Erythrocyte membrane lipids phospholipids

The erythrocyte-bound malaria parasite acquires host lipids by diverse methods and transports to the area of celluar growth. There they are remodeled significantly to meet the parasite s needs, including phospholipid head group alteration, modification of the fatty acid molecular species composition of certain phospholipids, and transformation of the erythrocyte membrane lipid composition in both infected and uninfected cells. 

The narrowness of these signals from isolated erythrocyte membrane lipids (Chapman et al., 1968a) in contrast with their broadness observed with 1 1 mixtures of egg-yolk lecithin and cholesterol (Chapman and Penkett, 1966) is readily explained if an optimum cholesterol/phospholipid molecular ratio, 1 1, is required for maximum broadening effect. Red cell phospholipids and other membrane lipids in excess of the optimum ratio, i.e., lipids which could not form a complex with cholesterol (Vandenheuvel, 1963), would then be responsible for observed narrow signals. 

Although the majority of the lipids in M. laidlawii membranes appear to be in a liquid-crystalline state, the system possesses the same physical properties that many other membranes possess. The ORD is that of a red-shifted a-helix high resolution NMR does not show obvious absorption by hydrocarbon protons, and infrared spectroscopy shows no ft structure. Like erythrocyte ghosts, treatment with pronase leaves an enzyme-resistant core containing about 20% of the protein of the intact membrane (56). This residual core retains the membrane lipid and appears membranous in the electron microscope (56). Like many others, M. laidlawii membranes are solubilized by detergents and can be reconstituted by removal of detergent. Apparently all of these properties can be consistent with a structure in which the lipids are predominantly in the bilayer conformation. The spectroscopic data are therefore insufficient to reject the concept of a phospholipid bilayer structure or to... 

In 1925, E. Gorter and F. Grendel (J. Exp. Med. 41, 439) reported measurements in which they extracted lipid from red blood cell membranes with acetone, spread the lipids as a monolayer, and measured the area of the compressed monolayer. They then estimated the surface area of an erythrocyte and calculated that the ratio of the lipids (as a monolayer) to the surface area of the red blood cell was 1.9-2.0. More modern experiments gave the following each erythrocyte membrane contains 4.5 x 10 16 mol of phospholipid and 3.1 x 10-16 mol of cholesterol. 

First, a mixture of synthetic or natural phospholipids, polymerizable lipids, and proteins can be converted to liposomes and then be polymerized. Second, polymerizable lipids are introduced into e.g. erythrocyte ghost cells by controlled hemolysis and subsequent polymerization as described by Zimmermann et al.61). This hemolysis technique is based on a reversible dielectric breakdown of the cell membrane. Dielectric breakdown provides a third possible path to the production of bi omembrane models. Zimmermann et al. could show that under certain conditions cells can be fused with other cells or liposomes61). Thus, lipids from artificial liposomes could be incorporated into a cell membrane. A fourth approach has been published by Chapman et al.20). Bacterial cells incorporate polymerizable diacetylene fatty acids into their membrane lipids. The diacetylene units can be photopolymerized in vivo. The investigations discussed in more detail below are based on approaches 1. and 3. 

Data on the proportions of different fatty acids in plasma lipid esters (cholesteryl esters, phospholipids, free fatty acids, or triacylglycerol), erythrocyte membranes, or adipose tissue may provide a more objective and accurate path to evaluating dietary fatty acid composition (Arab, 2003 Baylin and Campos, 2006). The fatty acid composition in blood and body tissues reflects the fatty acid composition of the diet at different time points after ingestion. Short and medium-term changes in the composition of dietary fatty acid intake are reflected in plasma lipids and erythrocyte membranes, weeks and months after intake, respectively. The incorporation of fatty acids in adipose tissue reflects long-term changes in the diet (years) (Baylin and Campos, 2006 Katan et al., 1997 Ma et al., 1995 Zock et al, 1997). 

