Metabolic fate of chemical mixtures. I. ?Shuttle oxidant? effect of lipoxygenase-generated radical of chlorpromazine and related phenothiazines on the oxidation of benzidine and other xenobiotics

Abstract

Many carcinogens, mutagens, teratogens, and other toxicants are known to be oxidized by lipoxygenases to potentially deleterious free radical intermediates. In this study, we tested for the first time the possibility that certain efficient substrates for lipoxygenase produce shuttle oxidants that stimulate the generation of reactive species from other chemicals. To evaluate the hypothesis, we investigated the metabolic interaction of two well-known substrates, chlorpromazine and benzidine, which have been shown to be oxidized by soybean lipoxygenase in the presence of hydrogen peroxide. The evidence presented here clearly indicates that the chlorpromazine cation radical generated by the lipoxygenase triggers a rapid oxidation of benzidine to benzidine diimine. Under the experimental conditions employed, the metabolic interaction resulted in a 42-fold stimulation in the rate of benzidine oxidation. The magnitude of stimulation of benzidine oxidation exhibited a dependence on the pH of the reaction medium, amount of the enzyme, and concentration of chlorpromazine, benzidine, and hydrogen peroxide. A number of other phenothiazines were also found to stimulate benzidine oxidation, albeit to a lesser degree. The chlorpromazine cation radical stimulated the oxidation of all six other xenobiotics tested. The highest stimulation (94-fold) was noted with tetramethyl phenylenediamine oxidation to the Wursters blue radical, while the lowest stimulatory response (2-fold) was observed with guaiacol. Preliminary data suggest that purified human term placental lipoxygenase also displays a similar stimulatory response in the benzidine oxidation in the presence of chlorpromazine. Although the toxicological significance of these in vitro findings remains to be established, it is worth pondering whether such a synergistic interaction occurs in humans in vivo. Teratogenesis Carcinog. Mutagen. 20:195-208, 2000.