Reaction kinetics of 4-methoxy ortho benzoquinone in relation to its mechanism of cytotoxicity: a pulse radiolysis study.

Rate constants quantifying the reactivity of 4-methoxy ortho benzoquinone, formed in the metabolic activation of 4-hydroxyanisole, a possible melanocytotoxic drug under current assessment as a treatment for malignant melanoma, have been determined by pulse radiolysis. The quinone is reactive towards the thiols cysteine (k = 3.5x10(5)M-1sec-1), glutathione (k = 3.1x10(5)M-1sec-1) and dithiothreitol (k = 3.5x10(5)M-1sec-1), but relatively unreactive towards other nucleophiles such as arginine (k less than or equal to 1M-1sec-1) and glutamine (k less than or equal to 1M-1sec-1). Redox exchange with ascorbate also occurs (k = 1.0x10(4)M-1sec-1). In view of the low reactivity of 4-methoxy ortho benzosemiquinone towards oxygen (k less than or equal to 10(5)M-1sec-1) and the model lipid trans-2-butenoic acid (k less than or equal to 2x10(5)M-1sec-1), it is unlikely that initiation of lipid peroxidation by the semiquinone is a major source of cytotoxicity. A more likely toxicity pathway appears to be covalent addition reactions of 4-methoxy ortho benzoquinone with cellular nucleophiles, especially thiols, and/or redox exchange reactions of the quinone leading to antioxidant depletion.