Lipid alcohols

The first example is the plasma-borne retinol-binding protein, RBP, which is a single polypeptide chain of 182 amino acid residues. This protein is responsible for transporting the lipid alcohol vitamin A (retinol) from its storage site in the liver to the various vitamin-A-dependent tissues. It is a disposable package in the sense that each RBP molecule transports only a single retinol molecule and is then degraded. 

Figure 17.2 Lipid peroxidation scheme. LH, a polyunsaturated fatty acid LOOM, lipid hydroperoxide LOH, lipid alcohol L, lipid radical LOO, lipid hydroperoxyl radical LO, lipid alkoxyl radical. Initiation the LH hydrogen is abstracted by reactive oxygen (e.g. lipid alkyl radical, lipid alkoxy radical, lipid hydroperoxyl radical, hydroxy radical, etc.) to produce a new lipid alkyl radical, L. Propagation the lipid alkyl, alkoxyl or hydroperoxyl radical abstracts hydrogen from the neighbouring LH to generate a new L radical.

Thiols are also important protection against lipid peroxidation. Glutathione (7-Glu-Cys-Gly) is used by several glutathione-dependent enzymes such as free-radical reductase (converts vitamin E radical to vitamin E), glutathione peroxidase (reduces hydrogen peroxide and lipid hydroperoxides to water and to the lipid alcohol, respectively), and others. In addition, the thiol group of many proteins is essential for function. Oxidation of the thiol of calcium ATPases impairs function and leads to increased intracellular calcium. Thiol derivatives such as the ovothiols (l-methyl-4-mercaptohistidines) (Shapiro, 1991) have been explored as therapeutics. 

Simon, S.A. and T.J. McIntosh. 1984. Interdigitated hydrocarbon chain packing causes the biphasic transition behavior in lipid/alcohol suspensions. Biochim Biophys Acta 773 169-172. 

An insertion mechanism for synthesis of cellulose. Using 14C "pulse and chase" labeling Han and Robyt found that new glucosyl units are added at the reducing ends of cellulose chains formed by cell membrane preparations from A. xylinum.135 This conclusion is in accord with the generalization that extracellular polysaccharides made by bacteria usually grow from the reducing end by an insertion mechanism that depends upon a polyprenyl alcohol present in the cell membrane.136 This lipid alcohol, often the C55... 

Insertion of monomer units at the base of a chain is a major mechanism of polymerization that is utilized for synthesis not only of polysaccharides but also of proteins (Chapter 29). For most carbohydrates the synthesis is dependent upon a polyprenyl lipid alcohol, hi bacteria this is often the 55-carbon undecaprenol or bactoprenol,136 which functions as a phosphate ester ... 

Lipid Alcohol Grade Ratio CP °c PP °C CFPP °C LTFT °C... 

Figure 29.12. Proposed mechanism of cephalosporin-induced nephrotoxicity. LOOH, lipid hydroperoxide LOH, lipid alcohol LO, lipid radical G-6-P, glucose-6-phosphate.

Citation of the classic chain reaction for lipid oxidation persists even though, as product analysis and studies of mechanisms have become more sophisticated, there is now considerable evidence that only Reactions 1, 2, and 5 (and perhaps also 6) of Figure 1 are always present. Research has shown that, although hydrogen abstraction ultimately occurs, it is not always the major fate of the initial peroxyl or alkoxyl radicals. Indeed, lipid alcohols from H abstraction are relatively minor products of lipid oxidation. There are many competing alternative reactions for LOO and LO that propagate the radical chain but lead to different kinetics and different products than expected from the classic reaction sequence (5, 6, 21). A more detailed consideration of each stage shows how this basic radical chain sequence portrays only a small part of the lipid oxidation process and products, and a new overall reaction scheme for lipid oxidation is needed. 

ROOH = LOOH, a lipid alcohol is released and no initiation or branching can occur. In aprotic solvents or acid environments, however, H abstraction is delayed and the radicals remain active. When the heme is myoglobin, hemoglobin, or a heme protein where an internal proton source is not available, the reaction mechanism is more likely to be homolytic, yielding alkoxyl radicals with no radical on the porphyrin. 

