CHEMICAL CHARACTERIZATION OF PHEOMELANOGENESIS STARTING FROM DIHYDROXYPHENYLALANINE OR TYROSINE AND CYSTEINE - EFFECTS OF TYROSINASE AND CYSTEINE CONCENTRATIONS AND REACTION-TIME

Two types of melanin pigment are produced in mammals; the brown-to-bla ck eumelanins and the yellow-to-reddish-brown pheomelanins. The switch from one type of melanin to the other appears to be regulated by the levels of tyrosinase and thiols, such as cysteine and glutathione. Thi s study examines the process of pheomelanin formation starting from di hydroxyphenylalanine (dopa) or tyrosine and cysteine. We prepared pheo melanins by tyrosinase oxidation of dopa or tyrosine in the presence o f cysteine. Experimental variables were reaction time, tyrosinase conc entration, and dopa or tyrosine to cysteine ratio. Following the react ions, we measured concentrations of tyrosine, dopa, cysteine and cyste inyldopas, amounts of total melanin (TM) by Soluene-350 solubilization and aminohydroxyphenylalanine (AHP), a specific indicator of pheomela nin, formed by hydriodic acid hydrolysis, and absorbance ratio, A(650) /A(500). It was found that (1) mixed melanogenesis is a heterogeneous process in which pheomelanogenesis proceeds first, followed by eumelan ogenesis, as shown by changes in the tyrosine and cysteinyldopa concen trations, the AHP/TM ratio, and the A(650)/A(500) ratio during the cou rse of melanogenesis and (2) lower tyrosinase concentration favors phe omelanogenesis even when the availability of cysteine is limited, as s hown by AHP/TM ratios that were higher than the corresponding tyrosine to cysteine ratios. These results indicate that the switch from eumel anogenesis to pheomelanogenesis can be achieved by lowering the tyrosi nase activity, which conforms to our proposal that tyrosinase activity is the major factor controlling the course of melanogenesis. (C) 1997 Elsevier Science B.V.