Requirements for superoxide-dependent tyrosine hydroperoxide formation in peptides

Superoxide reacts rapidly with other radicals, but these reactions have received little attention in the context of oxidative stress. For tyrosyl radicals, reaction with superoxide is 3-fold faster than dimerization, and forms the addition product tyrosine hydroperoxide. We have explored structural requirements for hydroperoxide formation using tyrosine analogues and di- and tri-peptides. Superoxide and phenoxyl radicals were generated using xanthine oxidase, peroxidase and the respective tyrosine derivative, or by γ-radiation. Peroxides were measured using FeSO₄/ Xylenol Orange. Tyrosine and tyramine formed stable hydroperoxides, but N-acetyltyrosine and p-hydroxyphenylacetic acid did not, demonstrating a requirement for a free amino group. Using [¹⁴C]tyrosine, the hydroperoxide and dityrosine were formed at a molar ratio of 1.8:1. Studies with pre-formed hydroperoxides, and measurements of substrate losses, indicated that, in the absence of a free amino group, reaction with superoxide resulted primarily in restitution of the parent compound. With dipeptides, hydroperoxides were formed only on N-terminal tyrosines. However, adjacent lysines promoted hydroperoxide formation, as did addition of free lysine or ethanolamine. Results are compatible with a mechanism [d'Alessandro, Bianchi, Fang, Jin, Schuchmann and von Sonntag (2000) J. Chem. Soc. Perkin Trans. II, 1862-1867] in which the phenoxyl radicals react initially with superoxide by addition, and the intermediate formed either releases oxygen to regenerate the parent compound or is converted into a hydroperoxide. Amino groups favour hydroperoxide formation through Michael addition to the tyrosyl ring. These studies indicate that tyrosyl hydroperoxides should be formed in proteins where there is a basic molecular environment. The contribution of these radical reactions to oxidative stress warrants further investigation.