Cholesterol - an essential component of mammalian cells - is important for the fluidity of membranes. With a single hydroxy group, cholesterol is only weakly am-phipathic. This can lead to its specific orientation within the phospholipid structure. Its influence on membrane fluidity has been studied most extensively in erythrocytes. It was found that increasing the cholesterol content restricts molecular motion in the hydrophobic portion of the membrane lipid bilayer. As the cholesterol content of membranes changes with age, this may affect drug transport and hence drug treatment. In lipid bilayers, there is an upper limit to the amount of cholesterol that can be taken up. The solubility limit has been determined by X-ray diffraction and is... 

List the four phospholipids that are commonly found in biological membranes. Explain how the single assay performed in this experiment could be used to quantify this entire group of lipids in the erythrocyte membrane. 

The implications of the existence of an enormous diversity of lipid species and fatty acid patterns in different membranes within one organism as well as the variations between different organisms have posed a allenge for a long time. Any model of lipid bilayer function has to take account of these variations. If we consider erythrocyte membrane phospholipids for example, the rat has about 50% phosphatidylcholine (PC) and only about 10% sphingomyelin (SM), whereas the sheep erythrocyte membrane contains more than 50% SM and no PC. By contrast the total membrane content of phosphatidylethanolamine (PE) + PC + PM in mammalian species is fairly constant, equal to about 15-20%, and the rest are charged lipids... 

The membrane constituents are lipids (phospholipids, glycosphingolipids, and cholesterol Figure 10-5), carbohydrates, and proteins. The ratio of protein lipid carbohydrate on a weight basis varies considerably from membrane to membrane. For example, the human erythrocyte membrane has a ratio of about 49 43 8, whereas myelin has a ratio of 18 79 3. The composition of the normal human erythrocyte membrane is shown in Table 10-2. All membrane lipids are amphipathic (i.e., polar lipids). The polar heads of the phospholipids may be neutral, anionic, or dipolar. The surface of the membrane bears a net negative charge. The distribution of lipid constituents in the bilayer is asymmetrical. For example, in the erythrocyte membrane, phosphatidylethanolamine and phosphatidylserine are located primarily in the internal monolayer, whereas phosphatidylcholine and sphingomyelin are located in the external monolayer. 

The transfer of radiolabeled phospholipids between vesicles and erythrocyte membranes could be used to assay lipid transfer activity. Intact erythrocytes are not an ideal substrate for routine measurements of transfer activity because some transfer proteins do not readily accelerate the transfer of phospholipids from these membranes. Van Meer et al. (1980) found that a very high concentration of the phosphatidylcholine-specific transfer protein was necessary to exchange the phosphatidylcholine of intact red blood cells. Erythrocyte ghosts are a more active substrate for this protein (Bloj and Zilversmit, 1976). However, the nonspecific transfer protein from bovine liver accelerates the exchange of phospholipid between intact erythrocytes and phosphatidylcholine vesicles (Crain and Zilversmit, 1980c). 

Net transfer of lipid occurs from the plasma to the erythrocyte membrane, presumably because of a shift in the equihbrium as the plasma lipoproteins become saturated with the excess cholesterol and phosphatidylcholine. This leads to membrane abnormalities and cholesterol-phospholipid ratios of up to 2 1. Changes in cellular physiology of the type referred to in section IV have also been reported [94,96,161]. These must reflect an alteration in lipid-protein interactions within the membranes. The molecular arrangement of the excessive amounts of cholesterol present in the cell membranes in diseased liver cells is not known. In model systems cholesterol is not present in molar amounts greater than 1 1. In liver disease a major change is in cellular morphology with the formation of abnormally shaped erythrocytes, as discussed earlier. 

Is lipid-assisted folding a widespread phenomenon and possibly applicable to soluble proteins The erythrocyte membrane contains about 20-mole % of PE that is almost exclusively localized in the inner leaflet and is in contact with highly concentrated heme-containing proteins. The refolding of the denatured soluble and heme-containing enzyme horseradish peroxidase (HRP) was followed in the presence and absence of liposomes made up of different phospholipids (Debnath et al., 2003). Remarkably, dimyristoyl-PE (a bilayer-forming... 

