In vivo effect of tunicamycin on the expression of rat small intestinal brush border membrane glycoproteins and glycoenzymes.

Tunicamycin, a known inhibitor of the lipid-dependent glycosylation of proteins, was used in vivo to study the biosynthesis of rat intestinal brush border membrane aminopeptidase N and dipeptidyl aminopeptidase IV. The incorporation of [3H]glucosamine into newly synthesized total protein of mucosal cell homogenates was inhibited by 60%, whereas incorporation of [3H]leucine was decreased only 21% by tunicamycin. This effect was much more pronounced in the brush border membrane fraction isolated from intestinal mucosal cells where incorporation of radiolabled leucine and glucosamine was reduced to 50 and 82% of control values respectively. An examination of the brush border membrane protein profile by sodium dodecyl sulfate-polyacrylamide gel electrophoresis showed that there was a marked selective decrease in the amount of glycoproteins of molecular weights greater than 130 kD. In addition, there were decreased levels of assayable aminopeptidase N, dipeptidyl aminopeptidase IV and disaccharidase activity in intestinal mucosal cell homogenates and brush border membranes of tunicamycin-treated rats. Though tunicamycin decreased incorporation of newly synthesized aminopeptidase N and dipeptidyl aminopeptidase IV protein into brush border membranes by 70-75%, the newly synthesized enzyme that was incorporated was indistinguishable from that of controls. Further, non-glycoslyated forms of both enzymes were not detected in any other subcellular fractions. These results show that tunicamycin, an inhibitor of glycosylation, significantly affected the expression of brush border membrane glycoproteins, suggesting that both polypeptide synthesis and degradation of these proteins may be altered in the presence of this drug.