title: Dimaprit analogues inhibit tyrosinase via a disulphide breakdown product independently of the histamine H2 receptor.
abstract: Histamine displayed specific and saturable binding to membrane fractions of the human melanoma cell line MM96E (Kd = 72.4 nM and Bmax = 487 fmol/mg protein). There was weak competition with isothioureas that inhibit tyrosinase in intact cells: dimaprit (an H2 agonist) nordimaprit and S-[2-(N,N-diisopropyl)ethyl]isothiourea (DINOR). Under culture conditions, rapid, pH-dependent hydrolysis of the isothioureas occurred, with cleavage to urea and a thiol which spontaneously oxidised to the disulphide. The H3 agonist imetit, which also inhibited tyrosinase, behaved similarly. The disulphide breakdown product of DINOR but not the thiol inhibited tyrosinase activity in intact MM96E cells to a similar extent as DINOR itself. Isothioureas with more bulky substituents, however, were stable in culture and did not inhibit tyrosinase. The results show that (a) certain histaminergic drugs exert effects via a disulphide hydrolysis product independently of the histamine H2 receptor, and (b) beta-aminoethyldisulphides are depigmenting agents.
authors.affiliation: Pharmacy Department, University of Queensland, Australia.
authors.forename: G A
authors.initials: GA
authors.lastname: Fechner
authors.forename: J J
authors.initials: JJ
authors.lastname: Jacobs
authors.forename: P G
authors.initials: PG
authors.lastname: Parsons
mesh_terms: D001711:Biotransformation; D002460:Cell Line; D017259:Dimaprit / Q000031:analogs & derivatives* / Q000378:metabolism / Q000494:pharmacology*; D004220:Disulfides / Q000378:metabolism; D006632:Histamine / Q000378:metabolism / Q000494:pharmacology; D006801:Humans; D007700:Kinetics; D008545:Melanoma; D014442:Monophenol Monooxygenase / Q000037:antagonists & inhibitors*; D011970:Receptors, Histamine H2 / Q000187:drug effects / Q000502:physiology*; D013438:Sulfhydryl Compounds / Q000032:analysis / Q000378:metabolism; D014407:Tumor Cells, Cultured
chemical_list: D004220:Disulfides; D011970:Receptors, Histamine H2; D013438:Sulfhydryl Compounds; C087684:S-(2-(N,N-diisopropylamino)ethyl)isothiourea; C025838:nordimaprit; D006632:Histamine; D014442:Monophenol Monooxygenase; D017259:Dimaprit