Phospholipids shifts

FIGURE 4.14 Effects of G-protein on the displacement of the muscarinic antagonist radioligand [sH]-L-quinuclidinyl benzylate by the agonist oxotremorine. Displacement in reconstituted phospholipid vesicles (devoid of G-protein sububits) shown in filled circles. Addition of G-protein (Go 5.9 nmol Py-subunit/3.4 nmol ao-IDP subunit) shifts the displacement curve to the left (higher affinity, see open circles) by a factor of 600. Data redrawn from [14]. 

The use of Europium shift reagent in combination with EDTA was used to study chelation to phospholipids.60... 

Section 3.3.4 pointed out that cosolvents alter aqueous ionization constants as the dielectric constant of the mixture decreases, acids appear to have higher pKa values and bases appear (to a lesser extent than acids) to have lower values. A lower dielectric constant implies that the force between charged species increases, according to Coulomb s law. The equilibrium reaction in Eq. (3.1) is shifted to the left in a decreased dielectric medium, which is the same as saying that pKa increases. Numerous studies indicate that the dielectric constant in the region of the polar head groups of phospholipids is 32, the same as the value of methanol. [381,446-453] Table 5.2 summarizes many of the results. 

Simultaneous decrease in the content of diene conjugates and increase in the content of Schiff bases evidence the quick shift of pro-/antioxidant equilibrium, generation of reactive radicals, and damage of cell membranes in EAC cells, because Schiff bases, generated as a consequence of interaction of malonic dialdehyde with aminogroups of phospholipids and proteins, are highly reactive compounds causing polycondensation of molecules and formation of intermolecular bonds. 

Lipids and proteins can shift easily within one layer of a membrane, but switching between the two layers ( flip/flop") is not possible for proteins and is only possible with difficulty for lipids (with the exception of cholesterol). To move to the other side, phospholipids require special auxiliary proteins (translocators, flipases ). 

The cholesterol excreted with the bile is poorly water-soluble. Together with phospholipids and bile acids, it forms micelles (see p. 270), which keep it in solution. If the proportions of phospholipids, bile acids and cholesterol shift, gallstones can arise. These mainly consist of precipitated cholesterol (cholesterol stones), but can also contain Ca " salts of bile acids and bile pigments (pigment stones). 

The coexistence of lipid and water solubility in the same molecule is essential for the action of a local anaesthetic drug. Lipophilicity permits the migration of drug across the phospholipid membrane of the nerve cell hydrophilicity is essential for the ionisation of the drug within the nerve. It follows that lipid and water solubility are the external and internal facilitators of local anaesthetic action in the nerve cell. Both within and without the nerve cell the unionised and ionised forms coexist in dynamic equilibrium. Outside the nerve, the active species is the unionised tertiary amine form. Conversely, inside the cell the ionised form predominates. The lower intracellular pH induces a shift in the equilibrium in favour of ionisation (Figure 5.5). 

Let us consider first lipid-lipid interaction. Urry et al, showed the existence of a positive CD band at 218 m/x and a negative CD band at about 192 m/z in phosphatidyl choline and phosphatidyl ethanolamine dissolved in trifluoroethanol (86). The 192-m/z band was not characterized in detail, but the 218-m/z band is of such position and shape that the addition of lipid and protein CD bands could produce a composite CD band, and hence an ORD Cotton effect, which is red shifted. As noted by Urry, the 218-m/z CD extremum of lecithin must arise from n — 7T transitions in the fatty acid ester groups. Although the optical activities of solutions of deproteinized membrane phospholipids determined at the same concentration as in the intact membrane are negligibly small, in membranes an ordered array of lipids could greatly enhance rotation. Such an effect could yield information on the nature of lipid-lipid association. This can be tested experimentally. Halobacterium cutirubrum offers a unique system since Kates has shown that the lipids in this extreme halophile contain ether bonds rather than ester bonds (43, 44), Hence, the n — tt transition essential to the CD band at 218 m/z in phospholipids does not exist. Nevertheless, we found that the ORD... 

As a second possibility, lipid-protein interaction must be considered. The red shift might be explained in terms of hydrophobic interaction of the hydrocarbon chains of phospholipids with the protein in such a way that the amide chromophores are transferred to a less polar environment (89). Again, the hypothesis can be tested by removal of lipid. The existence of the red shift in lipid-depleted mitochondria and in lipid-free mitochondrial structural protein shows that lipid-protein interaction is not necessary to produce the ORD spectra characteristic of membranes. It is possible that if some molecular rearrangement occurs during the extraction process, a red shift caused by hydrophobic lipid-protein association could be replaced with a red shift arising from hydrophobic protein-protein association. Such an explanation is unlikely, especially in view of the retention of the unit membrane structure in electron micrographs taken of extracted vesicles (30). On the basis of ORD, then, the most reasonable conclusion is that the red shift need not be assigned to lipid-protein association. 

A more complex case is the serum lipoprotein (74), shown in Figure 13. When sonicated into water, total lipids from both the low density (/ ) and high density (a) lipoproteins give rise to the high resolution spectra expected of molecules which have a high degree of motion. The spectra of the native lipoproteins show line widths nearly identical to those of the lipids alone, so that no additional motional constraints of the apolar portions of the phospholipids occur when the lipids are bound to the apoproteins of the blood lipoproteins. All the obvious peaks observed in the native lipoproteins can be accounted for by lipid protons, and no upheld shift of the methylene protons occurs. We can conclude that unlike the case of the lysolecithin-serum albumin system, the bonding of lipids to proteins is not apolar. In the serum lipoproteins the NMR results are consistent with a micellar structure and not with extensive apolar association of lipid with protein. 

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... 

Cells regulate the lipid compositions of their plasma membrane so that a reasonable membrane fluidity is main-tained. They do this by controlling fatty acid biosynthesis so as to vary the lengths of the fatty acid chains and the ratio of unsaturated to saturated fatty acids (see chapter 19). If cells are grown at low temperatures, their phospholipids contain more unsaturated fatty acids or fatty acids with shorter chains or both. These adjustments shift the Tm to lower temperatures, with the result that (to the extent that the melting transition is sharp enough to be measureable) the Tm re-... 

The transformation of squalene into lanosterol. The squalene monooxygenase reaction requires 02, NADPH, FAD, phospholipid, and a cytosolic protein. The cyclase reaction has no known cofactor requirements. The reaction proceeds by means of a protonated intermediate that undergoes a concerted series of trans-1,2 shifts of methyl groups and hydride ions to produce lanosterol. 

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