An Enzymatic and Electron Microscopic Characterization of a Variant of the Cloudman S-91 Melanoma.

A variant of the Cloudman 5-91 melanoma is described which is characterized by a low level of tyrosinase and dopa oxidase (phenolase complex) activity. On the ultrastructural level this is expressed by a preponderance of melanosomes (im mature melanin granules) rather than mature melanin particles. Extracts of the tumor exhibit only dopa oxidase activity. With tumor slices, tyrosinase activity can be demonstrated as well ; catalytic amounts of dopa have a stimulating effect on this reaction. Dopa oxidase activity is competitively inhibited by tyrosine. Inter-tumor variation with respect to melanogenesis is correlated with the presence, in some tumors, of a potent inhibitor of tyrosinase and dopa oxidase. The inhibitor can be demonstrated directly in enzymatic tests by mixing active and inhibitory ex tracts. Selection experiments showed that control of this inhibitor resides within the tumor rather than the host. With respect to both phenolase complex activity and ultrastructural characteristics, it can be concluded that this tumor is a genetic variant of the original S-91 melanoma and occupies a position intermediate between the melanotic and the ainelanotic variety. The development and the relationship to the endoplasmic reticulum of type A particles within this tumor are described.