Role of mixed oxidation states in the oxidation of hemerythrin species by ferricyanide ion

Abstract

The oxidation of deoxy- and oxyhemerythrin to methemerythrin by Fe(CN)6(3-) has been examined. The first step produces (semi-met)O, a half-oxidized form of hemerythrin which has different spectral and kinetic properties from (semi-met)R which is produced by one-electron reduction of methemerythrin. Further oxidation of (semi-met)O to methemerythrin usually occurs only indirectly via disproportionation of (semi-met)O to met and deoxy forms within the octameric framework. Oxidation of (semi-met)R by Fe(CN)6(3-) is a direct second-order reaction. (Semi-met)O is reduced rapidly by dithionite to deoxy, whereas that of (semi-met)R is a slow biphasic process. The oxidation of oxyhemerythrin occurs via the deoxy species, little, if any, reactivity being attributable to the oxy form. The oxidation of the azide adduct of (semi-met)R is biphasic, in which in step one N3(-) is removed during the oxidation to methemerythrin, which in the second step recombines with N3(-). Rate parameters for all these processes at pH 8.2 and 6.3 for protein from Phascolopsis gouldii and Themiste zostericola have been obtained. The implications of these findings to hemerythrin chemistry are discussed.

The reactions of semi-met forms of hemerythrin

“…methemerythrin + FlavinredH2 -* semi-methemerythrin + flavinH0 (9) 2flavinH°<=> flavin0, + flavinredH2 (10) Consistent with this mechanism, reaction 9 was measured independently and shown to be rapid (k = 1.4 X 105 M-1 s-1 at 25 °C and pH 8.2).…”

“…For the corresponding reaction of (semi-met)0, deoxyhemerythrin (20 µ ) was mixed with a Fe(CN)63-(0.1 mM), Nf (0.1-1.0 mM) mixture. 10 In both cases, the semi-met formed rapidly (within about 5-10 s) and the reaction with Nf was virtually complete before further reactions of semi-met or semi-met-Nf adducts with S2042-or FefCN)/occurred.9'10 When more than one monitoring method or wavelength was used, consistent rate data were obtained. Second-order reactions were carried out with pseudo-first-order conditions, and, in general, excellent first-order traces were obtained often up to 3-4 ii/2's.…”

“…Methemerythrin, (semi-met)R, and deoxyhemerythrin were prepared as described previously. 10 The (semi-met)o form was produced by mixing equimolar solutions of Fe-(CN)63-and deoxyhemerythrin (expressing the concentration of the latter on the basis of the monomeric unit, mol wt 13 500). The azide adduct of either semi-met form was prepared by adding azide ion to semi-met as soon as it had been prepared and in sufficient concentration (10.0 mM) to ensure completion of formation within seconds.…”

Comparisons of redox kinetics of methemerythrin and .mu.-sulfidomethemerythrin. Implications for interactions with cytochrome b5

“…(B) Oxidations. The kinetics of oxidation of P. gouldii (semi-met)RHr by K3Fe(CN)6 have been previously studied by Wilkins and co-workers (Bradic et al, 1980;Flarrington & Wilkins, 1983). In the present study the oxidation of jt-S2~semi-metHr and (semi-met)RHr by [Co(phen)3]Cl3 was carried out as well as the oxidation of /a-S2~semi-metHr by K3Fe(CN)6.…”

Reduction of methemerythrin in the erythrocytes of Phascolopsis gouldii

“…The oxidation kinetics of deoxy to metHr with ferricyanide have also been studied (23). The oxidation kinetics are also complex and the mechanism is incompletely understood.…”