The reaction of menadione with haemoglobin. Mechanism and effect of superoxide dismutase

Abstract

Menadione was found to react with both the haem groups and the β-93 thiol groups of haemoglobin. It oxidized the haem groups of oxyhaemoglobin, giving mainly methaemoglobin and a smaller amount of haemichrome. The reaction rate was decreased in the presence of catalase and markedly accelerated in the presence of superoxide dismutase. It is proposed that the overall reaction involves the initial reversible formation of methaemoglobin and the semiquinone, and that the effect of superoxide dismutase is to prevent the reverse reaction, by removing superoxide and hence the semiquinone via the equilibrium: semiquinone+O2-quinone+)2(-). E.s.r. evidence for the formation of the semiquinone and its reactions is presented. The reaction of menadione with the α-93 thiol groups of haemoglobin appeared to be similar to that with other thiols, forming the 3-thioether derivate of menadione, but it was also accompanied by reduction of methaemoglobin. This reduction was prevented by superoxide dismutase, but appeared to be caused by the semiquinone radical, which was produced as an intermediate. Reduced glutathione functioned only to a limited extent as a scavenger of the menadione semiquinone. Its main reaction was directly with menadione to form the thioether. Ascorbate was a more efficient scavenger, and accelerated the oxidation of oxyhaemoglobin by menadione. The significance of these findings in relation to menadione-induced erythrocyte haemolysis is discussed.