DESCRIPTION OF THE VARIOUS NATURAL LIPIDS |
Despite lipids are used by humans since ancient times, they were studied only through the nineteenth century by scientists probably because they are difficult to separate and identify.
No precise definition of the term lipid exists. In its broadest sense, "lipid" defines substances as oils, fats and waxes which can be only characterized by a large array of properties. They are in general
- coming from animals and plants living or fossilized,
- insoluble or immiscible with water but soluble in organic solvents such as chloroform,
ether, benzene, acetone,
- formed of long-chain hydrocarbon groups (carbon and hydrogen) but may also contain
oxygen, phosphorus, nitrogen and sulfur.
This definition covers a wide range of substances which contain moieties belonging
chemically to hydrocarbons, alcohols, aldehydes, acids, amines or glucides. The majority
forms molecules such as waxes, triacylglycerols, phospholipids and glycolipids. Other
substances are also considered as lipids while belonging to other classes of molecules,
e.g. fat soluble vitamins (A, D, E and K), coenzymes (ubiquinones), pigments
(carotenoids), terpenes, phenolics and sterols. They are equally considered because they
are extracted with the cellular or membrane constituents by extraction with
‘fat’ solvents. Lipids were also defined as natural substances having in their
molecular structure one chain of at least 10 carbon atoms.
Lipids are one of the three main classes of food, besides carbohydrates and proteins. They
are responsible for storing energy that animals do not immediately need.
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It can be considered that in 1786, the birth date of Chevreul
ME, the chemistry of lipids was unknown. The same year, in his treatise
"Eléments d'histoire naturelle et de chimie", Fourcroy AF
indicated that fats ("corps gras") may be classified in pure fatty
oils (olive, almond, rape), siccative oils (linseed, walnut, poppy-seed,
hemp-seed) and solid oils or butter (coprah, cocoa).
In 1823, in his famous treatise "Recherches
chimiques sur les corps gras d'origine animale",
Chevreul noted that the term "corps gras" (fats)
may be defined as compounds burning with an important flame and giving
lamp-black, soluble in ethanol but not in water. After making a distinction
between oils, butters, solid fats, tallow, waxes, resins, volatile oils and
balms, he said: "On voit, d'après ce qui précède, combien
l'expression de corps gras est vague, et l'impossibilité où l'on est d'en
donner une définition scientifique" ("It can be seen from the
previous description how the term corps gras is vague and that it appears
impossible to give a scientific definition of it"). Today, the situation is
not very different !!. Then, Chevreul gives what it may be considered as the
first scientific classification of fats, based on acidity, volatility and the
ability to be saponified. He defines two divisions and six groups. The first
division (acid fats) contains long-chain and medium-chain fatty acids, the
second one neutral fats) being divided into four groups : cholesterol, waxes,
long-chain and medium-chain triglycerides.
More than one century later, in 1947, Hilditch TP in
his famous book on "The chemical constitution of natural fats" (Chapman
and Hall Ltd, London) claimed that "unanimity has not yet been reached in the terminology to be adopted
in classifying the various types of naturally occurring compounds in which fatty acids are
present ... even a collective title for the whole group is not
completely settled". At that time
Hilditch proposed that the term lipid should be used, as it was
for the first time in
1926 (Lipid excretion III. Further studies of the quantitative relations of
the fecal lipids, Sperry WM, J Biol Chem 1926, 68, 357), discarding the terms lipoids
(British origin) and lipins
(American origin). It is important to remind that the earliest use of the word
protein was in 1868 and that of carbohydrate in 1869. Interestingly, the term "lipin" persits in "oxylipin"
and "cardiolipin". Hilditch stated that the collective name
"lipid" is for natural products in which
the higher fatty acids are present as essential compounds. He reserved the term fats
for natural triacylglycerols, solid or liquid. Lipids were organized into two sub-groups, the
simple lipids, including fats and waxes (ester waxes, sterols, alcohols),
and the compound or complex lipids, including phosphatides and
cerebrosides.
A lipid nomenclature was proposed by the International Union of Pure and Applied
Chemists and the International Union of Biochemistry and Molecular Biology (IUPAC-IUBMB)
Commission on Biochemical Nomenclature in 1976. Additional documents relating to
the nomenclature of glycolipids, prenols, and steroids have been released and
may be found on the IUPAC website (http://www.chem.qmul.ac.uk/iupac/).
To facilitate international communication, a comprehensive classification of
lipids was released to enable a cataloging which is compatible with other
chemical databases (Fahy E et al., J Lipid Res 2005, 46, 839-861). Thus,
lipids were divided into eight categories containing distinct classes and
subclasses of molecules which were given a 12 digit identifier.
A free PDF version of The Oily Press Book Lipid glossary 2 by Gunstone FD
and Herslof BG is available online at netlink : http://www.pjbarnes.co.uk/op/lg2pdf.htm
To simplify the study of this vast group of heteroclites molecules, we propose a
classification based on three major classes :
They are:
1 -
Simple
or neutral lipids
They contain one or two different types of compounds generally neutral except for fatty acids which are studied in this group (i.e. one sterol and/or one fatty acid, glycerol and fatty acid(s), one alcohol and/or one fatty acid, fatty acid and aminoacid...) and have apolar properties. Some classification restricts this group to molecules containing one alcohol and/or one fatty acid. Lipids containing sugar (Glycolipids) are excluded from that list and are classified as "complex lipids", even if they contain two compounds.
2 - Complex lipids
They contain frequently three
or more chemical identities (i.e. glycerol, fatty acids and sugar, one long chain base,
one fatty acid and one phosphate group...) and have polar properties.
Some contain only two components but including a sugar moiety.
They can be divided into three main groups:
- Phospholipids
- Glycolipids:
they include:
Glycoglycerolipids
-
- Lipoamino acids
3 - Proteolipids (fatty
acylated proteins)
This global classification is fairly
comprehensive but remains only a guide, some lipid compounds being labeled in class 1 by
someone and in class 2 by others.
Another ancient classification, frequently used in the lab, is founded on the well known
ability of some lipids to be hydrolyzed by basic solutions into compounds (i.e. one
glycerol and one, two or three fatty acids), they are said saponifiable. The others are
resistant to this type of treatment (they are unsaponifiable). There are several
crossing-over between the two classifications since polar lipids can be saponifiable or
not (the same for neutral lipids).