Absolute rate constants for the reaction of hypochlorous acid with protein side chains and peptide bonds

Hypochlorous acid (HOCl) is a potent oxidant, which is produced in vivo by activated phagocytes. This compound is an important antibacterial agent, but excessive or misplaced production has been implicated in a number of human diseases, including atherosclerosis, arthritis, and some cancers. Proteins are major targets for this oxidant, and such reaction results in side-chain modification, backbone fragmentation, and cross-linking. Despite a wealth of qualitative data for such reactions, little absolute kinetic data is available to rationalize the in vitro and in vivo data. In this study, absolute second-order rate constants for the reactions of HOCl with protein side chains, model compounds, and backbone amide (peptide) bonds have been determined at physiological pH values. The reactivity of HOCl with potential reactive sites in proteins is summarized by the series: Met (3.8 x 10(7) M-1 s(-1)) > Cys (3.0 x 10(7) M-1 s(-1)) >> cystine (1.6 x 10(5) M-1 s(-1)) approximate to His (1.0 x 10(5) M-1 s(-1)) approximate to alpha -amino (1.0 x 10(5) M-1 s(-1)) > Trp (1.1 x 10(4) M-1 s(-1)) > Lys (5.0 x 10(3) M-1 s(-1)) >> Tyr (44 M-1 s(-1)) approximate to Arg (26 M-1 s(-1)) > backbone amides (10-10(-3) M-1 s(-1)) > Gln(0.03 M-1 s(-1)) approximate to Asn (0.03 M-1 s(-1)). The rate constants for reaction of HOCl with backbone amides (peptide bonds) vary by 4 orders of magnitude with uncharged peptide bonds reacting more readily with HOCl than those in a charged environment. These kinetic parameters have been used in computer modeling of the reactions of HOCl with human serum albumin, apolipoprotein-Al and free amino acids in plasma at different molar excesses. These models are useful tools for predicting, and reconciling, experimental data obtained in HOCl-induced oxidations and allow estimations to be made as to the flux of HOCl to which proteins are exposed in vivo.