Abstract
Prostaglandin synthase is a multi-enzyme complex which catalyzes the oxygenation of arachidonic acid to the various prostaglandins. During the oxygenation, the enzyme is self-deactivated and, on the basis of ESR data, it has been proposed to form a self-destructive free radical. The free radical was suggested to form from the oxygen lost from prostaglandin G2 during its reduction to prostaglandin H2, and the destructive species was therefore thought to be an oxygen-centered free radical, tentatively identified as the hydroxy radical. We have reinvestigated this ESR signal (g = 2.005) and have concluded, with the aid of the known ESR parameters for the hydroxy and other oxygen-centered free radicals, that the free radical formed during the oxygenation is neither a hydroxy nor any known oxygen-centred radical. Prostaglandin synthase is thought to be a hemoprotein, so this unknown ESR signal was compared with the previously observed free radical formed by the reaction of H2O2 with methemoglobin. This comparison indicates that the free radical formed by the reaction of prostaglandin G2 with ram seminal vesicles is hemoprotein-derived and may be formed by the oxidation of an amino acid(s) located near the iron of the heme.
Highlights
The simplest explanation is an electron transfer from the electron-donating substrates, thereby oxidizing the cosubstrates and reducing the ram seminal vesicle free radical to its original state, which may be the active form of cyclooxygenase (24).Alternatively, since the metmyoglobin radical oxidizes its own aromatic amino acids, in the absence of alternate substrates (13), a similar process could account for the deactivation of prostaglandin synthase and its associated peroxidase activity
Once the similarity between the ram seminal vesicle free radical and the methemoglobin and metmyoglobin radicals is realized, a role for this free radical in the peroxidase activity of prostaglandin synthase can be proposed. It ispossible that the prostaglandin synthase radical is formed by the reaction of hydroperoxides,includinghydrogen peroxide, with the hemoprotein
A similar viewof the enzyme origin of the prostaglandin synthase radical has been suggested (34). These enzyme states of horseradish peroxidase and cytochrome c peroxidase contain an organic free radical in one-to-one stoichiometry with the heme (35, 36), and the chemical identity of these free radicals is an area of very active research
Summary
(such as inice) to lift this degeneracy, the ESRspectrum is so broad that the hydroxyl radical cannot be detected in a fluid. Arachidonic acid was obtained according to published From the arguments presented above, it is clear that the procedures (5, 9) If such an incubation is frozen in a 3-mm free radical generated during the reaction of hydroperoxides quartz sample tube containing a capillary of DPPH dispersed with ram seminal vesicles is not a freely rotating oxygenin KC1, the spectrum in Fig. 1 results. A g-valuein the range of 2.004 to 2.005 suggests that an radical were obtained by means of a dualcavity using Fremy’s organic free radical, possibly from one or more aromatic amino salt as the g-standard(Table I) This technique enables the acids, is responsible for the ESR signal of the unknown free separate recording of the ESRspectrum of the unknown free radical and the g-value standard so that the g-value of the sample of the unknown free radical can be determined without corrections for the overlap of two spectra (17).
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