Absolute rate constants for the reaction of hypochlorous acid with proteinside 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, bu t excessive or misplaced production has been implicated in a number of huma n diseases, including atherosclerosis, arthritis, and some cancers. Protein s are major targets for this oxidant, and such reaction results in side-cha in modification, backbone fragmentation, and cross-linking. Despite a wealt h of qualitative data for such reactions, little absolute kinetic data is a vailable to rationalize the in vitro and in vivo data. In this study, absol ute second-order rate constants for the reactions of HOCl with protein side chains, model compounds, and backbone amide (peptide) bonds have been dete rmined at physiological pH values. The reactivity of HOCl with potential re active 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)) app roximate 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 (peptid e bonds) vary by 4 orders of magnitude with uncharged peptide bonds reactin g more readily with HOCl than those in a charged environment. These kinetic parameters have been used in computer modeling of the reactions of HOCl wi th human serum albumin, apolipoprotein-Al and free amino acids in plasma at different molar excesses. These models are useful tools for predicting, an d reconciling, experimental data obtained in HOCl-induced oxidations and al low estimations to be made as to the flux of HOCl to which proteins are exp osed in vivo.