Regeneration of the tyrosyl radical in native or p-butoxyphenol-treated mouse ribonucleotide reductase R2 protein.

The regeneration of the tyrosyl radical in chemically reduced native or p-butoxyphenol-treated radical free forms of mouse ribonucleotide reductase R2 protein has been studied. Chemical reduction has been achieved by treatment with light-activated flavin compounds: deazaflavin, flavin mononucleotide, or deazaflavin with methylviologen as mediator. The admission of air to the flavin reduced mouse R2 protein results in regeneration of up to 59% of the initial tyrosyl radical contents, whereas not more than 6% could be regenerated in the p-butoxyphenol-treated form. The mixed-valent EPR signal generated in the p-butoxyphenol-treated mouse R2 protein is different from the spectrum observed after flavin reduction in the native mouse R2 protein, indicating that treatment of the protein with p-butoxyphenol results in a structural rearrangement of the diferric/radical site. The presence of 0.1 mM Fe(II) in the anaerobic protein/buffer solution significantly improves the regeneration of tyrosyl radical upon admission of air to the flavin reduced mouse R2 protein, but less to the protein treated with p-butoxyphenol.