Pulse radiolysis study of a yeast cytochrome c from Hansenula anomala.

The reduction of Hansenula anomala yeast cytochrome c by e-aq and CO-.2 was investigated by pulse radiolysis, at a high reductant to protein concentration ratio. The reactivity of the radicals was studied by observing absorbance changes in the cytochrome c spectrum over the wavelength range 280-600 nm. At pH 7, over the time scale of the radical decays (i.e. 0-4 microseconds for e-aq; 0-40 microseconds for CO-.2s) and beyond, the hemoprotein was reduced without any spectrally detected intermediate between ferri-and ferro-forms. This conclusion was reached by simulation studies based on the direct reduction of the yeast cytochrome c from the ferri- to the ferro-form, yielding a correct fit between experimental and calculated absorbance curves. The reduction rate constants were determined to be 1.0 +/- 01 X 10(10) M-1 S-1 for e-aq and 0.7 +/- 0.05 X 10(9) M-1 S-1 for CO-.2 at 0.16 M ionic strength, pH 7.0 and 20 degrees C, thus not significantly different from other values reported for horse heart cytochrome c. However, in the 360-390 nm region the generation of an additional radical species was noticed. The present experimental data were compared with previously published reports.

The reduction of flavocytochrome b2 by carboxylate radicals. A pulse radiolysis study.

The reaction of flavocytochrome b2 with carboxylate radicals has been studied by the pulse-radiolysis technique. The absorbance difference spectra observed 2, 10 and 90 mus after the pulse were very similar to the (reduced-minus-oxidized) difference spectrum of the cytochrome b2 core, a deflavoderivative prepared from flavocytochrome b2. The heme b2 group was reduced in a bimolecular reaction with a rate constant of (2.1 +/- 0.2) X 10(8) M-1 X s-1. Simulation data allowed us to assign this reduction mainly to a direct reaction of CO-2 on heme b2 without flavin involvement. However, this heme b2 reduction was accelerated in a flavocytochrome b2 sample of low molar activity. This observation seemed to reflect modifications of the heme b2 environment, in particular a closer contact with the solvent. Moreover, in such samples the flavin became reducible to the semiquinone state as deduced from absorbance difference spectra in the 440-490 nm region. The agreement between experimental and computed time-courses allowed to estimate the reaction rate of bound flavin to be equal to 2 X 10(8) M-1 X s-1 in this studied sample. Thus the reactivity of bound heme b2 and flavin within flavocytochrome b2 with CO-2 seems to be sensitive to the physical alterations of the polypeptide chain.

Saturation behavior of the manganese-containing superoxide dismutase from Paracoccus denitrificans.

A pulse radiolysis study of the Mn-superoxide dismutase from Paracoccus denitrificans has shown that, at concentration of 0(2)-. below 0.8 x 10(-4)M, the catalyzed dismutation of 0(2)-. is a first order reaction with regard to 0(2)-.. At concentration of 0(2)-. above 0.8 x 10(-4)M, the Mn-superoxide dismutase is shown to catalyze superoxide dismutation with a mechanism which exhibits saturation kinetics. This behavior was previously found in the bovine Cu/Zn-superoxide dismutase and in the Fe-superoxide dismutase from Photobacterium leiognathi. Two parameters of catalysis were determined from pH 5 to pH 11: the rate constant k was pH independent at basic pH. The variation of Km with pH indicated that the enzyme possessed an ionizable group with pK 9.8 which participates to the substrate binding.

The interaction of the membrane binding segment of cytochrome b5 with phospholipid vesicles. A pulse-radiolysis investigation.

The reactions of the radical anions Br2- and (SCN)2- produced by pulse-radiolysis have been used to study the interaction of dipalmitoyl phosphatidyl choline vesicles with the membrane binding segment (MBS) of cytochrome b5. Tryptophan oxidation by Br2- at neutral pH was characterized spectrophotometrically in detergent-solubilized solutions of MBS; the attack of tyrosine by (SCN)2- under alkaline conditions could also be observed directly. The incorporation of MBS into the lipid bilayer protected the tryptophan, tyrosine and methionine residues from oxidation by Br2- or (SCN)2-. Some structural implications of these results are discussed.