The function of superoxide dismutase during the enzymatic formation of the free radical of ribonucleotide reductase.

An enzyme system from Escherichia coli activates an inactive form of ribonucleotide reductase by transforming a tyrosine residue of the enzyme into a cationic free radical. The process requires NAD(P)H, a flavin, dithiothreitol, and oxygen and at least three proteins. After purification to near homogeneity two of the proteins were identified as superoxide dismutase and NAD(P)H:flavin oxidoreductase (Fontecave, M., Eliasson, R., and Reichard, P. (1987) J. Biol. Chem. 262, 12325-12331). The nature of the third protein, provisionally named Fraction b, is unknown. The flavin reductase is believed to reduce the ferric iron center of the ribonucleotide reductase as a prerequisite for radical generation. Here we demonstrate that the flavin reductase under aerobic conditions generates superoxide anions which inactivate ribonucleotide reductase. Superoxide dismutase protects the enzyme or a sensitive intermediate formed during the generation of the tyrosyl radical from the harmful effects of superoxide. Hydrogen peroxide, formed by superoxide dismutase, is also harmful. In this case, catalase present in Fraction b might protect the system. Fraction b has, however, an additional unknown function in the overall process of radical generation.