Membrane phospholipid surfaces

Xiang, T.-X. Anderson, B. D., Phospholipid surface density determines the partitioning and permeability of acetic acid in DMPC cholesterol bilayers, J. Membrane Biol. 148, 157-167 (1995). 

Isaacs, B.S., Husten, E.J., Esmon, C.T., and Johnson, A.E. (1986) A domain of membrane-bound blood coagulation factor Va is located far from the phospholipid surface. A fluorescence energy transfer measurement. Biochemistry 25, 4958-4969. 

The region at the surface of membranes and the underlying phospholipid head-group region are of particular interest since these regions can vary considerably with variations in the membrane phospholipid composition and under the influence of external molecules such as ions and hydrophobic molecules. The fluorescence anisotropy parameter tends to be less useful for examining this region since it is already intrinsically disordered and the... 

Methods used to demonstrate the existence of membrane phospholipid asymmetry, such as chemical labelling and susceptibility to hydrolysis or modification by phospholipases and other enzymes, are rmsuitable for dynamic studies because the rates of chemical and biochemical reactions are of a different order compared to the transmembrane translocahon of the phospholipids. Indirect methods have therefore been developed to measure the translocation rate which are consequent on the loss of membrane phospholipid asymmetry. Thus time scales appropriate to rates of lipid scrambling under resting conditions or when the forces preserving the asymmetric phospholipid distribution are disturbed can be monitored. Generally the methods rely on detecting the appearance of phosphatidylserine on the surface of cells. Methods of demonstrating Upid translocation in mammalian cells has been the subject of a recent review (Bevers etal., 1999). 

Reviews of the role of aminophospholipid translocase and scramblase (Schlegel et al, 2000) and the consequences of the appearance of phosphatidylserine on the cell surface (Williamson et al, 2001) in apoptosis of thymocytes have been published. The precise relationship between membrane phospholipid asymmetry and apoptosis is currently a topic of considerable interest. 

The control of the processes of phosphohpid translocation and scrambling by cytoplasmic calcium concentration is known to be one factor in regulating the appearance of phosphatidylserine on the cell surface. There is now evidence that other factors are involved in modulating membrane phospholipid distribution and for preserving phospholipid homeostasis. Current research is directed to clarify the role of these agents and to establish any connections with the initiation of apoptosis. 

Umeda, M., and Emoto, K., 1999, Membrane phospholipid dynamics during cytokinesis regulation of actin filament assembly by redistribution of membrane surface phosphohpid. Chem. Phys. Lipids, 101 81-91. 

Both pathways depend on the presence of activated thrombocytes, on the surface of which several reactions take place. For example, the prothrombinase complex (left) forms when factors Xa and 11, with the help of Va, bind via Ca "" ions to anionic phospholipids in the thrombocyte membrane. For this to happen, factors 11 and X have to contain the non-proteinogenic amino acid y-carboxygluta-mate (Gla see p. 62), which is formed in the liver by post-translational carboxylation of the factors. The Gla residues are found in groups in special domains that create contacts to the Ca "" ions. Factors Vll and IX are also linked to membrane phospholipids via Gla residues. 

On surface of platelet membrane phospholipid, phosphatidyl serine forms tenase and prothrombinase complexes... 

I agree with Professor McConnell that phospholipid phase transitions may play a role in controlling the activity of a membrane-bound enzyme. However, the case cited is somewhat ambiguous, since porcine phospholipase A2 is a soluble enzyme acting on a phospholipid surface. The major effect of the phase transition in this case is to alter the nature of the substrate rather than the intrinsic catalytic activity of the enzyme. 

A very brief description of biological membrane models, and model membranes, is given. Studies of lateral diffusion in model membranes (phospholipid bilayers) and biological membranes are described, emphasizing magnetic resonance methods. The relationship of the rates of lateral diffusion to lipid phase equilibria is discussed. Experiments are reported in which a membrane-dependent immunochemical reaction, complement fixation, is shown to depend on the rates of diffusion of membrane-bound molecules. It is pointed out that the lateral mobilities and distributions of membrane-bound molecules may be important for cell surface recognition. 

Cyt c is associated with the outer surface of the inner mitochondrial membrane. Phospholipids induce conformational changes in the protein and, in certain instances, the haem can convert to the high spin (S = 5/2) form, indicative of a weakening of the ligand field caused by displacement of the sixth ligand (Met-80). This has been associated with the detection of lipid radicals by direct EPR (at 11 K).65 Indeed, peroxidase-type activity is also evident in the reaction of cyt c with lipid hydroperoxides, as studied by spin trapping in conjunction with HPLC and MS.66... 

The sample is disrupted completely and distributed over the surface as a function of interactions with the support, the bonded phase, and the tissue matrix components themselves. The solid support acts as an abrasive that promotes sample disruption, whereas the bonded phase acts as a lipophilic, bound solvent that assists in sample disruption and lysis of cell membranes. The MSPD process disrupts cell membranes through solubilization of the component phospholipids and cholesterol into the Cis polymer matrix, with more polar substituents directed outward, perhaps forming a hydrophilic outer surface on the bead. Thus, the process could be viewed as essentially turning the cells inside out and forming an inverted membrane with the polymer bound to the solid support. This process would create a pseudo-ion exchange-reversed-phase for the separation of added components. Therefore, the Cis polymer would be modified by cell membrane phospholipids, interstitial fluid components, intracellular components and cholesterol, and would possess elution properties that would be dependent on the tissue used, the ratio of Cis to tissue employed and the elution profile performed (99-104). 

The N-acetylneuraminic acid in the RBC membrane can be removed by incubating RBCs with neuraminidase. Incubation of a dilute human RBC suspension (1% by volume) at 37°C for 1 hr with 15 /xg/mL of neuraminidase causes the complete removal of N-acetylneuraminic acid and a reduction of surface potential by 90% (24). The residual negative surface charges can be attributed to the phosphate groups of the membrane phospholipids or the carboxyl groups of the membrane proteins (28). 

With some drugs, renal damage may be related to the drugs biochemical mechanism of action. For example, the polymycins, such as amphotericin B, are surface-active agents that bind to membrane phospholipids, disrupting the integrity of the membrane and resulting in leaky cells. 

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