NAD(P)H:flavin oxidoreductase of Escherichia coli. A ferric iron reductase participating in the generation of the free radical of ribonucleotide reductase.

The active form of one subunit of Escherichia coli ribonucleotide reductase (protein B2) contains an organic free radical localized to tyrosine 122 of its polypeptide chain. When this radical is scavenged, e.g. by treatment with hydroxyurea, the enzyme is inactivated (protein B2/HU). E. coli contains an enzyme system consisting of at least three proteins that in the presence of NADPH, FMN, dithiothreitol, and oxygen introduce the tyrosyl radical into B2/HU (Eliasson, R., Jörnvall, H., and Reichard, P. (1986) Proc. Natl. Acad. Sci. U. S. A. 83, 2373-2377). One of the three proteins was identified as superoxide dismutase. We now identify a second protein, previously provisionally named Fraction c, as an NAD(P)H:flavin oxidoreductase (flavin reductase). After 4,000-fold purification the protein moved as a single band on sodium dodecyl sulfate gel electrophoresis with a molecular weight of 28,000-29,000. The enzyme contained no flavin but reduced riboflavin, FMN, and FAD by NADH, or riboflavin and FMN by NADPH. It is a powerful ferric iron reductase. We propose that its complementing activity during radical generation involves participation in the reduction of the ferric iron center of protein B2/HU. Radical formation is then linked to the reoxidation of iron by oxygen. The flavin reductase may also participate in other aspects of iron metabolism of E. coli.