Enzymes polar lipids

Figure 6.10. Phosphatidylinositol 3-hydroxy kinase activity. The enzyme phosphatidylinositol 3-hydroxy (Ptdlns 3-OH) kinase may phosphorylate inositol lipids as shown, to generate a series of novel polar lipids that may function in cell activation. See text for details.

Thus, the fat globules are surrounded, at least initially, by a membrane typical of eukaryotic cells. Membranes are a conspicuous feature of all cells and may represent 80% of the dry weight of some cells. They serve as barriers separating aqueous compartments with different solute composition and as the structural base on which many enzymes and transport systems are located. Although there is considerable variation, the typical composition of membranes is about 40% lipid and 60% protein. The lipids are mostly polar (nearly all the polar lipids in cells are located in the membranes), principally phospholipids and cholesterol in varying proportions. Membranes contain several proteins, perhaps up to 100 in complex membranes. Some of the proteins, referred to as extrinsic or peripheral, are loosely attached to the membrane surface and are easily removed by mild extraction procedures. The intrinsic or integral proteins, about 70% of the total protein, are tightly bound to the lipid portion and are removed only by severe treatment, e.g. by SDS or urea. 

The polar lipids of membranes undergo constant metabolic turnover, the rate of their synthesis normally counterbalanced by the rate of breakdown. The breakdown of lipids is promoted by hydrolytic enzymes in lysosomes, each enzyme capable of hydrolyzing a specific bond. When sphingolipid degradation is impaired by a defect in one of these enzymes (Fig. 1), partial breakdown products accumulate in the tissues, causing serious disease. 

When particulate enzyme preparations from Lemna minor were incubated with UDP-[U-14C]galacturonic acid (UDP-GalA), radioactive label was incorporated into pectic acid and into material soluble in organic solvents. This radioactive material had the solubility properties, and the chromatographic behavior on paper and DEAE-cellulose, of a lipid derivative of GalA however, confirmation awaits further characterization. Its formation is enhanced by UMP, and by addition of a polar, lipid fraction prepared from Lemna minor.98 Transfer of radioactivity from this material to pectic acid has been obtained.177... 

The stratum spinosum layer contains abundant desmosomes, lipid lamellar bodies (Odland bodies), keratinosomes, and membrane-coating granules (MCGs). Lipid lamellar bodies are parallel stacks of polar lipid-enriched disks enclosed in a trilaminar membrane.48 The lamellar bodies also contain hydrolytic enzymes capable of converting polar lipids such as glycolipids and phospholipids to nonpolar products such as ceramides and free fatty acids, respectively.46,49... 

Alternatively, the calcium stores may be concentrated by lamellar bodies from the intercellular fluids released during terminal differentiation. The lamellar bodies are thought to be modified lysosomes containing hydrolytic enzymes, and a potential source of the hyaluronidase activity. The lamellar bodies fuse with the plasma membranes of the terminally differentiating keratinocytes, increasing the plasma membrane surface area. Lamellar bodies are also associated with proton pumps that enhance acidity. The lamellar bodies also acidify, and their polar lipids become partially converted to neutral lipids, thereby participating in skin barrier function. 

A broad variety of enzymes have been used to catalyze organic reactions in microemulsions. In the majority of cases the enzyme retains both activity and stability in a satisfactory way. Special attention has been given to the use of lipases in W/O microemulsions where the enzyme is located in water droplets of a size not much larger than the hydrodynamic diameter of the protein. Such systems are biomimetic in the sense that lipases in biological systems operate at the interface between hydrophobic and hydrophilic domains, with these interfaces being stabilized by polar lipids and other natural amphiphiles. 

The same liquid phase (L3) as is formed by wheat lipids can be obtained in aqueous systems of rye lipids [2] and of oat lipids [3]. Oats are probably the food material richest in polar lipids, and it is therefore realistic to expect industrial production of oat lipids in the future in order to formulate microemulsions for foods. We have found (unpublished observations) that even large protein molecules can be encapsulated into this kind of L3 phase without being denatured. The incorporation of enzymes in L2 and L3 phases, for example, can provide protection against proteolytic degradation. 

Phospholipids, glycolipids, and cholesterol are the major components of mamalian cell-membrane lipids. They play important roles in cell-signaling transduction and cell-to-cell recognition or modification of enzyme functions. In this study, the TC-CCC system was applied to solve the problem of emulsification, and satisfactory stationary phase retention was obtained. Human brain lipids could then be separated. We established the solvent systems that are used for the separation of most of the phospholipids, glycolipids, and less-polar lipids of the human brain. Furthermore, we demonstrated that molecular species, which are derived from variations of the hydrophobic tail group, were separated by optimizing the composition of the solvents. TC-CCC is available for analyzing the hydrophobicities of various lipid molecules in biomembranes. 

True lipases from plants will hydrolyze these partial glycerides, but other enzymes that attack monoacylglycerols (but not triacylglycerols) have been described. However, in most cases the full substrate specificities of these enzymes have not been studied. In one case a lipolytic acyl hydrolase from potato tubers was shown to hydrolyze mono- and diacylglycerols in addition to a range of polar lipids. Thus, to avoid introducing a class of hydrolytic... 

In summary, the site of oleate is most likely the endoplasmic reticulum. This organelle contains, in addition, all the enzymes involved in phospholipid biosynthesis. Whether or not a specific polar lipid or an acyl-CoA or an acyl-ACP is directly or indirectly involved for linoleic synthesis remains for further investigations to clarify. There is indirect evidence suggesting that the monogalactosyldiglyceride in the outer envelope of the chloroplast may be involved in the conversion of linoleic acid to linolenic acid. Once linoleic acid and linolenate are formed, these acyl moieties must be transported to their specific sites. In the leaf cell, the principal site of these acids is the chloroplast lamellar membrane. At present, there is no direct evidence for the occurrence of polyunsaturation in these specific membranes or even in chloroplasts themselves (see Table II). Thus these acyl moieties must be presumably transferred directly or indirectly to their final site from their synthesizing site (see Section III for a discussion of transport mechanisms). 

The current findings of CLA effects on bone fatty acid composition are consistent with previously reported values. For example, CLA reduced 18 ln-9 and total monounsaturated fatty acids in both neutral and polar lipids, and reduced 20 4n-6 in the neutral fraction of liver. This could be due to reduced liver stearoyl-CoA desaturase (A9 desaturase, catalyze the formation of 18 ln-9 from 18 0) expression (16). CLA was reported to decrease the mRNA level of the A9-desaturase enzyme in both liver tissue and hepatocyte cultures (16). More interestingly, the c9,fll CLA isomer, which is implicated as the primary biologically active isomer (21), had no effect on the expression of A9 desaturase compared with the chemically synthesized CLA, which contained as much tl0,cl2 as c9,tll and small amounts of other isomers. This indicates that individual CLA isomers may have their own regulatory functions in a biological system. 

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