Lipids fatty acids, mobilization

Together with proteins, phospholipids are the most important structural components of biological membranes. Since mobility of the lipid segments fa vors molecular transport through a membrane and thereby increases its permeability, a marked increase in 7] along a lipid-fatty acid chain also reflects a more efficient molecular diffusion through the lipid layer of a membrane [175]. 

The nature of the fatty acids in TAGs determines their hydrophobicity/hydrophih-city and diffusional mobility. In an aqueous/hpid environment, such as adipose tissue or lipoproteins in plasma, the relative hydrophihcity of the TAGs determines their partitioning between the interfacial phase and the apolar phase. This may have far stretching consequences. For instance, the rate and selectivity of fatty acid mobilization from fat cells may affect levels and composition of the nonester-ified fatty acids in plasma. These in mrn affect lipid homeostasis. Rate and selectivity of fatty acid mobilization from adipose stores are not related to the positional distribution of fatty acids on the glycerol backbone (75). They are related, however, to triacylglycerol hydrophihcity and thus to TAG structure (76). 

ABA-induced chilling resistance may be due to an increase in the mobility of the membrane hydrocarbon acyl chains [18]. In intact seedlings and isolated cotyledonary discs of cotton, ABA reduced chilling injury by preventing a decrease in the content of reduced glutathione and this was closely correlated with membrane stabilization [28]. At an injurious chilling temperature desaturation of leaf polar lipids fatty acids in cucumber leaves was reduced. These observations support the suggestion that ABA stabilizes membranes [35]. 

Steinberg, D. Fatty acid mobilization-mechanisms of regulation and metabolic consequences — p. Ill—138. In J. K. Grant The Control of Lipid Metabolism. London Academic Press 1963. 

My first experiments with pancreatectomized ducks gave results which were basically in agreement with those reported by Miahle and in view of the limited information about the effect of glucagon on the serum lipids of birds available at the time, I turned my attention to this question. It was then observed that injection of glucagon caused a marked and rapid elevation of plasma FFA in all of the avian species examined and this prompted a study of the effects in birds of other hormones known to affect fatty acid mobilization in mammals. 

An excellent example of PLC applications in the indirect coupling version is provided by the works of Miwa et al. [12]. These researchers separated eight phospholipid standards and platelet phospholipids from the other lipids on a silica gel plate. The mobile phase was composed of methylacetate-propanol-chloro-form-methanol-0.2% (w/v) potassium chloride (25 30 20 10 10, v/v). After detection with iodine vapor (Figure 9.2), each phospholipid class was scraped off and extracted with 5 ml of methanol. The solvent was removed under a stream of nitrogen, and the fatty acids of each phospholipid class were analyzed (as their hydrazides) by HPLC. The aim of this study was to establish a standardized... 

The neurohormonal control of lipid metabolism chiefly affects the mobilization and synthesis of triglycerides in the fat tissue. The lipolysis in tissues is dependent upon the activity of triglyceride lipase. All the regulators that favour the conversion of the inactive (nonphosphorylated) lipase to the active (phosphoiylated) one, stimulate the lipolysis and the release of fatty acids into the blood. Adrenalin... 

Various combinations of hexane or light petroleum (40-60°C, bp) and diethyl ether, usually with a small amount of acetic acid (e.g. 90 10 1) or diisopropyl ether and acetic acid (98.5 1.5) are commonly used. The greater mobility is demonstrated by cholesterol esters followed by triacylglycerols, free fatty acids, cholestorol, diacylglycerols and monoacylglycerols, with complex polar lipids remaining unmoved. Double development in two solvents, e.g. diisopropyl... 

Fatty acids are clearly larger in size and show markedly slower diffusion velocity than the small water (or creatine) molecules which have been examined so far by diffusion weighted NMR spectroscopy. However, assessment of diffusion properties of lipids could be a key step for further experimental studies of skeletal muscle lipid metabolism. Diffusion properties of FFA and triglycerides are likely different due to differences in molecular weight. In addition, effects of temperature, chemical surroundings, and the mobility of small lipid droplets in the cytosol may also lead to measurable differences in the diffusion characteristics. 

The cholesterol required for biosynthesis of the steroid hormones is obtained from various sources, it is either taken up as a constituent of LDL lipoproteins (see p. 278) into the hormone-synthesizing glandular cells, or synthesized by glandular cells themselves from acetyl-CoA (see p. 172). Excess cholesterol is stored in the form of fatty acid esters in lipid droplets. Hydrolysis allows rapid mobilization of the cholesterol from this reserve again. 

