Phospholipids formation

Figure 20.3 Essential fatty acids in the diet, production of physiological essential acids and their roles in the cell cycle. Essential fatty adds in the diet are mainly linoleic and a-linolenic but they are converted by desaturation and elongation reactions to the essential acids that are used in phospholipid formation and synthesis of eicosanoids. (For details of the elongation and desaturation reactions and eicosanoid formation, see Chapter 11.).

Aminoglycoside binding to membrane acidic phospholipids Changes in content and metabolism of membrane phospholipids Displacement of calcium bound to phospholipids Formation of myeloid bodies... 

Triglyceride and phospholipid formation. This figure depicts the formation of triacylglycerol from a-glycerolphosphate and fatty-acyl CoA. The formation of phosphatidylethanolamine and phosphatidylcholine from scratch (i.e., from serine and methionine methyl groups) is also shown. The formation of phosphatidylcholine, starting with choline, is also depicted and is the major pathway for phosphatidylcholine synthesis. 

P. The enzyme catalyzing the formation of phosphatidylserine appears to occur naturally as an integral membrane protein of the ER. Some is also bound to ribosomes and to mitochondria. In contrast, most of the other enzymes of phospholipid formation are closely associated with or embedded in the cytoplasmic membrane. One of these, a pyruvoyl group dependent enzyme (Chapter 14, Section F), catalyzes decarboxylation of phosphatidylserine to phosphati-dylethanolamine (PE, step /, Fig. 21-4). This reaction had been thought unimportant in animals, but... 

Horrocks LA, Demediuk P, Saunders RD, Dugan L, Clendenon NR, Means ED, Anderson DK (1985) The degradation of phospholipids, formation of metabolites of arachidonic acid, and demyelination following experimental spinal cord injury. Cent Nerv Syst Trauma 2 115-120... 

Carr, A. C., J. J. van den Berg, and C. C. Winterboum. 1998. Differential reactivities of hypo-chlorous and hypobromons acids with pnrified Escherichia coli phospholipid Formation of haloamines andhalohydrins.1392(2-3) 254—64. 

Paclobutrazol is a triazole plant growth retardant which also has fungicidal properties [1]. It is an inhibitor of cytochrome P-450 dependant enzymes such as ent-kaurene oxidase and sterol 14a-methyl-demethylase [1]. The fungicidal activity of azole compounds is believed to result from the inhibition of sterol 14a-demethylation. This causes an accumulation of 14a-methylsterols and loss of ergosterol which may have adverse effects on membrane properties [2]. However, there is evidence for other essential requirements for sterol In cell proliferation in addition to a membrane structural role [3-6].For example, in yeast mutants traces of ergosterol stimulate phospholipid formation [7] and protein kinase activity [8]. This report describes the effects of paclobutrazol on the sterol and phospholipid compositions of membranes from plant and yeast cultures. 

These studies with bacteria indicate that, although animals and bacteria have a number of. similar pathways for phospholipid formation, there are several marked differences. Thus the major if not sole padiway for phosphatidylethanolamine (and phosphatidylserine) synthesis in bacteria is via CDP-diglyceride, while in animals this compound is formed from diglyceiide and CDP-ethanolamine (Keimedy and Weiss, 1956). Similarly, the major pathway for phosphatidylcholine sjmthesis in bacteria is via methylation of phosphatidylethanolamine, whereas in animals this compound is formed via diglyceride and CDP-choline (Kennedy and Weiss, 1956), as well as by methylation of phosphatidylethanolamine (Bremer and Greenberg, 1961). 

Figure 7.3 Relative importance of pathways for phospholipid formation. Thickness of lines indicate relative importance. For lipid abbreviations see Figure 7.1.

Cationic surfactants may be used [94] and the effect of salinity and valence of electrolyte on charged systems has been investigated [95-98]. The phospholipid lecithin can also produce microemulsions when combined with an alcohol cosolvent [99]. Microemulsions formed with a double-tailed surfactant such as Aerosol OT (AOT) do not require a cosurfactant for stability (see, for instance. Refs. 100, 101). Morphological hysteresis has been observed in the inversion process and the formation of stable mixtures of microemulsion indicated [102]. 

The effect is more than just a matter of pH. As shown in Fig. XV-14, phospholipid monolayers can be expanded at low pH values by the presence of phosphotungstate ions [123], which disrupt the stmctival order in the lipid film [124]. Uranyl ions, by contrast, contract the low-pH expanded phase presumably because of a type of counterion condensation [123]. These effects caution against using these ions as stains in electron microscopy. Clearly the nature of the counterion is very important. It is dramatically so with fatty acids that form an insoluble salt with the ion here quite low concentrations (10 M) of divalent ions lead to the formation of the metal salt unless the pH is quite low. Such films are much more condensed than the fatty-acid monolayers themselves [125-127]. 

Levels of free fatty acids are very low in the typical cell. The palmitate made in this process is rapidly converted to CoA esters in preparation for the formation of triacylglycerols and phospholipids.)... 

Formation from Template Surfaces Recently, a new method for the preparation of LUV was reported by Lasic et al. (1988). The method is based on a simple procedure which leads to the formation of homogeneous populations of LUV with a diameter of around L vim. Upon addition of solvent to a dry phospholipid film deposited on a template surface, vesicles are formed instantly without any chemical or physical treatment. The formation of multilamellar structures is prevented by inducing a surface charge on the bilayers. The size of the vesicles is controlled by the topography of the template surface on which the phospholipid film was deposited (Lasic, 1988). 

