Potential role of tryptophan and chloride in the inhibition of human myeloperoxidase

Abstract

Myeloperoxidase (MPO) binds H 2 O 2 in the absence and presence of chloride (Cl − ) and catalyzes the formation of potent oxidants through 1e − and 2e − oxidation pathways. These potent oxidants have been implicated in the pathogenesis of various diseases including atherosclerosis, asthma, arthritis, and cancer. Thus, inhibition of MPO and its byproducts may have a much wide application in biological systems. Using direct rapid kinetic measurements and H 2 O 2 -selective electrodes, we showed that tryptophan (Trp), an essential amino acid, was linked kinetically to the inhibition of MPO catalysis under physiological-like conditions. Trp inactivated MPO in the absence and presence of plasma levels of chloride (Cl − ), in various degrees, through binding to MPO forming the inactive complexes, Trp-MPO and Trp-MPO-Cl, respectively; and accelerating MPO Compound II formation, an inactive form of MPO. Inactivation of MPO was mirrored by the direct conversion of MPO-Fe(III) to MPO Compound II without any sign of Compound I accumulation. This behavior indicates that Trp binding modulates the formation of MPO intermediates and their decay rates. Importantly, Trp is a poor substrate for MPO Compound II and has no role in destabilizing complex formation. Thus, the overall MPO catalytic activity will be limited by: 1) the dissociation of Trp from Trp-MPO and Trp-MPO-Cl complexes; 2) the affinity of MPO Compound I towards Cl − versus Trp; and 3) the slow conversion of MPO Compound II to MPO-Fe(III). Importantly, Trp dependentinhibition of MPO occurred with a wide range of concentrations that span various physiological and supplemental ranges.

Human Indoleamine 2,3-Dioxygenase Is a Catalyst of Physiological Heme Peroxidase Reactions

“…7) and previously (26) is further evidence for formation of an Fe IV ϭO heme in IDO. Together, our data show that as for other heme enzymes (32,52), L-Trp is a reactive substrate for IDO compound I but not compound II.…”

“…9). In contrast, L-Trp, as a poor compound II substrate (32,52), inhibited H 2 O 2 consumption by rIDO (Fig. 2) that coincided with the detection of IDO compound II as the major steady-state heme species detected in the presence of L-Trp and H 2 O 2 (Figs.…”

“…These findings are indicative of IDO exhibiting a heme peroxidase function, where enzyme turnover and consequently H 2 O 2 consumption is accelerated by effective substrates for compound II (i.e. ascorbate and tyrosine) (31,53) and inhibited by L-Trp, a poor compound II reductant (32,52). Spectroscopic studies (see below) support this proposal as IDO compound II is the major heme species detected in the presence of H 2 O 2 and L-Trp.…”

“…As well as binding avidly to the IDO active site, L-Trp is a known substrate for compound I, but not compound II, of peroxidases (32,52). In light of this, we also examined if other peroxidase substrates could modulate H 2 O 2 -induced inhibition of dioxygenase activity.…”

Melatonin Prevents Myeloperoxidase Heme Destruction and the Generation of Free Iron Mediated by Self-Generated Hypochlorous Acid

“…Detailed mechanistic insight into how exogenously added or self-generated HOCl mediates the MPO heme moiety has recently been elucidated [32,34]. Therefore, factors that influence rates of HOCl removal are of growing interest [20,[35][36][37][38][39][40]. Here, we examine the ability of MLT to prevent HOCl-mediated heme destruction and subsequent iron release.…”