Tyrosine Nitration by Simultaneous Generation of ⋅NO and O⨪2 under Physiological Conditions

Abstract

Radiation chemical experiments demonstrate that the reaction of tyrosyl radical (TyrO ) with NO 2 yields 45 3% 3-nitrotyrosine and that a major product of the reaction of TyrO with NO is 3,3-dityrosine. Radiolysis was used to generate NO and O 2 . in the presence of tyrosine 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 radiolysis was identical to that obtained with authentic peroxynitrite. The proposed mechanism that accounts for the data is as follows: (i) In the presence of CO 2 the reaction of NO with O 2 . yields 33% NO 2 and CO 3 . , where the latter reacts rapidly with tyrosine to form TyrO ; (ii) The formation of 3-nitrotyrosine takes place via the reaction of NO 2 with TyrO , which is the main process at high dose rates; and (iii) Under continuous generation of NO and O 2 . , the formation of 3-nitrotyrosine is strongly suppressed because of efficient scavenging of NO 2 by tyrosine. The proposed model shows that the highest nitration yield is obtained for similar fluxes of NO and O 2 . and is completely inhibited upon excess production of O 2 . because of efficient scavenging of TyrO by O 2 . . The biological implications of these findings are discussed.