Subfractionation and properties of rat kidney cortex microsomal fraction

Abstract

1. 1. When isolated by conventional methods, the rat kidney cortex microsomal fraction is of heterogeneous composition and contains mitochondria, brush borders, lysosomes and Golgi elements in addition to the membranes deriving from the endoplasmic reticulum. 2. 2. The microsomal fraction from rat kidney cortex in this study, contains chiefly fragments of the endoplasmic reticulum, but is contamined by 4–8% of the mitochondria and by about 10% of the plasma membrane and brush border fragments. 3. 3. Using rate differential centrifugation in a gradient containing both sucrose and Ficoll it was possible to separate microsomal subfractions of different cellular origin and properties. 4. 4. One subfraction was shown on the basis of morphological, chemical and enzymatic data to derive to a very large extent from the endoplasmic reticulum of kidney cortex epithelial cells, primarily the proximal tubule cells. 5. 5. The ER-membranes of the rat kidney cortex seem to have many properties in common with the rat liver microsomal fraction including a similar phospholipid/protein ratio and a similar enzymatic pattern. On the other hand, the kidney microsomes have a much lower ribosome content and a 50% higher cholesterol/phospholipid ratio. 6. 6. The following enzymes or enzyme activities have been localized in the ER membranes of rat kidney cortex epithelial cells: glucose-6-phosphatase, inosine-5′-diphosphatase, NADPH-cytochrome c reductase, NADH-cytochrome c reductase, cytochromes P-450 k and b 5, fatty acid hydroxylase and 3,4-benzo(α)pyrene hydroxylase.