Endoplasmic reticulum membrane isolated from small-intestinal epithelial cells: enzyme and protein components

Endoplasmic reticulum membrane-rich fraction was obtained by subfractionation of the light microsomes from mouse jejunal mucosal epithelial cells. It was marked by high glucose-6-phosphatase, NADPH-cytochrome c reductase, and NADH-cytochrome c reductase activities and low Na+,K+-ATPase activity. The enrichment of Na+,K+-ATPase was 180-fold higher in the basolateral membranes than in the endoplasmic reticulum membrane-rich fraction relative to glucose-6-phosphatase. The protein peak that was phosphorylated in a Na-dependent manner was prominent in the basolateral membranes while it was a minor peak in the endoplasmic reticulum membrane-rich fraction. Under the electron microscope the fraction was seen to be composed of homogeneous small vesicles with thin smooth membranes.