In the present study, the effects of hypochlorous acid (HOCl), monochloramine (NH2Cl), glutamine-chloramine (Glu-Cl) and taurine-chloramine (Tau-Cl) on the formation of 12-lipoxygenase (LOX) metabolite, 12-HETE, and cyclooxygenase (COX) metabolites, TXB2, and 12-HHT, from exogenous arachidonic acid (AA) in rat platelets were examined. Rat platelets (4 x 10(8)/ml) were preincubated with drugs for 5 min at 37 degrees C prior to the incubation with AA (40 mu M) for 2 min at 37 degrees C. HOCl (50-250 mu M) showed an inhibition on the formation of LOX metabolite (12-HETE, 5-67% inhibition) and COX metabolites (TXB2, 33-73% inhibition; 12-HHT, 27-74% inhibition). Although Tau-Cl and Glu-Cl up to 100 mu M were without effect on the formation of 12-HETE, TXB2 and 12-HTT, NH2Cl showed a strong inhibition on the formation of all three metabolites (10-100 mu M NH2Cl, 12-HETE, 21-92% inhibition; TXB2, 58-94% inhibition; 12-HHT, 36-92% inhibition). Methionine reversed a reduction of formation of LOX and COX metabolites induced by NH2Cl, and taurine restoring that induced by both NH2Cl and HOCl. These results suggest that NH2Cl is a more potent inhibitor of COX and LOX pathways in platelets than HOCl, and taurine and methionine can be modulators of NH2Cl-induced alterations in the COX and LOX pathways in vivo. (c) 2006 Elsevier Inc. All rights reserved.