These results and those now discussed which show the ability of resorcinolic lipids to incorporate and modulate phospholipid bilayer properties, suggest a possible similar role in biological membrane-related enzymatic activities. It has been demonstrated that at a concentration of 10 5 M, long-chain resorcinolic lipids, caused a decrease of apparent acetylocholinesterase activity in the erythrocyte membrane while simultaneously stimulating the activity of Ca2+-dependent ATPase [348]. The inhibition of erythrocyte acetylcholinesterase has been also observed for other phenolic lipids (Stasiuk and Kozubek - unpublished work) and a similar effect of one of the homologues, namely tridecylresorcinol shown towards Na+-K+ ATPase [157]. a-Glucosidase and aldolase were also inhibited by resorcinolic lipids isolated from cashew [283]. 

For P. falciparum, and the related parasite Babesia divergens, high density lipoproteins (HDL) are a major source of preformed lipid, in particular PC, significant amounts of which were converted into PE (24,25). A unidirectional flux of PC to the erythrocyte membrane from HDL and subsequently to the intracellular parasite was proposed for both parasites. Malaria-infected erythrocytes can acquire phospholipids (PC, PE, PS) directly from lipid vesicles this phenomenon, which is enhanced by PhL transfer proteins, may involve exchange with the erythrocyte membrane (26). 

As shown in a Swedish cohort of healthy 50-year-old men with 20 years follow-up, proportion of 14 0 and 16 0 in serum cholesterol esters predicted the development of metabolic syndrome, independently of other metabolic and lifestyle factors [105]. Besides serum lipids, proportion of PA in adipose tissue is also related to insulin sensitivity. Unlikely 14 0 and 18 0, which were positively associated with insulin sensitivity, PA inversely correlated with insulin sensitivity in 59 healthy British men and women [134]. Since the authors excluded the effect of dietary intake for any of these SFA, they concluded that the reason is de novo lipogenesis in adipose tissue. In human skeletal muscle phospholipids, SFA [135,135] and especially PA [137] have been negatively associated with insulin sensitivity and Type 2 diabetes, [138] which could partly reflect dietary intake [139], In a Finnish cohort study of 4 years follow-up, impaired fasting glucose and Type 2 diabetes incidence were associated with serum nonesterified 16 0 levels, but were not associated with baseline dietary 16 0 intakes assessed from dietary records [140], Recently published prospective follow-up study showed that erythrocyte membrane fatty acids nominaly predict incident type 2 diabetes [141], In the American Atherosclerosis Risk in Commimities (ARIC) study, 2909 middle-aged men and women were followed for 9 years. The incidence of Type 2 diabetes was associated with total SFA levels of plasma cholesterol esters (also observed for 16 0 independently) and phospolipids (also for 16 0 and 18 0) [101]. In a more recent 4-year case-... 

The effects described here in a model membrane may have some implications for membrane fusion. When a fusogenic lipid is introduced into the asymmetric bilayer structure of an erythrocyte membrane (Zwaal et al., 1973) it may initially interact with choline-containing phospholipids in the outer half of the bilayer (Maggio Lucy, 1975). This will alter the phospholipid head groups and... 

It is known that cholesterol interacts with erythrocyte phospholipids reducing its molecular area (Demel et al., 1967) and, as a consequence, a decrease in the local fluidity of the lipid matrix of the erythrocyte membrane occurs, as it was determined by electron spin resonance studies (Kroes et al., 1972). The results obtained for the inhibition by F of the erythrocyte membrane acetylcholinesterase and (Na" ", KT ")-ATPase from rats fed corn oil and corn oil-plus-cholesterol supplemented diet, respectively, are presented in Table 5. In the case of acetylcholinesterase, the values of n change from 1.5 to 1.0 because of cholesterol dietary effect. Consequently, in the (Na , KT ")-ATPase the values of n vary in an inverse manner (from 2.0 to 3.6). (Bloj et al., 1973 ). 

A notable feature of the lipid regions of biological membranes is that the different phospholipid types may be asymmetrically distributed across the bilayer. For the erythrocyte membrane for example, it has been demonstrated by surface labelling and phospholipase digestion that the sphingomyelin and phosphatidylcholine are located in the outer half of the bilayer, whereas the phosphatidylethanolamine and phosphatidylserine are localized to the inner half (Zwaal et al., 1973). 

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