Biosynthesis of brush border glycoproteins by human small intestinal mucosa in organ culture

The incorporation of [14C]glucosamine into brush border glycoproteins by human small intestinal mucosa in organ culture has been investigated. The experiments were based on the observations that (1) isolated brush border membrane fragments from cultured explants showed an unchanged pattern of protein bands and brush border enzyme activities on sodium dodecyl sulfate/polyacrylamide gels after electrophoresis and (2) the rate of overall [14C]glucosamine incorporation measured in the tissue homogenate remained constant up to 48 h. After 24 h of culture, the radioactivity peaks on gels due to incorporation of [14C]glucosamine were found exclusively in the high molecular weight region and corresponded to protein bands identified as maltase-glucoamylase, lactase, sucrase-isomaltase, enterokinase and alkaline phosphatase. Enzymatic activity could not be assigned to the three remaining labelled bands. Most of these glycoproteins were already labelled after 5 h. Newly glycosylated brush border enzymes remained predominantly associated with the brush border membrane of intact cells with little release into the medium up to 24 h.