Tyrosine nitration by simultaneous generation of ·NO and O₂/·^- under physiological conditions. How the radicals do the job

Radiation chemical experiments demonstrate that the reaction of tyrosyl radical (TyrO·) with ·NO₂ 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 O₂/·^- 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 CO₂ the reaction of ·NO with O·₂ yields 33% ·NO₂ and CO₃/·^-, where the latter reacts rapidly with tyrosine to form TyrO·; (ii) The formation of 3-nitrotyrosine takes place via the reaction of ·NO₂ with TyrO·, which is the main process at high dose rates; and (iii) Under continuous generation of ·NO and O₂/·^-, the formation of 3-nitrotyrosine is strongly suppressed because of efficient scavenging of ·NO₂ by tyrosine. The proposed model shows that the highest nitration yield is obtained for similar fluxes of ·NO and O₂/·^- and is completely inhibited upon excess production of O₂/·^- because of efficient scavenging of TyrO· by O₂/·^-. The biological implications of these findings are discussed.