Reductive nitrosylation and peroxynitrite-mediated oxidation of heme-hemopexin.
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 beta-propeller domains. The heme is bound between the two beta-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) x 10(1) m(-1).s(-1), at pH 7.0 and 10.0 degrees C. NO binding to HPX-heme(III) is rate limiting. In the absence and presence of CO2 (1.2 x 10(-3) m), excess peroxynitrite reacts with HPX-heme(II)-NO (2.6 x 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) x 10(4) and (1.2 +/- 0.2) x 10(6) m(-1).s(-1) in the absence and presence of CO2, respectively, at pH 7.0 and 10.0 degrees C. The CO2-independent value of the first-order rate constant for HPX-heme(III)-NO denitrosylation is (4.3 +/- 0.4) x 10(-1) s(-1), at pH 7.0 and 10.0 degrees 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).
Ascenzi
Paolo
P
Department of Biology and Interdepartmental Laboratory for Electron Microscopy, University Roma Tre, Rome, Italy. ascenzi@bio.uniroma3.it
Bocedi
Alessio
A
Antonini
Giovanni
G
Bolognesi
Martino
M
Fasano
Mauro
M
D000818:Animals; D002245:Carbon Dioxide / Q000737:chemistry; D006418:Heme / Q000737:chemistry*; D006466:Hemopexin / Q000737:chemistry*; D006801:Humans; D007700:Kinetics; D008956:Models, Chemical; D008958:Models, Molecular; D008968:Molecular Conformation; D009569:Nitric Oxide / Q000737:chemistry; D009584:Nitrogen / Q000737:chemistry*; D010100:Oxygen / Q000737:chemistry*; D030421:Peroxynitrous Acid / Q000737:chemistry*; D011817:Rabbits; D013997:Time Factors
D002245:Carbon Dioxide; D030421:Peroxynitrous Acid; D009569:Nitric Oxide; D006418:Heme; D006466:Hemopexin; D009584:Nitrogen; D010100:Oxygen