Enzymatic conversion of proteins to glycoproteins by lipid-linked saccharides: a study of potential exogenous acceptor proteins.

Previous studies have shown that a membrane preparation from hen oviduct catalyzes transfer of oligosaccharide from oligosaccharide-P-P-dolichol to denatured RNase and alpha-lactalbumin. To gain further insight into the structural requirements of a protein that allow it to serve as a substrate for glycosylation, the acceptor ability of a variety of other modified proteins containing the tripeptide sequence-ASN-X-(SER/THR)-has been investigated. Of 7 proteins tested, 2 (ovine prolactin and rabbit muscle triosephosphate isomerase) could be enzymatically glycosylated by a particulate preparation from hen oviduct. The remaining 5 proteins, assayed as either S-carboxymethylated or S-aminoethylated derivatives, were inactive as carbohydrate acceptors. However, cyanogen bromide treatment of 2 of the inactive proteins, bovine catalase and concanavalin A from jack bean, yielded peptide fragments which served as substrates for glycosylation. These results suggests that for some proteins, disruption of the tertiary structure is sufficient to allow attachment of carbohydrate. Other denatured proteins may possess additional restrictions imposed by their secondary structure. In certain cases, these restrictions are removed when the polypeptide chain is fragmented.