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The one-electron reduction of uroporphyrin I by rat hepatic microsomes

Authors
Journal
Archives of Biochemistry and Biophysics
0003-9861
Publisher
Elsevier
Publication Date
Volume
257
Issue
2
Identifiers
DOI: 10.1016/0003-9861(87)90567-4
Keywords
  • Oxygenases
  • Microsomes
  • And P-450 Reactions

Abstract

Abstract Uroporphyrin I, which accumulates in body tissues of congenital erythropoietic porphyria patients, can undergo an enzymatic one-electron reduction to the porphyrin anion radical when a suitable reducing cofactor is present. We have demonstrated, in the absence of light, that anaerobic microsomal incubations containing NADPH and uroporphyrin I give an electron spin resonance spectrum consistent with the enzymatic formation of a porphyrin anion free radical. This radical undergoes a second-order decay ( k 2 ~ 10 5 M −1 s −1) due to nonenzymatic disproportionation of the radical. Aerobic microsomal incubations were also investigated for the reduction of oxygen to superoxide by monitoring oxygen consumption and the spin-trapping of superoxide. These experiments demonstrate that electron transfer from the porphyrin radical to molecular oxygen does occur, but due to the slow formation of the radical anion, no oxygen consumption above the basal level could be detected in the microsomal incubations. The photoreduction of uroporphyrin I in aerobic and anaerobic incubations was also investigated.

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