The sodium and potassium activated ATPase of intestinal epithelium. I. Location of enzymatic activity in the cell

The location of sodium and potassium activated Mg‐ATPase (Na‐K ATPase) was studied in columnar epithelial cells of the small intestine of rats. Cell components were segregated in centrifuge fractions by a mild procedure (sucrose medium), which preserved mitochondria and vesicular inclusions, and by a drastic procedure, designed to preserve the striated borders selectively. The contents of fractions were characterized by phase contrast and electron microscopy and by the assay of alkaline phosphatase (E.C.3.1.3.1), cytochome oxidase (E.C.1.9.3.1), invertase (E.C.3.2.1.26) and Mg‐ATPase (E.C.3.6.1.4). Na‐K ATPase was found to be most concentrated in fractions containing mitochondria on one hand, and striated borders on the other. Its distribution differed from the distributions of the other four enzymes. The physiological implications of finding the “sodium pump” enzyme in the membrane at the apical pole of the epithelial cell were discussed.