Tyrosine nitration by simultaneous generation of (NO)-N-center dot and O-2(center dot) under physiological conditions - How the radicals do the job

Radiation chemical experiments demonstrate that the reaction of tyrosyl rad ical (TyrO(.)) with (NO2)-N-. yields 45 +/- 3% 3-nitrotyrosine and that a m ajor product of the reaction of TyrO(.) with (NO)-N-. is 3,3'-dityrosine. R adiolysis was used to generate (NO)-N-. and O-2(-.) in the presence of tyro sine and bicarbonate at pH 7.5 +/- 0.1. The nitration yield was found to be dose rate-dependent, and the yield per radical produced by pulse radiolysi s was identical to that obtained with authentic peroxynitrite, The proposed mechanism that accounts for the data is as follows: (i) In the presence of CO2 the reaction of (NO)-N-. with O-2(-.) yields 33% (NO2)-N-. and CO3-. w here the latter reacts rapidly with tyrosine to form TyrO; (ii) The formati on of 3-nitrotyrosine takes place via the reaction of (NO2)-N-. with TyrO(. ), which is the main process at high dose rates; and (iii) Under continuous generation of (NO)-N-. and O-2(-.) the formation of 3-nitrotyrosine is str ongly suppressed because of efficient scavenging of NO2, by tyrosine. The p roposed model shows that the highest nitration yield is obtained for simila r fluxes of (NO)-N-. and O-2(-.) and is completely inhibited upon excess pr oduction of O-2(-.) because of efficient scavenging of TyrO(.) by O-2(-.). The biological implications of these findings are discussed.