The effect of oxygen, antioxidants, and superoxide radical on tyrosine phenoxyl radical dimerization

Dimerization of tyrosine phenoxyl radical yields bityrosine (BT) which can easily be monitored by its characteristic fluorescence at 400 nm. The reactivity of tyrosine phenoxyl radical with O2 was examined by a variety of techniques. BT fluorescence was measured as a function of O2 concentration. Over a range of pH values (4-12) there was no effect of oxygen on BT production ([O2] less than or equal to 0.72 mM). In addition, oxygen uptake by the phenoxyl radical was measured directly with an oxygen electrode. It was determined by this technique that oxygen does not react with the phenoxyl radical with a rate constant greater than 10(3) M-1 s-1. Tyrosine phenoxyl radical "repair" by superoxide and physiological antioxidants was examined by BT fluorescence quenching as well as pulse radiolysis. Implications of these results as to the fate of tyrosine phenoxyl radicals produced in biological systems is discussed.