Disparate behaviour of two melanosomal enzymes (alpha-mannosidase and gamma-glutamyltransferase).
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.
Borovanský
J
J
Department of Medical Chemistry and Biochemistry, Charles University, Prague, Czech Republic. jborov@lf1.cuni.cz
Hach
P
P
D000818:Animals; D003902:Detergents / Q000494:pharmacology; D007527:Isoenzymes / Q000378:metabolism*; D008361:Mannosidases / Q000378:metabolism*; D008546:Melanoma, Experimental / Q000201:enzymology / Q000473:pathology; D020460:Melanosomes / Q000187:drug effects / Q000378:metabolism*; D051379:Mice; D008810:Mice, Inbred C57BL; D009363:Neoplasm Proteins / Q000378:metabolism; D009368:Neoplasm Transplantation; D043323:alpha-Mannosidase; D005723:gamma-Glutamyltransferase / Q000378:metabolism*
D003902:Detergents; D007527:Isoenzymes; D009363:Neoplasm Proteins; D005723:gamma-Glutamyltransferase; D008361:Mannosidases; D043323:alpha-Mannosidase