Hemopexin (HPX), which serves as a scavenger and transporter of toxic plasma heme, has been postulated to play a key role in the homeostasis of NO. In fact, HPX-heme(II) reversibly binds NO and facilitates NO scavenging by O 2. HPX-heme is formed by two four-bladed β-propeller domains. The heme is bound between the two β-propeller domains, residues His213 and His266 coordinate the heme iron atom. HPX-heme displays structural features of heme-proteins endowed with (pseudo-)enzymatic activities. In this study, the kinetics of rabbit HPX-heme(III) reductive nitrosylation and peroxynitrite-mediated oxidation of HPX-heme(II)-NO are reported. In the presence of excess NO, HPX-heme(III) is converted to HPX-heme(II)-NO by reductive nitrosylation. The second-order rate constant for HPX-heme(III) reductive nitrosylation is (1.3 ± 0.1) × 101 m -1·s-1, at pH 7.0 and 10.0 °C. NO binding to HPX-heme(III) is rate limiting. In the absence and presence of CO2 (1.2 × 10-3 m), excess peroxynitrite reacts with HPX-heme(II)-NO (2.6 × 10-6 m) leading to HPX-heme(III) and NO, via the transient HPX-heme(III)-NO species. Values of the second-order rate constant for HPX-heme(III)-NO formation are (8.6 ± 0.8) × 10 4 and (1.2 ± 0.2) × 106 m -1·s-1 in the absence and presence of CO 2, respectively, at pH 7.0 and 10.0°C. The CO2- independent value of the first-order rate constant for HPX-heme(III)-NO denitrosylation is (4.3 ± 0.4) × 10-1 s-1, at pH 7.0 and 10.0°C. HPX-heme(III)-NO denitrosylation is rate limiting. HPX-heme(II)-NO appears to act as an efficient scavenger of peroxynitrite and of strong oxidants and nitrating species following the reaction of peroxynitrite with CO2 (e.g. ONOOC(O)O-, CO3 -, and NO2).
- Nitric oxide
- Peroxynitrite-mediated oxidation
- Rabbit hemopexin
- Reductive nitrosylation
ASJC Scopus subject areas