A histochemical study of
alpha-D-mannosidase revealed that normal human melanocytes (resting state, activated,
lentigo simplex) exhibit either no or just detectable activity, as do melanocytes in the initial phase of
lentigo maligna. Junctional, or occasionally zone A naevocytes displayed a very low
enzyme activity. On the other hand, melanocytes in the initial stage of neoplastic transformation (dysplastic naevi, advanced stage of
lentigo maligna) and also
melanoma cells in disorders of low malignant potential (initial naevogenic
melanoma, superficial spreading
melanoma) displayed a high activity uniformly throughout the cell population. In the malignant forms (nodular
melanoma, recurrences,
metastases), the
enzyme activity was remarkably heterogeneous, suggesting a breakdown of uniformity during malignant transformation. The significance of
alpha-mannosidase activity induction in the course of melanocyte neoplastic transformation is not clear at present. The results of biochemical assays suggest that the lysosomal
isoenzyme is mainly responsible. Other lysosomal
enzymes, and
dehydrogenases studied concomitantly, did not display any comparable phenomena of induction or similar behaviour. However, the results of a comparison of
alpha-mannosidase with the melanocyte reference
enzyme tyrosinase suggested activity patterns in the
enzyme pair which may provide a better insight into the biochemical differentiation of human melanocytes in neoplastic disorders. The possible relationship of
alpha-mannosidase to melanogenesis is also discussed.