INTRODUCTION
TO
LIPID PEROXIDATION
Lipid peroxidation can be defined as the oxidative deterioration of lipids containing
any number of carbon-carbon double bonds.
1 - INTRODUCTION
free oxygen an thiyl radicals ?
2 - HISTORY
3 - What are the
CLICK HERE
4 - How are generated peroxidized lipids ?
5 - What are the
primary products of- fatty acid peroxidation ? CLICK HERE
- cholesterol peroxidation ? CLICK HERE
CLICK HERE6 - What are the secondary products of lipid peroxidation ?
7 - Antioxidants: Protection against oxidation
in preparation
Lipid hydroperoxides are non-radical intermediates derived from unsaturated fatty acids, phospholipids, glycolipids, cholesterol esters and cholesterol itself. Their formation occur in enzymatic or non-enzymatic reactions involving activated chemical species known as "" (ROS) which are responsible for toxic effects in the body via various tissue damages. These ROS include among others hydroxyl radicals, lipid oxyl or peroxyl radicals, singlet oxygen, and peroxinitrite formed from nitrogen oxide (NO), all these groups of atoms behave as a unit and are now named "free radical". These chemical forms are defined as any species capable of independent existence that contains one or more unpaired electrons (those which occupy an atomic or molecular orbital by themselves). They are formed either by the loss of a single electron from a non-radical or by the gain of a single electron by a non-radical. They can easily be formed when a covalent bond is broken if one electron from each of the pair shared remains with each atom, this mechanism is known as homolytic fission. In water, this process generates the most reactive species, hydroxyl radical OH.. Chemists know well that combustion which is able at high temperature to rupture C-C, C-H or C-O bonds is a free-radical process. The opposite of this mechanism is the heterolytic fission in which, after a covalent break, one atom receives both electrons (this gives a negative charge) while the other remains with a positive charge.
In eukaryotic organisms, ROS are mainly generated during the
normal respiration process involving oxygen, oxidases and electron transports in
mitochondria or endoplasmic reticulum.
Oxygen is known since a long time as poisonous as well in plants as in animals or
bacteria, mainly when supplied at concentrations greater than those in normal air. Thus,
oxygen is able to damage plant tissues in inhibiting chloroplast development, seed
viability and root growth. The growth of bacteria (Escherichia coli) was shown to
be slowed down by pure oxygen, the toxic effect being enhanced by ionizing radiations.
Very early, it appears that the effects of oxygen and those of ionizing radiations on
organisms have many similarities. In man, the toxicity of oxygen was studied in relation
with diving, life in submarines and spacecrafts, treatment of some pathologies such as
cancer, sclerosis or gangrene. Exposure of man to oxygen at 1 atmosphere pressure causes
chest soreness, cough, sore throat, damage to lung alveoli and acute central nervous
system toxicity. This toxicity was at the origin of the frequent blindness observed in the
early 1940s and solved only around 1954 among infants born prematurely and kept in
incubators fed with high oxygen concentrations. Several observations led Gershman
R and Gilbert DL to propose in 1954, that most of the damaging
effects of oxygen could be attributed to the formation of free oxygen radicals (ROS). (Gilbert
DL 1981, Oxygen and living processes: an inter-disciplinary approach, Springer, NY).
Oxygen-dependent deterioration of lipids, known as rancidity, has been noticed
since antiquity as a major problem in the storage of oils (mainly olive oil) but
was also considered useful as far back as the 15th century in preparing
siccative oil paints and printing
inks. The same oxidation process is also
considered important today for natural products used in human consumption such as fats,
oils, dressings or margarine but also for chemical and industrial products such as
paints, inks, resins, varnishes or lacquers.
The first studies of this lipid oxidation problem was those of the Swiss chemist
Nicolas-Théodore de Saussure who observed around
1800, using a simple mercury manometer, that a layer of walnut oil exposed to air was able
to absorb about 150 times its own volume of oxygen during a one year period. Parallel with
these changes, oil became viscous and had a bad smell. Later, Berzelius (who discovered
selenium) suggested that this oxidation might be involved in the spontaneous ignition of
wool lubrified with linseed oil in textile mills. Parmentier AA, a pharmacist
who introduced potato culture in France, hypothesized also that oxygen in
combining with fats was the agent of rancidity (cited by Braconnot H, 1815).
Much information concerning the mechanism of autoxidation of lipid compounds has been
obtained by the study of the oxidation of simple non fatty products such as
cyclohexene. Thus, in 1928 Stephens reported the isolation of a peroxide of this
compound (Stephens HN, J Am Chem Soc 1928, 50, 568). The correct structure of
this peroxide was established only in 1942 (Farmer EH, Trans Faraday Soc 1942, 38, 340).
Systematic studies of lipid autoxidation may be considered to have begun around the
40s since Criegee et al (Ber 1939, 72, 1799) established that
hydroperoxides are the primary products of hydrocarbon oxidation. The major credit for
developing the hydroperoxide hypothesis of lipid autoxidation is due to Farmer
and co-workers (Farmer EH, J Chem Soc 1943, p.119 and 541). Bolland
(Q Rev 1949, 3, 1) established that the primary autoxidation products of linoleic
acid are hydroperoxides (on carbon atom 9 or 13) containing conjugated dienes (Review
in Autoxidation and antioxidants, Lundberg WO Ed, Wiley, NY 1961). Since the early
1960's, our understanding of the oxidation of unsaturated lipids has advanced considerably
as a result of the application of new analytical tools. Detailed studies of the products
of polyunsaturated fatty acids were initiated in the 70's by several research groups,
revealing more complex mixtures than those previously proposed (Porter NA et al.,
J Am
Chem Soc 1980, 102, 5597). With the help of HPLC, several hydroperoxide products could
be separated after autoxidation of arachidonic acid (Porter NA et al., Biochem Biophys
Res Comm 1979, 89, 1058), including products of lipoxygenase action (Porter NA et
al., J Am Chem Soc 1979, 101, 4319). The first demonstration of free radical oxidation
of membrane phospholipids was given in 1980 (Porter NA et al Lipids 1980, 15, 163)
leading to a new fruitful era with a continuous flow of innumerable works devoted to
chemistry, biochemistry and medicine.
