Analytical characterization and purification of plasma membrane from cultured hepatoma cells (HTC cells)

The subcellular sites of sphingomyelin synthesis in BHK cells

The subcellular distributions of the enzymes which synthesise sphingomyelin (SM) and glucosylceramide (GluCer) from ceramide have been assessed in BHK cells. On a sucrose density gradient GluCer synthase (a marker of the cis/medial Golgi apparatus) and the trans-Golgi marker galactosyltransferase showed an similar monotonic distribution. In contrast, SM synthase showed two peaks of activity, a minor one which migrated with the Golgi markers and a major one which had a density close to that of plasma membrane markers (sphingomyelin, cholesterol, PtdSer, ganglioside GM3 and alkaline phosphodiesterase). When cell homogenates were treated with digitonin, the sedimentation characteristics of the Golgi markers was largely unaffected whereas the plasma membrane markers and the main peak of SM synthase activity were shifted to higher density. In contrast, when cells were treated with brefeldin A (BFA) the Golgi markers were shifted to higher density but not the plasma membrane markers or the main peak of SM synthase. These results suggest that the bulk of SM synthase activity in BHK cells is not associated with the Golgi cisternae but with a cell compartment which is relatively rich in cholesterol (e.g., plasma membrane, endosomes or trans-Golgi network.) Further experiments in which cells were treated with sphingomyelinase provided evidence that SM synthase activity was in an internal compartment and not at the plasma membrane.

Studies on regenerating liver and hepatoma plasma membranes--II. Membrane fluidity and enzyme activity

1. Rat hepatocyte plasma membranes isolated from Morris hepatoma 7288C, normal and regenerating liver were labelled with the fluorescent probe 1,6-diphenyl-1,3,5-hexatriene. 2. Steady-state fluorescence polarisation measurements indicated an increased fluidity of the membranes in the early stages of regeneration, returning to normal levels after 48 hr. 3. There was a decrease in hepatoma plasma membrane fluidity compared to normal hepatocytes. Changes in fluorescence polarisation with temperature (Arrhenius studies) indicate an increase in the lower critical temperature for the membrane lipid thermotropic transition of hepatoma compared to normal liver plasma membranes. 4. These changes in membrane lipid fluidity alter the activation of some intrinsic and extrinsic membrane bound enzymes.

Thromboplastin (tissue factor) in plasma membranes of human monocytes

The synthesis of thromboplastin, a potent trigger of blood coagulation, can be induced in human peripheral blood monocytes. Indirect evidence suggests that newly synthesized thromboplastin becomes in part available on the cell surface. We have attempted to study the localization and availability of thromboplastin more directly by isolating plasma membranes from isolated human peripheral blood monocytes. The specific activities of the plasma membrane markers increased 16-22-fold in these preparations with a recovery of about 15%. The contamination by mitochondria, lysosomes, nuclei and endoplasmic reticulum was low as estimated by marker enzymes and electron microscopy. In both unstimulated and stimulated monocytes thromboplastin was largely recovered in this plasma membrane fraction, providing direct evidence for its membrane localization. Phospholipase C (E.C. 3.1.4.3) is a potent inactivator of thromboplastin through its hydrolysis of the phospholipids necessary for thromboplastin activity [Otnaess, Prydz, Bjørklid & Berre (1972) Eur. J. Biochem. 27, 238-243]. About 70% of the total membrane thromboplastin activity was inactivated when whole cells were treated with phospholipase C and the membranes subsequently isolated. Following stimulation to induce thromboplastin synthesis, the plasma membranes showed a shift in their relative content of phosphatidylcholine and phosphatidylethanolamine consistent with a transmethylation process.

Biochemical and morphological characterization of a plasma membrane-enriched fraction from bovine parathyroid cells

A fraction of enriched plasma membranes from bovine parathyroid cells has been prepared by differential centrifugation. Biochemical characterization shows that this fraction has a specific activity enrichment of 7.2-fold in ouabain-sensitive Na+-K+ ATPase, and 3.5-fold in 5'-nucleotidase. Less than 4% of the total mitochondria and lysosomes are present within the plasma membranes, while microsomal contamination accounts for 14% of total specific activity. Parathyroid hormone radioimmunoassay also reveals the presence of some secretory granules within the plasma membrane fraction. The characteristic morphological aspect of the unusual surface membrane is shown by freeze-fracture electron microscopy. In the enriched pellets, vesicles identified as having a plasma membrane origin have variable sizes, and 50% show an inside-out conformation. Even though the plasma membrane fraction described herein is not absolutely free from contamination by other subcellular components, this protocol represents the first attempt to purify surface membrane from parathyroid tissue and provide the starting material for understanding, at a molecular level, the properties of extracellular Ca2+ regulation and its coupling with secretion of parathyroid hormone.