Journal
FREE RADICAL RESEARCH
Volume 46, Issue 9, Pages 1150-1156Publisher
INFORMA HEALTHCARE
DOI: 10.3109/10715762.2012.695868
Keywords
tert-butoxyl; tyrosyl; kinetics; rate constant; pulse radiolysis
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Funding
- Cancer Research UK
- Comision Sectorial de Investigacion Cientifica, Universidad de la Republica
- National Institutes of Health [2 R01Hl063119-05, 1R01AI095173-01]
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Oxidation of tyrosine moieties by radicals involved in lipid peroxidation is of current interest; while a rate constant has been reported for reaction of lipid peroxyl radicals with a tyrosine model, little is known about the reaction between tyrosine and alkoxyl radicals (also intermediates in the lipid peroxidation chain reaction). In this study, the reaction between a model alkoxyl radical, the tert-butoxyl radical and tyrosine was followed using steady-state and pulse radiolysis. Acetone, a product of the beta-fragmentation of the tert-butoxyl radical, was measured; the yield was reduced by the presence of tyrosine in a concentration-and pH-dependent manner. From these data, a rate constant for the reaction between tert-butoxyl and tyrosine was estimated as 6 +/- 1 x 10(7) M-1 s(-1) at pH 10. Tyrosine phenoxyl radicals were also monitored directly by kinetic spectrophotometry following generation of tert-butoxyl radicals by pulse radiolysis of solutions containing tyrosine. From the yield of tyrosyl radicals (measured before they decayed) as a function of tyrosine concentration, a rate constant for the reaction between tert-butoxyl and tyrosine was estimated as 7 +/- 3 x 10(7) M-1 s(-1) at pH 10 (the reaction was not observable at pH 7). We conclude that reaction involves oxidation of tyrosine phenolate rather than undissociated phenol; since the pK(a) of phenolic hydroxyl dissociation in tyrosine is similar to 10.3, this infers a much lower rate constant, about 3 x 10(5) M-1 s(-1), for the reaction between this alkoxyl radical and tyrosine at pH 7.4.
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