The Chemistry of Melanin Iii. Mechanism of the Oxidation of Dihydroxyphenylalanine by Tyrosinase* by Howard S. Mason
نویسنده
چکیده
When 3,4-dihydroxyphenylalanine is oxidized in the presence of phenol oxidases (2,3) or by silver oxide (4), a red pigment is formed. Enzymically, this pigment is converted to a melanin. In the absence of enzyme, it has been shown that the pigment may rearrange to either Zcarboxy-5,6-dihydroxyindole or to 5,6-dihydroxyindole (5). The structure of the red pigment has therefore been inferred to be either 2-carboxy-2,3dihydroindole-5,6-quinone or the tautomeric Zcarboxy-2,3-dihydro-6-hydroxyindole-1,8quinonimine (5-7), and on this basis Raper has proposed the accompanying hypothesis for the enzymic conversion of 3 ,Cdihydroxyphenylalanine to synthetic dopa melanin (5, 6). This study was undertaken to determine whether or not the postulated reactions beyond the formation of the red pigment do take place in the presence of oxygen and enzyme. The enzymic oxidation of 3,4-dihydroxyphenylalanine was followed spectrophotometrically. The process was observed to proceed in three chromophoric phases, the first corresponding to the formation of the red pigment, the second to an intermediate purple pigment, and the third to the formation of melanin. By comparison of the observed spectra with those of known substances, it was possible to show that a rearrangement of the red pigment does occur during the enzymic oxidation, that synthetic dihydroxyphenylalanine melanin is probably a polymer of indoled, 6-quinone, and that the inferred o-quinonoid formulation of the red pigment is correct.
منابع مشابه
The Chemistry of Melanin v. Oxidation of Dihydroxyphenylalanine by Tyrosinase*
The sequence of reactions which occurs when 3,4-dihydroxyphenylalanine is oxidized in the presence of tyrosinase is now considered to be as in the accompanying diagram (1, 2). This process has been reported to require from 3.2 to 4.12 atoms of oxygen per molecule of dihydroxyphenylalanine (3-5). Detection of the carbon dioxide which should be evolved during this reaction sequence has not, to ou...
متن کاملThe chemistry of melanin; oxidation of dihydroxyphenylalanine by tyrosinase.
The sequence of reactions which occurs when 3,4-dihydroxyphenylalanine is oxidized in the presence of tyrosinase is now considered to be as in the accompanying diagram (1, 2). This process has been reported to require from 3.2 to 4.12 atoms of oxygen per molecule of dihydroxyphenylalanine (3-5). Detection of the carbon dioxide which should be evolved during this reaction sequence has not, to ou...
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The accompanying diagram illustrates a mechanism proposed for the enzymic oxidation of catechol (l-5). In the present study the oxidations of catechol and hydroxyhydroquinone at varying concentrations of substrate, enzyme, and hydrogen ion have been followed spectrophotometrically. Step 1, first proposed by Raper on chemical grounds (6), has been confirmed. Further results indicate that the cat...
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