In addition to tyrosinase and its related proteins melanosomes contain a variety of further enzyme activities. Using spectrophotometric methods alpha-mannosidase and gamma-glutamyl-transferase (GGT) were studied in B 16 melanoma, in isolated melanosomes and in tumour host (mice C57BL6J) sera. When compared to the original melanoma tissue (12.8-26.7 nkat/g TP) isolated melanosomes exhibited much higher alpha-mannosidase activity [227-420 nkat/g of total proteins (TP)]. Strong activation by Zn2+ and no influence of Co2+ ions suggested that the dominant form of alpha-mannosidase of the enzyme present in melanosomes was of the acid (lysosomal) type. The GGT activity of isolated melanosomes (168-244 nkat/g TP) was comparable with that of the whole melanoma tissue (203-375 nkat/g TP) . Treatment of melanosomes with detergents (0.1% Triton X-100, 0.5% deoxycholate) revealed striking extractibility differences between the two enzymes investigated in relation to their localization: alpha-mannosidase remained immobilized in the melanized matrix of melanosomes whereas the membrane bound GGT was easily released. Unlike the alpha-mannosidase the GGT serum levels were increasing in relation to the melanoma growth. The demonstration of acid form of alpha-mannosidase in melanosomes is consistent with their lysosomal ranking; the presence of GGT is in keeping with its expected roles both in protection against oxidative stress and in melanogenesis.

Department of Medical Chemistry and Biochemistry Charles University Prague Czech Republic

Melanoma-Bearing Libechov Minipig (MeLiM): The Unique Swine Model of Hereditary Metastatic Melanoma