Wolfe has presented an excellent description of the systematic application of stable and radioactive isotope tracers in determining the kinetics of intestinal fat absorption, hepatic triglyceride synthesis, lipid mobilization, triglyceride-fatty acid recycling, and cholesterol turnover. 

The idea of using a nonpolar stationary phase and a polar mobile phase was first put forward in the literature by Boscott ( ), who advocated the use of cellulose acetate as the stationary phase. Shortly thereafter Bol-dingh (4) separated Cg-Cu fatty acids by using moderately vulcanized rubber saturated with benzene as the stationary phase and water-methanol as the mobile phase. This was rapidly followed by other work (5-7) and the method was widely practiced by lipid chemists until the advent of HPLC. The main problem with the use of rubber for the stationary phase is that the degree of swelling is extremely critical in determining the... 

Membrane conformational changes are observed on exposure to anesthetics, further supporting the importance of physical interactions that lead to perturbation of membrane macromolecules. For example, exposure of membranes to clinically relevant concentrations of anesthetics causes membranes to expand beyond a critical volume (critical volume hypothesis) associated with normal cellular function. Additionally, membrane structure becomes disorganized, so that the insertion of anesthetic molecules into the lipid membrane causes an increase in the mobility of the fatty acid chains in the phospholipid bilayer (membrane fluidization theory) or prevent the interconversion of membrane lipids from a gel to a liquid form, a process that is assumed necessary for normal neuronal function (lateral phase separation hypothesis). 

Increased synthesis of lipid or uptake. Increased synthesis of lipid may be the cause of fatty liver after hydrazine administration as this compound increases the activity of the enzyme involved in the synthesis of diglycerides. Hydrazine also depletes ATP and, however, inhibits protein synthesis. Large doses of ethanol will cause fatty liver in humans, and it is believed that this is partly due to an increase in fatty acid synthesis. This is a result of an increase in the NADH/NAD"1" ratio and therefore of the synthesis of triglycerides. Changes in the mobilization of lipids in tissues followed by uptake into the liver can also be another cause of steatosis. 

While bovine milk is a rich source of xanthine oxidase, milks from some species do not necessarily contain appreciable amounts of enzymatically active xanthine oxidase. For example, human milk contains only traces of xanthine oxidase activity as measured by oxidation of xanthine or hypoxanthine (Zikakis and Treece 1971 Zikakis et aL 1976), yet a band corresponding in electrophoretic mobility to xanthine oxidase is a major constituent of human milk lipid globule membrane (Freudenstein et al. 1979 Murray et cd. 1979). Evidence that the membrane-bound form of xanthine oxidase in bovine lipid globule membrane contains small amounts of tightly bound fatty acid has been obtained (Keenan et al. 1982). Whether this property promotes attraction between the membrane or membrane-associated coat and the surface of the globule core remains to be determined. 

Fatty acids are carried to tissues for use in synthesis of triacylglycerols, phospholipids, and other membrane lipids. The mobilization of fatty acids from triacylglycerol stores and from cholesterol esters depends upon hormone-sensitive lipase (p. 635).53b/ 53c This enzyme is activated by cAMP-dependent phos-... 

Figure D1.6.2 TLC-FID separation of lipids recovered from the gastric contents of a hooded seal pup. The mobile phase was 91 6 3 1 (v/v/v/v) hexane/ethyl acetate/diethyl ether/formic acid. Time refers to scanning time of the Chromarod. Abbreviations DG, 1,2-diglyceride FFA, free fatty acid MG, monoglyceride IS, internal standard TG, triglyceride. Reproduced from Ackman and Heras (1997) with permission from AOCS Press.

Low-wavelength UV detection (200-210 nm) is more sensitive and permits the use of gradients but precludes the use of certain common lipid solvents, such as chloroform and acetone, which are opaque in the UV region of interest. With low-wavelength UV detection, the response will also be somewhat dependent on fatty acid composition. For these reasons the mobile phases used in lipid analysis by HPLC may seem rather strange to workers familiar with the Thin Layer Chromatography (TLC) or open column separations. 

Fig. 30 Silver ion high-performance liquid chromatography (Ag-HPLC-FID) with flame ionization detector (FID) analysis of the triacylglycerols of chromatographed Crepis alpina seed oil. Ag-HPLC-FID conditions 0.5-mg sample 5-micron Chromspher Lipids column (Chrompack International, Middelburg, The Netherlands) (4.6 X 250 mm) mobile phase 0.5% acetonitrile in hexane (v/v) flow rate 1.0 ml/min FID. Chromatogram peak triacylglycerol fatty acid abbreviations S, saturated (palmitic and stearic) O, oleic L, linoleic and Cr, crepenynoic fatty acids.

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