The characterization of rat kidney plasma membranes prepared by zonal centrifugation

1. Plasma membranes were isolated from a 10000g-min pellet prepared from a renal cortical homogenate in 20mm-NaHCO₃ by isopycnic centrifugation in a linear sucrose gradient in an ‘A’-type zonal rotor. 2. The preparation was characterized by electron microscopy, and alkaline phosphatase, 5′-nucleotidase, l-leucine β-naphthylamidase and l-leucine p-nitroanilidase activities were found to be selectively associated with the renal plasma membrane. 3. The preparation had a high degree of purity, as indicated by the presence of low activities of marker enzymes associated with subcellular organelles. A preliminary chemical analysis indicated that the chemical composition resembled that of plasma membranes of other tissues. 4. Plasma membranes were also prepared from tubular fragments and their enzyme contents were found to be similar to those of plasma membranes prepared from cortical homogenates. 5. l-Leucine β-naphthylamidase, l-leucine p-nitroanilidase and 5′-nucleotidase were not enriched to the same extent as alkaline phosphatase in the preparation of plasma membranes from tubular fragments. A possible explanation for this finding is discussed.