Lysosomal hydrolases are present in melanosomes and are elevated in melanizing cells

Melanosomes, the subcellular site of melanin synthesis and deposition, may be related to the endolysosomal lineage of organelles. To determine if melanosomes contain lysosomal hydrolases, we examined the subcellular distribution of five of these enzymes in melanocytes cultured from C57BL/6J mice. Analyses of Percoll gradient density centrifugations demonstrated that beta-hexosaminidase, beta-galactosidase, beta-glucuronidase, and cathepsins B and L all co-sedimented with tyrosinase-rich densely sedimenting melanosomes. The melanosomal distribution of these enzymes was confirmed in studies of melanocytes cultured from albino mice and of melanocytes rendered amelanotic by transfection with the v-rasHa oncogene (which lack dense, melanized melanosomes). In these cells, only a less dense peak of activity for each hydrolase was present. The level of each hydrolase was elevated in black cells when compared with albino cells. Metabolic labeling studies confirmed that the increase in beta-glucuronidase in black versus albino cells resulted mainly from increased synthesis of this enzyme. The data suggest that melanosomes represent specialized lysosomes present within melanocytes, that they contain a broad array of lysosomal hydrolases, and that the levels of these hydrolases are elevated in cells actively engaged in pigment production.