Kinetics of oxidation of tyrosine by a model alkoxyl radical.
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 β-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 × 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 × 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 ≈ 10.3, this infers a much lower rate constant, about 3 × 10(5) M(-1) s(-1), for the reaction between this alkoxyl radical and tyrosine at pH 7.4.
Gray Institute for Radiation, Oncology and Biology, Department of Oncology, University of Oxford, Oxford, UK. Lisa.Folkes@oncology.ox.ac.uk
Lisa K
LK
Folkes
Silvina
S
Bartesaghi
Madia
M
Trujillo
Rafael
R
Radi
Peter
P
Wardman
D000438:Alcohols / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008956:Models, Chemical; D010084:Oxidation-Reduction; D014443:Tyrosine / Q000737:chemistry*
D000438:Alcohols; C059688:alkoxyl radical; D014443:Tyrosine