title: O2-mediated oxidation of hemopexin-heme(II)-NO.
abstract: Hemopexin (HPX), serving as scavenger and transporter of toxic plasma heme, has been postulated to play a key role in the homeostasis of NO. Here, kinetics of HPX-heme(II) nitrosylation and O2-mediated oxidation of HPX-heme(II)-NO are reported. NO reacts reversibly with HPX-heme(II) yielding HPX-heme(II)-NO, according to the minimum reaction scheme: HPX-heme(II)+NO kon<-->koff HPX-heme(II)-NO values of kon, koff, and K (=kon/koff) are (6.3+/-0.3)x10(3)M-1s-1, (9.1+/-0.4)x10(-4)s-1, and (6.9+/-0.6)x10(6)M-1, respectively, at pH 7.0 and 10.0 degrees C. O2 reacts with HPX-heme(II)-NO yielding HPX-heme(III) and NO3-, by means of the ferric heme-bound peroxynitrite intermediate (HPX-heme(III)-N(O)OO), according to the minimum reaction scheme: HPX-heme(II)-NO+O2 hon<--> HPX-heme(III)-N(O)OO l-->HPX-heme(III)+NO3- the backward reaction rate is negligible. Values of hon and l are (2.4+/-0.3)x10(1)M-1s-1 and (1.4+/-0.2)x10(-3)s-1, respectively, at pH 7.0 and 10.0 degrees C. The decay of HPX-heme(III)-N(O)OO (i.e., l) is rate limiting. The HPX-heme(III)-N(O)OO intermediate has been characterized by optical absorption spectroscopy in the Soret region (lambdamax=409 nm and epsilon409=1.51x10(5)M-1cm-1). These results, representing the first kinetic evidence for HPX-heme(II) nitrosylation and O2-mediated oxidation of HPX-heme(II)-NO, might be predictive of transient (pseudo-enzymatic) function(s) of heme carriers.
authors.affiliation: Department of Structural and Functional Biology and Center of Neuroscience, University of Insubria, Via Alberto da Giussano 12, I-21052 Busto Arsizio (VA), Italy.
authors.forename: Mauro
authors.initials: M
authors.lastname: Fasano
authors.forename: Giovanni
authors.initials: G
authors.lastname: Antonini
authors.forename: Paolo
authors.initials: P
authors.lastname: Ascenzi
mesh_terms: D006418:Heme / Q000031:analogs & derivatives / Q000737:chemistry / Q000378:metabolism*; D006466:Hemopexin / Q000737:chemistry / Q000378:metabolism*; D006706:Homeostasis / Q000502:physiology*; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D009569:Nitric Oxide / Q000737:chemistry / Q000378:metabolism; D010084:Oxidation-Reduction; D010100:Oxygen / Q000378:metabolism*; D011485:Protein Binding; D013696:Temperature
chemical_list: D009569:Nitric Oxide; D006418:Heme; C027417:nitrosylprotoheme; D006466:Hemopexin; D010100:Oxygen