γ-Glutamyl Transpeptidase, Tyrosinase, and 5-S-Cysteinyldopa Production in Melanoma Cells

The distribution of γ-glutamyl transpeptidase (γ-GTP), tyrosinase, and 5-S-cysteinyldopa (5-S-CD) within melanoma cells has been studied in vitro as well as in vivo. Sodium periodate treatment of intact B-16 melanoma cells has been found to inhibit γ-GTP present as an ectoenzyme, However, these periodate-treated cells in the presence of 10-5 M dopa and glutathione have been found to continue to secrete large quantifies of 5-S-CD in their medium. The large-granule fraction of Greene's melanotjc melanoma contains substantial amounts of both tyrosinase and γ-GTP. However, further separation of the large-granule fraction into sub-fractions indicates that tyrosinase and γ-GTP seem to co-exist with premelanosome. It is suggested that glutathione-dependent 5-S-CD genesis proceeds within premelanosomes through the formation of glutathione-dopa. The excess of glutathione-dopa and 5-S-CD, unutilized for pheomelanin formation overflow into the cytoplasm