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Problems concerning the biochemical action of superoxide dismutase (erythrocuprein)
Abstract:
The decay of the tetraperoxochromate- (V) complex (CrO83theta) was examined to study the substrate specificity of erythrocuprein (super-oxide dismutase). The decay of CrO83theta proved rather complex in aqueous solutions. Apart from the two known oxygen species O2theta and singlet oxygen (1 deltagO2), H2O2 and probably OH radicals were formed. No unequivocal evidence for the appearance of superoxide was obtained. The possible electron transfer from Cr5 to Fe3 (cytochrome c) was also discussed. In Tris buffer, pH 7.8, there were absolutely no signs of superoxide or OH radical formation. In fact, pulse radiolysis measurements employing a homogeneous OH source demonstrated that the Tris and OH radicals react with each other. One mol of H2O2 was generated from 1 mol of CrO83theta in Tris buffer. By contrast, only 0.5 mol H2O2 could be determined when the CrO83theta decay was carried out in 2-[4-(2-hydroxyethyl)-1-piperazinyl]-ethanesulfonic acid (HEPES) buffer, pH 7.8. The phenomenon of reducing oxidized cytochrome c could not fully be assigned to a superoxide-mediated reduction, since erythrocuprein was unable to inhibit this cytochrome c reduction efficiently. The energetic oxygen species (1deltag O2, OH etc.) appearing during the CrO83theta decay gave rise to a clearly detectable chemiluminescence. In this system, erythrocuprein was very active regardless of which buffer was used. Even in the absence of a chemiluminescent mediating agent, which might have interferred with the enzyme, erythrocuprein proved capable of inhibiting the CrO83theta-induced chemiluminescence in a rather specific way. No such specificity was seen in the presence of low molecular weight Cu-chelates including Cu(Tyr)2, Cu(Lys)2 and Cu(His)2. The ability to suppress chemiluminescence was approximately 3 orders of magnitude less pronounced than that of the native enzyme. It is presumed that erythrocuprein reacts with oxygen species other than the superoxide radical.