Reactivity of thiols towards derivatives of 2- and 6-methyl-1,4-naphthoquinone bioreductive alkylating agents.

The stoichiometry and kinetics of the anaerobic reactions between some thiols and derivatives of 2- and 6-methyl-1,4-naphthoquinones in water were measured using stopped flow spectrophotometry. The stoichiometry of the reaction with representative compounds was 1:2 thiol:quinone, a finding consistent with the formation of a hydroquinone as well as a thioether in the reaction. The first-order dependence of rate on thiol concentration, and the pH-dependent rate constants indicated that the thiolate anion was involved in the rate-limiting step, with rate constants at pH 7.6 generally increasing in the order glutathione (GSH) less than cysteamine less than dithiothreitol (DTT) less than cysteine. Despite the lower reactivity of GSH, the half-lives of the uncatalyzed conjugation reaction of these quinones at typical biological concentrations of GSH (e.g. 2 mM) ranged from about 2.0 to 20 s at pH 7.6 and 25 degrees C. The implications of these reactions in the use of naphthoquinones as potential bioreductive alkylating agents and as hypoxic cell radiosensitizers are discussed.