Oxidation of melatonin by oxoferryl hemoglobin: A mechanistic study

Reaction of melatonin with the hypervalent iron centre of oxoferryl hemoglobin, produced in aqueous solution from methemoglobin and H2O2, has been investigated at 37degreesC and pH 7.4, by absorption spectroscopy. The reaction results in reduction of the oxoferryl moiety with formation of a heme-ferric containing hemoprotein. Stopped-flow spectrophotometric measurements provide evidence that the reduction of oxoferryl-Hb by melatonin is first-order in oxoferryl-Hb and first-order in melatonin. The bimolecular reaction constant at pH 7.4 and 37degreesC is 112 +/- 1.0 M(-1)s(-1). Two major oxidation products from melatonin have been found by gas chromatography-mass spectroscopy: the cyclic compound 1,2,3,3a,8,8ahexahydro-1-acetyl-5-methoxy-3a-hydroxypyrrolo[2,3-b]indole (cyclic 3-hydroxy-melatonin), and N-acetyl-N-1-formyl 5-methoxykynuramine (AFMK). The percentage yield of the two major products appears dependent on the ratio [oxoferryl-Hb]: [melatonin]-the higher the ratio the higher the yield of AFMK. The observed stoichiometry oxoferryl-Hb(reduced):melatonin(consumcd) is 2, when the ratio [oxoferryl-Hb]:[melatonin] is 1:1, but appears >2 at higher molar ratios. The reduction of the hypervalent iron of the oxoferryl moiety may be consistent with an oxidation of melatonin by two one-electron steps.