Reduction of hypervalent states of myoglobin and hemoglobin to their ferrous forms by thymoquinone: The role of GSH, NADH and NADPH

The reactivity of thymoquinone towards different redox states of hemoglobin and myoglobin in the presence of GSH, NADH, and NADPH was evaluated by optical spectral analysis. Thymoquinone reduces the ferryl forms (Hb(IV)/Mb(IV)) of both met-hemoglobin (Hb(III)) and metmyoglobin (Mb(III)) to oxy-hemoglobin (Hb(II)O(2)) and oxy-myoglobin (Mb(II)O(2)) under physiological conditions. The reaction is mediated by the intermediate quinone forms of TQ, that is, glutathionyl-dihydrothymoquinone (DHTQ-GS) and dihydrothymoquinone (DHTQ), formed from direct interaction of TQ with GSH or NADH (NADPH). In vitro incubation of oxidized human erythrocytes with TQ, DHTQ, and the GSH/TQ mixture reduces the intracellular met-Hb at different rates. In the present study, we report that TQ and its reduced derivatives can also prevent lipid peroxidation induced by the MbFe(III)/H2O2 system. In this system, lipid peroxidation is induced by Mb(IV) or a putative Mb(IV)/Mb(VI) composite; it is plausible that the antioxidant function of TQ derivatives is related to their ability to reduce these oxidizing species. This is of particular biological significance, as natural quitiones may participate in reducing processes that lead to recovery of hemoglobin and myoglobin during oxidative stress. (C) 2007 Elsevier B.V. All rights reserved.