Chemical and biochemical reactions of superoxide O2sup(-.)

In recent work we have shown that O2sup(-.) produced by radiolytic and enzymatic methods can oxidise the haem groups in oxyhaemoglobin and reduce those in methaemoglobin: 2H+ + O2sup(-.) + haem-Fe2+-O2 → haem-Fe3+ + H2O2 + O2 (1) O2sup(-.) + haem-Fe3+ → haem-Fe2+-O2 (2). From equilibrium and kinetic studies we find that k1 = (4 +- 1) x 103 M-1s-1 and K1/L2 = 0.7 +- 0.2 at pH 7. Reaction (1) could put some stress on the red blood cell in which O2sup(-.) is produced, and it can be shown from these rate constants that sufficient superoxide dismutase (SOD) occurs in the red blood cell to prevent this. This seems a trivial and special need for SOD, and a more general role is being sought. (author)