Hydration of Phospholipids with Solutions of Very Low Ionic Strength Very large unilamellar and oligolamellar vesicles can be prepared when a thin lipid film is dispersed in a solution of very low ionic strength (Reeves and Dowben, 1969). The formation of vesicles with diameters up to 300 pm enclosing latex beads with a diameter of 20 pm have been reported (Antanavage et al., 1978). 

Sonication of MLV dispersions above the Tc of the lipids results in the formation of SUV (Saunders, et al., 1962). Sonication can be performed with a bath sonicator (Papahadjopoulos and Watkins, 1967) or a probe sonicator (Huang, 1969). During sonication the MLV structure is broken down and small unilamellar vesicles with a high radius of curvature are formed. In case of SUV with diameters of about 20 nm (maximum radius of curvature), the outer monolayer can contain over 50% of the phospholipids and in the case of lipid... 

Injection of phospholipid dissolved in ethanol into excess water heated above the Tc of the lipids results in the formation of mainly unilamellar vesicles (Batzri and Korn, 1973). The remaining ethano can be removed by dialysis or by gel filtration (Nordlund et al., 1981). 

Ueno, M., Tanford, C., and Reynolds, J. A. (1984). Phospholipid vesicle formation using nonionic detergents with low monomer solubility. Kinetic factors determine vesicle size and polydis-persity. Biochemistry, 3070-3076. 

Figure 14-22. Formation of lipid membranes, micelles, emulsions, and liposomes from am-phipathic lipids, eg, phospholipids.

Rats fed a purified nonlipid diet containing vitamins A and D exhibit a reduced growth rate and reproductive deficiency which may be cured by the addition of linoleic, a-linolenic, and arachidonic acids to the diet. These fatty acids are found in high concentrations in vegetable oils (Table 14-2) and in small amounts in animal carcasses. These essential fatty acids are required for prostaglandin, thromboxane, leukotriene, and lipoxin formation (see below), and they also have various other functions which are less well defined. Essential fatty acids are found in the stmctural lipids of the cell, often in the 2 position of phospholipids, and are concerned with the structural integrity of the mitochondrial membrane. 

Figure 24-2. Biosynthesis of triaq/lglycerol and phospholipids. ( , Monoacylglycerol pathway (D, glycerol phosphate pathway.) Phosphatidylethanolamine may be formed from ethanolamine by a pathway similar to that shown for the formation of phosphatidylcholine from choline.

HDL concentrations vary reciprocally with plasma triacylglycerol concentrations and directly with the activity of lipoprotein lipase. This may be due to surplus surface constituents, eg, phospholipid and apo A-I being released during hydrolysis of chylomicrons and VLDL and contributing toward the formation of preP-HDL and discoidal HDL. HDLj concentrations are inversely related to the incidence of coronary atherosclerosis, possibly because they reflect the efficiency of reverse cholesterol transport. HDL, (HDLj) is found in... 

Figure 51-1. The pathways of blood coagulation. The intrinsic and extrinsic pathways are indicated. The events depicted below factor Xa are designated the final common pathway, culminating in the formation of cross-linked fibrin. New observations (dotted arrow) include the finding that complexes of tissue factor and factor Vila activate not only factor X (in the classic extrinsic pathway) but also factor IX in the intrinsic pathway, in addition, thrombin and factor Xa feedback-activate at the two sites indicated (dashed arrows). (PK, prekallikrein HK, HMW kininogen PL, phospholipids.) (Reproduced, with permission, from Roberts HR, Lozier JN New perspectives on the coagulation cascade. Hosp Pract [Off Ed] 1992Jan 27 97.)...

Modeling Pardaxin Channel. The remarkable switching of conformation in the presence of detergents or phospholipid vesicles (5) suggests that pardaxin is a very flexible molecule. This property helps to explain the apparent ability of pardaxin to insert into phospholipid bilayers. In addition, it is consistent with the suggestion that the deoxycholate-like aminoglycosteroids (5,7) present in the natural secretion from which pardaxin is purified (5) serve to stabilize its dissociated conformation. The question of the mechanism by which pardaxin assembles within membranes is important for understanding pore formation and its cytolytic activity (5). 

Adsorption chromatography using small particle silica or alumina has also been employed in the separation of biologically meaningful substances. Phospholipids, for example, have been separated on silica (38). One of the big problems for such substances is detection, since many of the compounds are not U.V. active. Generally, the refractive index detector is employed for isocratic operation, and the moving wire detector for gradient operation. Formation of U.V.-active derivatives is also possible (39). 

In this in vitro system, the presence of serum in cell culture medium is not necessary, but the type of transwell is important (the total amount of H-triglycerides secreted was two-fold higher when using 3 pm versus 1 pm pore size transwells), and oleic acid supplementation is required for the formation and secretion of CMs as well as the transport of 3-carotene through Caco-2 cells. Finally, the presence of Tween 40 does not affect CM synthesis and secretion in this in vitro cell culture system. Thus, CMs secreted by Caco-2 cells were characterized as particles rich in newly synthesized H-triglycerides (90% of total secreted) containing apolipoprotein B (30% of total secreted) and H-phospholipids (20% of total secreted) and with an average diameter of 60 nm. These characteristics are close to those of CMs secreted in vivo by enterocytes. ... 

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