Contrasting oxygen-effects in the inactivation of ribonuclease A by N.3, (SCN).-2 and .OH radicals.

N.3 exhibits higher efficiency than .OH in the inactivation of RNase in de-acerated (neutral) aqueous solution. In O2-saturated solution the .OH-induced inactivation is enhanced, but N.3 and (SCN).-2 become remarkably inefficient. Our results suggest that semi-oxidized tyrosine, the predominant initial defect induced by N.3 and (SCN).-2 but not by .OH2 can be re-reduced upon reaction with O.-2 or cysteine.