Consecutive halogen transfer between various functional groups induced by reaction of hypohalous acids: NADH oxidation by halogenated amide groups

Cyclic dipeptides (c-Gly(2), c-Ser(2), c-Gly-Phe, etc.) were used as simple protein models to investigate the HOCl-induced generation and reactivity o f chlorinated amide groups. The pH dependence of the kinetics of amide chlo rination reveals that ClO- (not HOCl) is the reactive agent. N-Chlorinated cyclopeptides are stable up to 30 min, they exhibit narrow absorption bands around 215 nm, and they are capable of oxidizing certain biological substr ates, the reactivity decreasing in the order GSH > ascorbate > methionine > NADH >> GSSG. The chloroamide is less reactive, but much more selective in its reactions, than HOCl or ClO-; thus, with formation of the chloroamide prolonged oxidative effects, directed toward specific target molecules, can be expected. Chlorination of NADH, yielding a catalytically inactive speci es (NAD/Cl), was investigated in most detail because it is likely to be an important and highly lethal process. The chloroamide group is far more reac tive toward NADH than chloroamines derived from primary amines. Chloronucle otides formed by reaction of ClO- with inosine, GMP, TMP, or UMP are capabl e of quantitative chlorine transfer to cyclopeptides; however, no chlorine transfer between the amide nitrogen and primary amines is detectable, in ei ther direction. The results presented enable prediction of chlorine transfe r cascades induced by HOCl/ClO-, involving nucleotides, peptide amide group s, and final target molecules. Chlorinated NAD(P)H, as a stable terminal pr oduct of consecutive chlorine transfer reactions, might be a useful biologi cal marker for assessing the role of HOCl in inflammatory events. Brominati on by BrO- of cyclopeptides is more than two orders of magnitude faster tha n chlorination by ClO-, and the reactivity of bromoamide with NADH exceeds that of chloroamide by more than four orders of magnitude, (C) 1999 Academi c Press.