title: Indirect oxidation of 6-tetrahydrobiopterin by tyrosinase.
abstract: 6-Tetrahydrobiopterin is known to bind to an allosteric site of tyrosinase to directly inhibit the enzyme. However, simultaneous measurements of ultraviolet-visible absorption spectra and oxygen consumption led us to conclude that the inhibition was due to oxidation of 6-tetrahydrobiopterin by dopaquinone. Immediately after addition of 6-tetrahydrobiopterin, tyrosinase stopped producing dopachrome from either tyrosine or dopa. Duration of inhibition was proportional to the concentration of added 6-tetrahydrobiopterin and the enzyme activity was fully restored after the inhibition. Surprisingly, there was a rapid consumption of oxygen during the inhibition period. In addition, absorption spectra indicated that the only reaction that occurred during the inhibition was oxidation of 6-tetrahydrobiopterin to 7,8-dihydrobiopterin. In the absence of tyrosine or dopa, tyrosinase did not oxidize 6-tetrahydrobiopterin, suggesting that a reaction intermediate between dopa and dopachrome was a target for the inhibition. We propose a new mechanism in which dopa is oxidized to dopaquinone and the latter, instead of producing dopachrome, is reduced back to dopa by 6-tetrahydrobiopterin.
authors.affiliation: Department of Biochemisry, College of Natural Sciences, Kangwon National University, 200-701, Chunchon, Republic of Korea.
authors.forename: Jae Hyung
authors.initials: JH
authors.lastname: Jung
authors.forename: Shin Wook
authors.initials: SW
authors.lastname: Choi
authors.forename: Sanghwa
authors.initials: S
authors.lastname: Han
mesh_terms: D001708:Biopterins / Q000031:analogs & derivatives* / Q000378:metabolism*; D004295:Dihydroxyphenylalanine / Q000378:metabolism; D014442:Monophenol Monooxygenase / Q000037:antagonists & inhibitors / Q000378:metabolism*; D010084:Oxidation-Reduction; D013056:Spectrophotometry, Ultraviolet; D013379:Substrate Specificity
chemical_list: D001708:Biopterins; D004295:Dihydroxyphenylalanine; D014442:Monophenol Monooxygenase; C003402:sapropterin