Intestinal Epithelial Cell Surface Membrane Glycoprotein Synthesis

Abstract

Abstract Cell surface glycoproteins, which have been implicated in the control and expression of cell turnover, differentiation, and malignant transformation in tissue culture studies, were examined for a possible similar role in the intestinal epithelial cell. This cell is characterized by a rapid cell turnover and by a gradient of differentiation from crypt to villus. A method was developed which isolated epithelial cells from different levels of crypt and villus areas and when radioactive labeled d-glucosamine, l-fucose, or d-galactosamine were given intraperitoneally into a rat, a sharp gradient of radioactive incorporation paralleled the specific activity gradients of sucrase and alkaline phosphatase. Isolated epithelial cells incubated with d-[1-14C]glucosamine also showed that the upper villus cells had a much higher incorporation of d-[1-14C]glucosamine than did crypt cells. Subcellular fractionation of the intestinal cells labeled in vivo demonstrated that the highest specific activity was in the purified microvillus plasma membranes and the lowest specific activity was in the microsomal and cytosol fractions. Sodium dodecyl sulfate polyacrylamide gel electrophoresis of microvillus membranes showed peaks of d-[1-14C]glucosamine incorporation which coincided with peaks of sucrase and alkaline phosphatase activities and most of the bands which stained for protein also stained for glycoprotein. Microvillus core proteins separated on sodium dodecyl sulfate polyacrylamide did not show peaks of d-[1-14C]glucosamine incorporation and did not stain for glycoprotein. These results suggest that the more differentiated upper villus cells incorporate more labeled sugar precursors into membrane glycoproteins than the less differentiated lower villus or crypt cells and that these membrane glycoproteins are associated with membrane-bound enzymes.