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Abstract
For the first time, the enzymatic one-electron oxidation of several naturally occurring and synthetic water-soluble porphyrins by peroxidases was investigated by ESR and optical spectroscopy. The ESR spectra of the free radical metabolites of the porphyrins were singlets ( g = 2.0024, ΔH = 2–3 G), which we assigned to their respective porphyrin pi-cation free radicals. Several porphyrins were investigated and ranked by the intensity of their ESR spectra (coproporphyrin III > coproporphyrin I > deuteroporphyrin IX > mesoporphyrin IX > Photofrin II > protoporphyrin IX > uroporphyrin I > uropor-phyrin III > hematoporphyrin IX). The porphyrins were oxidized by several peroxidases (horseradish peroxidase, lactoperoxidase, and myeloperoxidase), yielding the same type of ESR spectra. From these results, we conclude that porphyrins are substrates for peroxidases. The changes in the visible absorbance spectra of the porphyrins during enzymatic oxidation were monitored. The two-electron oxidation product, which was assigned to the dihydroxyporphyrin, was detected as an intermediate of the oxidation process. The optical spectrum of the porphyrin pi-cation free radical was not detected, probably due to its low steady-state concentration.
Published Version
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