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Absolute rate constants for the reaction of the thiyl radicals derived from cysteamine, cysteine, glutathione, penicillamine, and 2-mercaptopropionylglycine (Thiola) with the phenothiazines, promethazine and chlorpromazine and with ascorbate ion, have been measured by pulse radiolysis and have been found to lie in the range 0.03-1.3 x 109 I mol-I s-I. The reactions are considered to occur by an electron transfer mechanism. The possibility that, in biochemical systems, glutathione can act as a catalyst linking hydrogen atom and electron transfer reactions is suggested.Phenothiazines are widely used in psychiatric therapy. However, in spite of their considerable success, they may present several undesirable side effects, such as skin photosensitisation and visual disturbance.'e2 In vitro studies have shown that the radical-cation formed by one-electron oxidation of chlorpromazine (CZ") is a potent inhibitor of the enzymes ATPase and uridine diphosphate glucose NAD + oxido-red~ctase.~'~

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Absolute rate constants for the reaction of a variety of electrophilic free radicals with the pyrazoline derivatives aminopyrine and antipyrine have been measured by pulse radiolysis. In the case of aminopyrine the resulting radical cation is a particularly stable species ( E ~~~ 5.35 x 1 O3 dm3 mol-' cm-').Both compounds are readily oxidised to their respective radical-cations with the one-electron oxidation potential of antipyrine (E" 1 .I-I .6 V) being higher than that of aminopyrine (F 0.26-0.5 V). Studies of the reaction of the radical-cations with reducing agents suggest that aminopyrine in particular may prove to be a useful reference compound in studies of free radical one-electron oxidations.Free-radical intermediates have now been detected during the metabolism of many xenobiotics. ' In particular, the chemical