We characterized the melanogenic apparatus in a family of transplantable gerbil
melanomas (melanotic and amelanotic) using a combination of biophysical, ultrastructural and biochemical methods. Melanotic
melanomas produced pure
eumelanin but in vesiculo-globular melanosomes ('pheomelanosomes'); the eumelanosomes, characteristically ellipsoidal in shape with fibrillar or fibrillo-lamelar matrix, were never noticed. Melanotic
melanomas also had significant
tyrosinase activity and Zn, Pb/S, Ca and P content; all higher than in the amelanotic variants. The amelanotic variant, which was devoid of
melanin pigment and melanosomes, had clearly detectable
tyrosinase activity (albeit at 20% of that in the melanotic variant). Thus, with these multidirectional approaches we demonstrate that pure
eumelanin can be synthesized in organelles ultrastructurally defined as pheomelanosomes, but a defect in the formation of melanosomes can prevent in vivo
melanin synthesis despite the presence of detectable
tyrosinase activity. We conclude that this
melanoma system provides an excellent experimental model for the study of molecular components determining pheo- and/or eumelanogenesis. The information generated can be used for defining the roles of melanogenesis and of
tyrosinase expression in the regulation of
melanoma behavior and the effect of their modification on the course of the disease.