Folding of intestinal brush border enzymes. Evidence that high-mannose glycosylation is an essential early event

The dynamics of polypeptide folding of pig intestinal enzymes was studied using a polyvalent antiserum for protein precipitation. The introduction of fructose produced a decrease in protein folding of the bruch border enzymes, aminopeptidase N and sucrase-isomaltase by interfering with glycosylation. Malglycosylated and high-mannose-glycosylatedforms are retained in the endoplasmic reticulum and degraded by proteolysis. The findings give importance to correct sequencing of glycosylation and polypeptide folding in intestinal brush border enzyme kinetics.