title: Studies on the reactions between human tyrosinase, superoxide anion, hydrogen peroxide and thiols.
abstract: Human tyrosinase (5.5 mg) has been purified from a single human melanotic melanoma metastasis (50.5 g). In the presence of dioxygen, L-tyrosine proved to be a very poor substrate for this enzyme with barely detectable activity compared to L-dopa. However, saturating superoxide anion (i.e., greater than 5 x 10(-3) M) enhanced the oxidation rate of L-tyrosine to dopachrome 40-fold. Hydrogen peroxide was shown to be a competitive inhibitor of tyrosinase when L-tyrosine was the substrate. This reversible inhibition is based on a slow pseudocatalase activity for tyrosinase. Monothiols and dithiols inhibit tyrosinase by different mechanisms. Reduced human thioredoxin and 2,3-dithiopropanol are allosteric inhibitors of tyrosinase yielding bis-cysteinate complexes with one of the copper atoms in the enzyme active site. Bis-cysteinate tyrosinase activity is down-regulated to 30% of native enzyme activity in the L-dopa assay; suggesting a true regulatory role for dithiols. Monothiols such as reduced glutathione and beta-mercaptoethanol are much less reactive with tyrosinase although 10(-3) M monothiol totally inhibits enzyme activity. Reduced thioredoxin inhibits tyrosinase 23-fold more than reduced glutathione under the same experimental conditions.
authors.affiliation: Department of Dermatology, University of Hamburg, F.R.G.
authors.forename: J M
authors.initials: JM
authors.lastname: Wood
authors.forename: K U
authors.initials: KU
authors.lastname: Schallreuter
mesh_terms: D000838:Anions; D002374:Catalase / Q000378:metabolism; D002848:Chromatography, DEAE-Cellulose; D006801:Humans; D006861:Hydrogen Peroxide / Q000494:pharmacology*; D045563:Indolequinones; D007211:Indoles / Q000378:metabolism; D007980:Levodopa / Q000378:metabolism; D008545:Melanoma / Q000201:enzymology; D014442:Monophenol Monooxygenase / Q000037:antagonists & inhibitors / Q000302:isolation & purification / Q000378:metabolism*; D010084:Oxidation-Reduction; D011809:Quinones / Q000378:metabolism; D013379:Substrate Specificity; D013438:Sulfhydryl Compounds / Q000494:pharmacology*; D013481:Superoxides / Q000494:pharmacology*
chemical_list: D000838:Anions; D045563:Indolequinones; D007211:Indoles; D011809:Quinones; D013438:Sulfhydryl Compounds; D013481:Superoxides; C001123:dopachrome; D007980:Levodopa; D006861:Hydrogen Peroxide; D002374:Catalase; D014442:Monophenol Monooxygenase