GRAHAM, DOYLE G.(1978) Oxidative pathways for catecholamines in the genesis of neuromelanin and cytotoxic quinones. Mol. Pharmacol., 14, 633-643

The autoxidation, periodate oxidation, and tyrosinase-mediated oxidation of 6-hydroxydopamine, dopamine, norepinephrine, and epinephrine were studied by absorption spec-troscopy. Autoxidation and tyrosinase-mediated oxidation of the three catecholamines resulted in dopachrome analogues-aminochrome from dopamine, noradrenochrome from norepinephrine, and adrenochrome from epinephrine-without evidence for the expected intermediates, the o-quinones and the corresponding leukochromes. The use of periodate as an oxidant, on the other hand, allowed visualization of the o-quinone intermediates and the subsequent conversion to the dopachrome analogues. Cyclization of the o-quinones appeared to occur in the order epinephrine> norepinephrine> dopamine, while the rate of autoxidation occurred in the reverse order. The oxidation of 6-hydroxydopamine to its p-quinone was visualized under all three oxidizing conditions. However, the oxidation of 6-hydroxydopamine by periodate gave evidence for a transient intermediate, the o-quinone, which rapidly tautomerized to thep-quinone. The p-quinone product of 6-hydroxydopamine was seen to undergo cyclization to aminochrome, with subsequent polymerization.