,4-naphthoquinone. For the 2-methyl-1,4-naphthoquinone (menadione) series the following order was observed: 5-hydroxy-2-methyl-1,4-naphthoquinone greater than 3-glutathionyl-5-hydroxy-2-methyl-1,4-naphthoquinone greater than 3-glutathionyl-2-methyl-1,4-naphthoquinone greater than 2-methyl-1,4-naphthoquinone greater than 3-hydroxy-2-methyl-1,4-naphthoquinone. The autoxidized naphthohydroquinone derivatives were re-reduced by DT-diaphorase, thus closing a cycle of enzymic reduction in equilibrium autoxidation. This was expressed as an excess of NADPH oxidized over the initial concentration of quinone present as well as H2O2 formation. These findings demonstrate that glutathionyl conjugates of 1,4-naphthoquinone and 2-methyl-1,4-naphthoquinone and those of their respective 5-hydroxy derivatives are able to act as substrates for DT-diaphorase and that they also autoxidize at rates higher than those for the unsubstituted parent compounds. These results are discussed in terms of the cellular role of DT-diaphorase in the reduction of hydroxy- or glutathionyl-substituted naphthoquinones as well as the further conjugation of these hydroquinones with glucuronide or sulphate within the cellular milieu, thereby facilitating their disposal from the cells.