Fast repair of protein radicals by urate

Abstract

The repair of tryptophan and tyrosine radicals in proteins by urate was studied by pulse radiolysis. In chymotrypsin, urate repairs tryptophan radicals efficiently with a rate constant of 2.7×108M−1s−1, ca. 14 times higher than the rate constant derived for N-acetyltryptophan amide, 1.9×107M−1s−1. In contrast, no repair of tryptophan radicals was observed in pepsin, which indicates a rate constant smaller than 6×107M−1s−1. Urate repairs tyrosine radicals in pepsin with a rate constant of 3×108M−1s−1―ca. 12 times smaller than the rate constant reported for free tyrosine―but not in chymotrypsin, which implies an upper limit of 1×106M−1s−1 for the corresponding rate constant. Intra- and intermolecular electron transfer from tyrosine residues to tryptophan radicals is observed in both proteins, however, to different extents and with different rate constants. Urate inhibits electron transfer in chymotrypsin but not in pepsin. Our results suggest that urate repairs the first step on the long path to protein modification and prevents damage in vivo. It may prove to be a very important repair agent in tissue compartments where its concentration is higher than that of ascorbate. The product of such repair, the urate radical, can be reduced by ascorbate. Loss of ascorbate is then expected to be the net result, whereas urate is conserved.