Classes of lipids

A common feature of all lipids is that, biologically, their hydrocarbon content is derived from the polymerization of acetate followed by reduction of the chain so formed. (However, this process also occurs for the synthesis of some compounds that are not lipids and therefore cannot be used as a definition of lipids.) For example, polymerization of acetate can give rise to the following  

The products are fatty acids, CH3(CH2) COOH, which in turn can give rise to amines and alcohols. Lipids containing fatty acids include the glycerolipids, the sphingolipids, and waxes. 

Branched-chain hydrocarbons via a five-carbon intermediate, isopentene (isoprene)  

These are called acetogenins (or sometimes polyketides). Many of these compounds are aromatic, and their pathway of formation is the principal means of synthesis of the benzene ring in nature. Not all are lipids, because partial reduction often leaves oxygen-containing groups, which render the product soluble in water. 

For the lipids in Example 6.1, the routes of synthesis (just discussed) are  

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Steroids (1) are members of a large class of lipid compounds called terpenes that are biogenicaHy derived from the same parent compound, isoprene, C Hg Steroids contain or are derived from the perhydro-l,2-cyclopentenophenanthrene ring system (1) and are found in a variety of different marine, terrestrial, and synthetic sources. The vast diversity of the natural and synthetic members of this class depends on variations in side-chain substitution (primarily at C17), degree of unsaturation, degree and nature of oxidation, and the stereochemical relationships at the ring junctions. 

The lipids found in biological systems are either hydrophobic (containing only nonpolar groups) or amphipathic, which means they possess both polar and nonpolar groups. The hydrophobic nature of lipid molecules allows membranes to act as effective barriers to more polar molecules. In this chapter, we discuss the chemical and physical properties of the various classes of lipid molecules. The following chapter considers membranes, whose properties depend intimately on their lipid constituents. 

Eicosanoids and terpenoids are still other classes of lipids. Eicosanoids, of which prostaglandins are the most abundant kind, are derived biosynthetically from arachidonic acid, are found in all body tissues, and have a wide range of physiological activity. Terpenoids are often isolated from the essential oils of plants, have an immense diversity of structure, and are produced biosynthetically from the five-carbon precursor isopentenyl diphosphate (IPP). lsopentenyl diphosphate is itself biosynthesized from 3 equivalents of acetate in the mevalonate pathway. 

Lipid Modifications. Figure 1 Major classes of lipid-modified proteins. 

Experiments with monkeys given intramuscular injections of a mineral oil emulsion with [l-14C] -hexa-decane tracer provide data illustrating that absorbed C-16 hydrocarbon (a major component of liquid petrolatum) is slowly metabolized to various classes of lipids (Bollinger 1970). Two days after injection, substantial portions of the radioactivity recovered in liver (30%), fat (42%), kidney (74%), spleen (81%), and ovary (90%) were unmetabolized -hexadecane. The remainder of the radioactivity was found as phospholipids, free fatty acids, triglycerides, and sterol esters. Essentially no radioactivity was found in the water-soluble or residue fractions. One or three months after injection, radioactivity still was detected only in the fat-soluble fractions of the various organs, but 80-98% of the detected radioactivity was found in non-hydrocarbon lipids. 

Each phospholipid class in a given tissue has a characteristic fatty acid composition. Though the same fatty acid may be present in a number of lipids, the quantitative fatty acid composition is different for each class of lipids and remains fairly constant during the growth and development of the brain. A typical distribution profile of the major fatty acids in rat brain phospholipids is given in Table 3.1. Not only do the phosphoglycerides differ in the structure of the polar head groups, or phospholipid... 

Tocopherols are the second major class of lipid-soluble antioxidants inphotosynthetic membranes. Their primary function is to protect cells from hpid peroxidation. The water-soluble ascorbic acid also significantly acts against hpid peroxidation and DNA damage. However, these low-molecular weight antioxidants are chemically consumed and hence not considered the most efficient detoxifying agents. 

Lipids can be extracted from biological samples using a variety of organic solvents. A chloroform-methanol solvent is suitable for all lipids but it is possible to extract different classes of lipid selectively on the basis of their solubility in different organic solvents. This may be achieved by the addition of a solvent that will effect either the precipitation or the extraction of the lipids of interest. An example of the former is the precipitation of high concentrations of phospholipids with cold, dry acetone, and of the latter, the extraction of fatty acids into ether or heptane at an acid pH. However, like all solvent extraction procedures these are not entirely specific. 

