Solubilization of oligosaccharide transferase and glucosidase activities from thyroid rough microsomes.

In previous studies from this laboratory [ 1,2], an oligosaccharide transferase activity was demonstrated in thyroid rough microsomes: the membranous enzyme catalyzes the transfer of oligosaccharides from exogenously supplied oligosaccharide lipids to the asparagine residue of synthetic Asn-X-Thr containing peptide acceptors. This transferase was also recently described in hen oviduct [3], rat liver [4] and yeast [5] membranes. As a prerequisite for purification, this paper examines the conditions which allow total extraction of the enzyme from membranes while maintaining full activity. Although a few other glycosyltransferases involved in the early lipid-linked pathway of N-glycosylation have already been solubilized [6,7], this is the first report dealing with the oligosaccharide transferase. Several authors have suggested that glucose-containing oligosaccharide lipids constitute the physiological donor of the oligosaccharide transferase [8,9]. Glucose residues might be a signal in the transfer reaction before being removed by specific glucosidases. The present study points out that the oligosaccharide transferase is extracted together with two glucosidases of distinct specificities.