Dihydro-1,4-benzothiazine-6,7-dione, the ultimate toxic metabolite of 4-S-cysteaminylphenol and 4-S-cysteaminylcatechol

Biochem Pharmacol. 1997 May 15;53(10):1435-44. doi: 10.1016/s0006-2952(97)00075-0.


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.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • Antineoplastic Agents / metabolism*
  • Antineoplastic Agents / pharmacology
  • Catalase / metabolism
  • Chromatography, High Pressure Liquid
  • Cysteamine / analogs & derivatives*
  • Cysteamine / metabolism
  • Cysteamine / pharmacology
  • Drug Screening Assays, Antitumor
  • Melanocytes / drug effects*
  • Melanoma / drug therapy
  • Melanoma / enzymology
  • Mice
  • Mice, Inbred C57BL
  • Monophenol Monooxygenase / metabolism
  • Spectrophotometry, Atomic
  • Superoxide Dismutase / metabolism
  • Thiazines / metabolism*
  • Thiazines / pharmacology
  • Tumor Cells, Cultured


  • 4-S-cysteaminylcatechol
  • Antineoplastic Agents
  • Thiazines
  • dihydro-1,4-benzothiazine-6,7-dione
  • Cysteamine
  • Catalase
  • Monophenol Monooxygenase
  • Superoxide Dismutase
  • 4-S-cysteaminylphenol