It is difficult to separate the wide range of complex lipids in a single solvent system but the task is simplified if the sample has already been partially purified (e.g. by a column technique) and only one class of lipid is present. Even so, it is often necessary to perform two-dimensional chromatography and silica gel without binder is often preferred. 

The stratum corneum consists of separated, nonviable, cornified, almost nonpermeable corneocytes embedded into a continuous lipid bilayer made of various classes of lipids, for example, ceramides, cholesterol, cholesterol esters, free fatty acids, and triglycerides [6], Structurally, this epidermis layer is best described by the so-called brick-and-mortar model [7], The stratum corneum is crucial for the barrier function of the skin, controlling percutaneous absorption of dermally applied substances and regulating fluid homeostasis. The thickness of the stratum corneum is usually 10-25 /an, with exceptions at the soles of the feet and the palms, and swells several-fold when hydrated. All components of the stratum corneum originate from the basal layer of the epidermis, the stratum germinativum. 

Fatty acids are key constituents of several structural classes of lipids triglycerides, glycerophospholipids, and glycolipids. 

Lipids can be classified into substances that are either hydrolyzable— i. e., able to undergo hydrolytic cleavage—or nonhydrolyzable. Only a few examples of the many lipids known can be mentioned here. The individual classes of lipids are discussed in more detail in the following pages. 

Table 19.1 reports a summary of application of HPLC dealing with analysis of different classes of lipids in foods. 

Often used in the past for problematical compounds but with gradual improvement of reverse phases increasingly less used. Useful for chromatography of very lipophilic compounds such as in the separation of different classes of lipids and in the analysis of surfactants, which tend to form micelles under the conditions used for reverse-phase chromatography A moderately polar phase often used for the analysis of sugars and surfactants... 

Isoprostanes are a relatively new class of lipids and are produced in vivo principally by a free radical-catalyzed peroxidation of polyunsaturated fatty acids. Isoprostanes... 

The third class of lipids is steroids. Included in this category of lipids are cholesterol, bile salts, and sex hormones. Steroid structures contain fused rings consisting of three six-carbon rings and a five-carbon ring ... 

Lipids are naturally occurring organic molecules, isolated from animal or plant cells by extraction with nonpolar organic solvents. This definition defines lipids in terms of a physical property (solubility) and differs from structural definitions used for proteins or carbohydrates. Not surprisingly, lipids are highly varied in their structure from the medicinal chemistry perspective, there are five classes of lipids ... 

The composition of lipids from the silk and cuticule has been reviewed by Schulz (1997a, 1999). These lipids consist primarily of alkanes, as found in other arthropods, with 2-methylalkanes with an even number of carbon atoms in the chain being most abundant, with lesser amounts of alcohols, acids, aldehydes, and wax esters. Recently, a thorough analysis of the silk lipids of N. clavipes (Schulz, 2001) revealed a unique class of lipids from spider silk and cuticle, consisting of straight-chain and branched methyl ethers (1-methoxyalkanes, Fig. 4.4) with chain lengths between 25 and 45 carbon atoms. 

Schulz, S. and Toft, S. (1993a). Branched long chain alkyl methyl ethers a new class of lipids from spider silk. Tetrahedron 49 6805-6820. 

Certain classes of lipids are susceptible to degradation under specific conditions. For example, all ester-linked fatty acids in triacylglycerols, phospholipids, and sterol esters are released by mild acid or alkaline treatment, and somewhat harsher hydrolysis conditions release amide-bound fatty acids from sphingolipids. Enzymes that specifically hydrolyze certain lipids are also useful in the determination of lipid structure. Phospholipases A, C, and D (Fig. 10-15) each split particular bonds in phospholipids and yield products with characteristic solubilities and chromatographic behaviors. Phospholipase C, for example, releases a water-soluble phosphoryl alcohol (such as phosphocholine from phosphatidylcholine) and a chloroform-soluble diacylglycerol, each of which can be characterized separately to determine the structure of the intact phospholipid. The combination of specific hydrolysis with characterization of the products by thin-layer, gas-liquid, or high-performance liquid chromatography often allows determination of a lipid structure. 

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