Properties isolated plasma membranes

Primarily using isolated plasma membrane vesicles as an experimental preparation, the functional properties of Na /H exchangers have been elucidated. The important kinetic properties include (1) stoichiometry (one-for-one) (2) reversibility (3) substrate specificity (monovalent cations Na, H, Li, NH4, but not K, Rb, Cs, choline) (4) modes of operation (Na -for-H, Na -for-Na Li " -for-Na, Na -for-NH4 ) (5) existence of an internal site for allosteric activation by (6) reversible inhibition by amiloride (Af-amidino-5-amino-6-chloropyr-azine carboxamide) and its 5-amino-substituted analogs and (7) competitive nature... 

Coleman, R., and Finean, J. B., 1966, Preparation and properties of isolated plasma membranes from guinea-pig tissues, Biochim. Biophys. Acta 125 197. 

There have been rather few studies of the location of probes in whole cells. DPH incorporates into most subcellular fractions (see, e.g, Ref. 64), whereas with TMA-DPH, early after introduction only the plasma membranes appear to be labeled/64,65) There is considerable interest in examining the lipid motional properties of living cells by fluorescence techniques. In this type of study the location of the probe has to be carefully checked before conclusions can be drawn. This is carried out by separate measurements of the recovery of probe from intact labeled cells in isolated subcellular fractions and/or by fluorescence microscopy. 

Lipoproteins or glycoproteins are the macromolecules that most commonly form receptors. They are often firmly embedded in the plasma membrane or cell-organelle membrane as intrinsic proteins (see section 7.1). At times, this renders their isolation and subsequent functional reconstitution difficult, as their structure may be dependent upon the surrounding membrane. Isolation of such a receptor molecule may cause its structural collapse, even to the extent that specific binding properties are lost. 

Chemical analyses of membranes isolated from various sources reveal certain common properties. Each kingdom, each species, each tissue or cell type, and the organelles of each cell type have a characteristic set of membrane lipids. Plasma membranes, for example, are enriched in cholesterol and contain no detectable cardiolipin (Fig. 11-2) in the inner mitochondrial membrane of the hepatocyte, this distribution is reversed very low cholesterol and high cardiolipin. Cardiolipin is essential to the function of certain proteins of the inner mitochondrial membrane. Cells clearly have mechanisms to control the kinds and amounts of membrane lipids they synthesize and to target specific lipids to particular organelles. In many cases, we can surmise the adaptive advantages of distinct combinations of membrane lipids in other cases, the functional significance of these combinations is as yet unknown. 

A low molecular weight protein, different from metallothionine which reversibly binds iron with high affinity has been isolated from rabbit reticulocyte cytosol (54, 55, 56). Although very little is yet known about its physiological properties, the molecular weight is around 6000, and iron appears to be reversibly bound under physiological conditions. This protein may be able to mobilize iron from the plasma membrane and donate it for heme and ferritin biosynthesis (56), but no definitive physiological role for siderochelin has been established. 

A standardized approach has been developed for arriving at a method for isolation of purified plasma membrane from each of various smooth muscles (31-41 see 26,33,34). It starts with a very careful dissection to remove all possible cells other than smooth muscle. Studies have shown that what appears to be a minor contamination by non-smooth muscle components (eg. fat cells or adventitia of blood vessels) may markedly affect the overall properties of the subsequent membrane fractions isolated (36). Since many smooth muscle organs contain two or more layers of muscle which often differ in their physiological (42), pharmacological (43) and isolated membrane properties (44), it is always desirable to separate muscle layers and to isolate their membranes individually. There are inevitably a few fibroblasts and nerves left in the most carefully dissected smooth muscle tissue so far, no one has compared these properties of membranes from smooth muscle tissue to those from freshly isolated smooth muscle cells to determine if these residual contaminent membranes affect the properties of the membranes from tissues we think not. 

Yermiyahu, U., Nir, S., Ben-Hayyim, G., Kafkafi, U., Scherer, G.F.E., Kinraide, T.B., 1999. Surface properties of plasma membrane vesicles isolated from melon (Cucumus melo L.) root cells differing in salinity tolerance. CoUoids Surfaces B 14, 237-249. 

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