Volume 272, Number 25, Issue of June 20, 1997 pp. 15796-15803
©1997 by The American Society for Biochemistry and Molecular Biology, Inc.

Inhibition of N-Glycan Processing in B16 Melanoma Cells Results in Inactivation of Tyrosinase but Does Not Prevent Its Transport to the Melanosome

(Received for publication, February 10, 1997, and in revised form, March 15, 1997)

Stefana M. Petrescu , Andrei-J. Petrescu , Haralambie N. Titu , Raymond A. Dwek and Frances M. Platt

From the Glycobiology Institute, University of Oxford, South Parks Road, OX1 3QU Oxford, United Kingdom and the  Institute of Biochemistry, Splaiul Independentei 296, 77700 Bucharest 17, Romania

Tyrosinase is the key enzyme in melanin biosynthesis, catalyzing multiple steps in this pathway. The mature glycoprotein is transported from the Golgi to the melanosome where melanin biosynthesis occurs. In this study, we have investigated the effects of inhibitors of N-glycan processing on the synthesis, transport, and catalytic activity of tyrosinase. When B16 mouse melanoma cells were cultured in the presence of N-butyldeoxynojirimycin, an inhibitor of the endoplasmic reticulum-processing enzymes -glucosidases I and II, the enzyme was synthesized and transported to the melanosome but almost completely lacked catalytic activity. The cells contained only 2% of the melanin found in untreated cells. Structural analysis of the N-glycans from N-butyldeoxynojirimycin-treated B16 cells demonstrated that three oligosaccharide structures (Glc₃Man_7-9) predominated. Removal of the glucose residues with -glucosidases I and II failed to restore enzymatic activity, suggesting that the glucosylated N-glycans do not sterically interfere with the enzyme's active sites. The mannosidase inhibitor deoxymannojirimycin had no effect on catalytic activity suggesting that the retention of glucosylated N-glycans results in the inactivation of this enzyme. The retention of glucosylated N-glycans does not therefore result in misfolding and degradation of the glycoprotein, as the enzyme is transported to the melanosome, but may cause conformational changes in its catalytic domains.

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