Glycosylation of ribonuclease A catalysed by rabbit liver extracts

Crude extracts of rabbit liver catalyse in vitro the transfer of N acetylglucosamine from uridine diphosphate N acetylglucosamine to bovine pancreatic ribonuclease A. The enzymic activity is contained in rough endoplasmic reticulum. It has an absolute requirement for a bivalent metal ion: Co²⁺>Mn²⁺>Ni²⁺ Mg²⁺ is ineffective. There is enzymic activity in the absence of detergent, but increased activity is observed in the presence of Triton X 100. The site of glycosylation of ribonuclease A is asparagine 34, and glycosylation occurs only at this point. These findings agree with the hypothesis that the sequence Asn X Thr(Ser) (where X may be one of a number of types of amino acid) is a necessary, but not sufficient, condition for N acetylglucosaminylation of a protein bound asparagine residue.