Lipoid substances

The term lipid is frequently used to denote a wide variety of natural products fatty , oily or waxy substances of animal and vegetable origin that are easily soluble in organic solvents readily satisfy this loose definition. Thus, such diverse compounds as fatty acids and their derivatives, triglycerides, sterols, phosphatides and sphingolipids, carotenoids, bile acids, vitamins A, D, E, and K, long-chain alcohols, terpenes, etc., may be included. Studies of these substances by GC are indeed numerous a comprehensive survey of this field is beyond the scope of this 

The lipids are among the most important components of human diet and occur widely in nature. However, it is the biochemical role of lipids as the basic components of various cellular membranes and the lipid-protein complexes (lipoproteins) that bring them into the focus of highly important scientific activities. Major clinical interest has concentrated on blood lipid chemistry as related to atherosclerosis, lipid storage diseases, diabetes, and other metabolic conditions. 

Investigations and determinations of various types of lipids usually employ a number of analytical techniques of which GC is just one approach. The large lipid mixtures are first fractionated into different classes (cholesteryl esters, triglycerides, phospholipids, etc.), while various forms of liquid chromatography are typically used to separate further the individual molecular species from each other. A controlled chemical degradation may subsequently be applied to generate molecular fragments, such as fatty acids, that are amenable to GC. 

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Hydrophobic and ionic interactions between immunoglobulins and lipoid substances in fatty tissue. Nonspecific staining of fatty tissue rarely interferes with interpretation of specific staining and can usually be disregarded. 115-121... 

Narcotics suppress many activities of living cells and they also inhibit the action of some enzymes.1 Their action may be an adsorption on the active patches, or possibly they may, through their solubility in lipoid substances, enter into the structure underlying the active patches and distort these so that they can no longer perform their specific functions. 

The starting point for synthesis of fatty acids and other lipoid substances such as carotenoids and terpenes is acetyl coenzyme A. Very little labeled acetic acid can be isolated from the photo-synthesizing cell. It is presumed that the concentration of acetyl coenzyme A is very small and that the small pool turns over very rapidly. 

Table 1 Selected Examples of TLC Separations of Various Lipoidal Substances ...

These substances, together with the sterols (Chapt. XIV), are called lipoid (fat-like) becaxise of their solubility properties. Lipoid substances serve primarily as structural components they form hydrophobic (water-repelling) layers. Membranes frequently contain protein-lipoid double layers. 

The actual mechanism or process involved in the operation of smelling is not exactly known. The most important investigation in this direction is that of Backmann. He observed that in order that a substance may be odorous it must be sufficiently soluble in both water and in the lipoid fats of the nose cells. The odours of the saturated aliphatic alcohols first increase as the molecular weight increases and then decrease. The lower alcohols are comparatively odourless because of their low degree of solubility in the lipoid fats, while on the other hand the highest members are odourless because of their insolubility in water. The intermediate alcohols which are soluble in both fats and water have powerful odours. Backmann used olive oil in his experiments as a substitute for the lipoid fats. 

In this connection it is interesting to recall Kremer s experiments. By means of a spectroscopic method, Kremer demonstrated that when air saturated with an odoriferous substance such as pyridine or camphor is bubbled through a liquid containing a lipoid—such as a suspension of lecithin of a fatty animal tissue in Ringer s solution—more of the odoriferous substance is adsorbed than when the saturated air passes through water only. 

It seems likely that the sequence of events in the process of smelling is, after the odoriferous substance has reached the nostrils, first for the substance to dissolve in the aqueous outer layer, thence passing to the lipoid fats, wherein an addition reaction takes place, causing a change of energy which produces a sensation perceptible to the nervous centre. 

It will be realised that the strength of an odour may suffer successive diminutions in the process of smelling. It will be governed firstly, by the vapour pressure. of the odoriferous body, secondly, by the degree of solubility of the substance in water, thirdly, to its relative solubility in the lipoid fats with respect to that in water, and, lastly, to the speed of the chemical reaction. To a less extent the type of odour is similarly governed and this may account for the many shades of odour that exist. It is obvious that too much importance must not be placed on the chemical aspect of the problem, especially as regards the strength of an odour. 

No mention was made of the alcohol v. ether extraction of foods but in their paper reporting the results of feeding fat-free diets Osborne and Mendel stated clearly that none of the foods had been extracted with hot alcohol. Furthermore, they found it difficult to believe (this was in mid-1912) that skimmed milk could contain an important lipoid (fat-like substances such as lecithin, e.g.) in any adequate amount, "while... butter, which must contain some compounds of this type [is] inadequate." (32)... 

ANTIGEN. A substance, usually a protein, a polysaccharide, or a lipoid, which when introduced into the body stimulates the production of antibodies. Bacteria, their toxins, red blood corpuscles, tissue extracts, pollens, dust, and many other substances may act as antigens. See Antibody. 

The various lipoid barriers of the gastrointestinal tract, the kidney tubules, and the central nervous system (CNS) allow the absorption of essential nutrients, guard against the uncontrollable disposal of electrolytes and other substances, and prevent the entrance of potentially toxic materials. 

Lipid-soluble substances traverse the membrane by dissolving in the lipoid phase, and the lipid-insoluble substances penetrate only when they are small enough to pass through the pores. The absorption of large lipid-insoluble substances such as sugars and amino acids is accomplished by specialized transport processes. 

A passage of the blood-brain barrier has, however, not to be expected due to their polarity and molecular size. Due to the particular structure of the brain capillaries only lipoid-soluble low molecular substances are able to enter the brain vessels from the blood. This fact is called the blood-brain or blood-liquor barrier which indeed cannot be overcome by polymers but by free low-molecular active substances like mezcalin or aminoantipyrine. 

The term lipid or lipoid is used to describe a wide variety of natural origin chemical substances. All lipids contain mostly non-polar groups. This feature causes the most obvious common property of these molecules, which is the similar behavior to some organic solvents (i.e. chloroform, diethylether, hexane etc)... 

This concept allows the classification of molecules such as carotenoids, lipophilic vitamins, steroids, terpenes etc, as lipoids . Most researchers consider those molecules as separate groups and confine the term lipid to fatty acids, their derivatives and to substances that are closely related to their synthesis and function [1,2],... 

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