Reactions of hypochlorous acid with tyrosine and peptidyl-tyrosyl residuesgive dichlorinated and aldehydic products in addition to 3-chlorotyrosine

The toxicity of hypochlorous acid (HOCl) generated from activated neutrophi ls has been associated with several pathological processes such as atherosc lerosis. Formation of 3-chlorotyrosine (Cl-Tyr) has been used as a marker f or assessing the involvement of HOCl in such processes. In this study, we a imed to investigate the formation of Cl-Tyr from reaction of HOCl with tyro sine (both free and peptide-bound) and the fate of Cl-Tyr under such condit ions. Tyrosine, N-acetyltyrosine, bovine serum albumin, and human low densi ty lipoproteins were incubated with a range of reagent hypochlorite concent rations for varying periods in 10 mM phosphate buffer (pH 7.4) at 22 degree s C. The reaction products, and several biological samples, were hydrolyzed (in the case of proteins), isolated, and purified by high pressure liquid chromatography and characterized or quantitated by mass spectrometry and NM R, A significant amount of 3,5-dichlorotyrosine (diCl-Tyr) was obtained fro m the bovine serum albumin, low density lipoprotein, and some biological sa mples, in addition to CI-Tyr, indicating that Cl-Tyr competes effectively f or HOCl even when tyrosine is present in great excess. Cl-Tyr and diCl-Tyr were also formed from free tyrosine but then reacted further with HOCl. Thi s finding differs from a claim in the literature that Cl-Tyr was not formed in such a system. The further reaction products of Cl-Tyr and diCl-Tyr wit h HOCl were elucidated as their corresponding mono- and dichlorinated 4-hyd roxyphenylacetaldehydes. These results indicate the importance of assessing other products of HOCl action in addition to Cl-Tyr.