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

Journal of Biological Chemistry, ISSN: 0021-9258, Vol: 275, Issue: 5, Page: 3031-3036
2000
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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.

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