title: Regulation of tyrosinase by tetrahydropteridines and H2O2.
abstract: Recently two alternative mechanisms have been put forward for the inhibition of tyrosinase by 6R-l-erythro 5,6,7,8-tetrahydrobiopterin (6BH(4)). Initially allosteric uncompetitive inhibition was demonstrated due to 1:1 binding of 10(-6)M 6BH(4) to a specific domain 28 amino acids away from the Cu(A) active site of the enzyme. Alternatively it was then shown that 10(-3)M 6BH(4) inhibit the reaction by the reduction of the product dopaquinone back to l-dopa. In the study presented herein we have used two structural analogues of 6BH(4) (i.e., 6,7-(R,S)-dimethyl tetrahydrobiopterin and 6-(R,S)-tetrahydromonapterin) confirming classical uncompetitive inhibition due to specific binding of the pyrimidine ring of the pterin moiety to the regulatory domain on tyrosinase. Under these conditions there was no reduction of l-dopaquinone back to l-dopa by both cofactor analogues. Inhibition of tyrosinase by 6BH(4) occurs in the concentration range of 10(-6)M after preactivation with l-tyrosine and this mechanism uncouples the enzyme reaction producing H(2)O(2) from O(2). Moreover, a direct oxidation of 6BH(4) to 7,8-dihydrobiopterin by tyrosinase in the absence of the substrate l-tyrosine was demonstrated. The enzyme was activated by low concentrations of H(2)O(2) (<0.3 x 10(-3)M), but deactivated at concentrations in the range 0.5-5.0 x 10(-3)M. In summary, our results confirm a major role for 6BH(4) in the regulation of human pigmentation.
authors.affiliation: Clinical and Experimental Dermatology, Department of Biomedical Sciences, University of Bradford, West Yorkshire BD7 1DP, UK.
authors.forename: John M
authors.initials: JM
authors.lastname: Wood
authors.forename: Bhaven
authors.initials: B
authors.lastname: Chavan
authors.forename: Idris
authors.initials: I
authors.lastname: Hafeez
authors.forename: Karin U
authors.initials: KU
authors.lastname: Schallreuter
mesh_terms: D001665:Binding Sites; D001708:Biopterins / Q000031:analogs & derivatives* / Q000737:chemistry*; D004789:Enzyme Activation; D004791:Enzyme Inhibitors / Q000737:chemistry; D006861:Hydrogen Peroxide / Q000737:chemistry*; D014442:Monophenol Monooxygenase / Q000037:antagonists & inhibitors / Q000737:chemistry*; D010084:Oxidation-Reduction; D011485:Protein Binding; D011621:Pteridines / Q000737:chemistry; D014443:Tyrosine / Q000737:chemistry*
chemical_list: D004791:Enzyme Inhibitors; D011621:Pteridines; D001708:Biopterins; D014443:Tyrosine; D006861:Hydrogen Peroxide; D014442:Monophenol Monooxygenase; C003402:sapropterin