Dihydro-1,4-benzothiazine-6,7-dione, the ultimate toxic metabolite of 4-S-cysteaminylphenol and 4-S-cysteaminylcatechol.
4-S-Cysteaminylphenol (4-S-CAP) and the corresponding catechol 4-S-cysteaminylcatechol (4-S-CAC) have been evaluated for melanocytotoxicity. It was shown recently that tyrosinase oxidation of these substrates produces a violet pigment, dihydro-1,4-benzothiazine-6,7-dione (BQ). In this study we examined whether BQ is the ultimate toxic metabolite produced in melanoma cells from 4-S-CAP/4-S-CAC. Biochemical experiments showed that (1) BQ was formed by autoxidation of 4-S-CAC as well as by tyrosinase oxidation of 4-S-CAP/4-S-CAC, (2) BQ reacted rapidly with thiols such as reduced glutathione (GSH), and (3) BQ inhibited the activity of alcohol dehydrogenase, an SH enzyme. In vitro experiments showed that (1) the cytotoxicity of 4-S-CAC was mostly prevented by catalase and superoxide dismutase, (2) BQ was highly cytotoxic to B16 melanoma cells (IC50 being 3.9 microM as compared with 507 microM for 4-S-CAP), (3) BQ was metabolized rapidly to a GSH adduct in melanoma cells, and (4) the same GSH adduct was also formed upon incubation of melanoma cells with 4-S-CAP, the reaction being tyrosinase dependent. In vivo experiments showed that intratumoral administration of BQ (0.5 micromol) inhibited the subcutaneous growth of B16 melanoma nearly as effectively as 4-S-CAP/4-S-CAC (20 micromol). These results indicate that BQ is the ultimate toxic metabolite produced by tyrosinase oxidation of 4-S-CAP/4-S-CAC. BQ deprives melanoma cells of GSH and may inactivate SH enzymes essential for DNA synthesis and cell proliferation by covalent binding through their cysteine residues, thereby exerting melanocytotoxicity. Cytotoxicity of 4-S-CAC depends mostly on autoxidation producing BQ and active oxygens.
Hasegawa
K
K
School of Medicine, Fujita Health University, Toyoake, Aichi, Japan.
Ito
S
S
Inoue
S
S
Wakamatsu
K
K
Ozeki
H
H
Ishiguro
I
I
D000818:Animals; D000970:Antineoplastic Agents / Q000378:metabolism* / Q000494:pharmacology; D002374:Catalase / Q000378:metabolism; D002851:Chromatography, High Pressure Liquid; D003543:Cysteamine / Q000031:analogs & derivatives* / Q000378:metabolism / Q000494:pharmacology; D004354:Drug Screening Assays, Antitumor; D008544:Melanocytes / Q000187:drug effects*; D008545:Melanoma / Q000188:drug therapy / Q000201:enzymology; D051379:Mice; D008810:Mice, Inbred C57BL; D014442:Monophenol Monooxygenase / Q000378:metabolism; D013054:Spectrophotometry, Atomic; D013482:Superoxide Dismutase / Q000378:metabolism; D013843:Thiazines / Q000378:metabolism* / Q000494:pharmacology; D014407:Tumor Cells, Cultured
C094171:4-S-cysteaminylcatechol; D000970:Antineoplastic Agents; D013843:Thiazines; C107729:dihydro-1,4-benzothiazine-6,7-dione; D003543:Cysteamine; D002374:Catalase; D014442:Monophenol Monooxygenase; D013482:Superoxide Dismutase; C043037:4-S-cysteaminylphenol