Current information raises questions about the literal application of the classic free radical chain sequence to lipid oxidation. Observed products do not match those predicted Many studies have now shown that hydroperoxides are not exclusive products in early stages and lipid alcohols are not even major products after hydroperoxide decomposition. Product distributions are consistent with multiple pathways that compete with each other and change dominance with reaction conditions and system composition. Rate constants show no strong preference for H abstraction, cyclization, addition, or scission, which partially explains the mixmre of products usually observed with oxidizing lipids. It could be argued that the reactions in Figure 1 accurately describe early processes of lipid oxidation, but LOO rate constants considerably higher for cyclization than for abstraction contradict this. 

Lipid peroxidation of biological membranes is a destructive process, proceeding via an autocatalytic chain reaction mechanism [73]. Membrane phospholipids contain hydrogen atoms adjacent to unconjugated olefinic bonds, which make them highly susceptible to free radical oxidation. This is characterised by an initiation step, one or more propagation steps and a termination step [1], which may involve the combination of two radical species or interaction with an antioxidant molecule such as vitamin E. The products formed from such reactions include lipid peroxides, lipid alcohols and aldehydic by-products such as malondialdehyde and 4 hydroxynonenal [73]. 

In other areas of the world, waste from the sugar industry (molasses), starch, waste lipids, alcohols such as methanol (Bourque et al. 1995) and especially lignocellulosic feedstocks are available in quantities that are appropriate for industrial process demands. 

Such oscillations in a lipid-alcohol-doped milipore filter for the existence of deterministic chaos can be studied using standard methods from non-linear dynamic theory, inspite of the fact that reliable Lyapunov exponent could be obtained from the data using low values of the evolution time. A possible source of the noise in the system could be spatial non-uniformities or erratic variations in thickness of the gel, which is not easy to study. 

Chitathione pcragdilase catalyzes the reduction of lipid peroxidases (oxidants) to lipid alcohols. 

JV-Glycolyl-8-O-sulfoneuraminic acid, a component of a sea urchin sialosphin-golipid, has been synthesized from neuraminic acid via its O-benzyl glycoside, and some neuraminic acid lipid conjugates have been prepared as neuritogenic agents by sialylation of lipid alcohols. ... 

Lipide Alcohols.—Glycerol (glycerin) and higher solid alcohols. 

Aliphatic Alcohols.—Natural fats and oils are esters of the trihydroxy alcohol glycerol the other ahphatic alcohols occur either free as solutes in oils and fats or combined as waxes. Five groups of these lipide alcohols may be recognised, four of which belong to the straight-chain type of compounds. 

Sterols, or Alcohols of the Cholane Series.— These differ from the other lipide alcohols in the possession of a complex... 

A similar strategy, involving the sequential addition of the two different lipid alcohols to POCI3 followed hy the addition of the amine, provided cationic lipophosphoramidates (36) possessing two different lipid chains/ ... 

Vitamin A is a fat-soluble micronutrient that is required by all vertebrates to maintain vision, epithelial tissues, immvme functions, reproduction, and for life itself. It was discovered in 1913 as a minor component in eggs, butter, whole milk, and fish liver oils. It soon became apparent that vitamin A exists in two chemically distinct yet structurally related forms. The first form to be characterized was retinol, a lipid alcohol that is present only in foods of animal origin. Retinol is also known as preformed vitamin A because it can be metabolized directly into compovmds that exert the biological effects of vitamin A. A second form of vitamin A, present in deep-yellow vegetables, was characterized as /3-carotene, which is synthesized only by plants but can be converted to retinol during absorption in the small intestines. These carotenoids are sometimes referred to as provitamin A. The nutritional requirement for vitamin A can be met by preformed retinol, provitamin A carotenoids, or a mixture, and therefore it is possible to obtain a sufficient intake of vitamin A from carnivorous, herbivorous, or omnivorous diets. 

Retinol, the parent molecule of the vitamin A family, is a fat-soluble lipid alcohol (C20H30O, molecular mass 286.4) composed of a methyl substituted cyclohexenyl (yS-ionone) ring, an 11-carbon conjugated tetraene side chain, and a terminal hydroxyl group (Figure lA, Ri). Most of the double bonds can exist in either trans or cis conformation. All-traws-retinol is the most stable and most prevalent form in foods and tissues, but small amounts of other geometric isomers such as 9-cis- and 13-c/s-retinol are found in some cells. The terminal hydroxyl group of retinol can be free or esterified with a... 

---