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 “reactive oxygen species” (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 transformation of the physical properties of vegetal oil under the action of air was firstly reported in 1791 by the famous Swiss scientist Jean Senebier (Encyclopédie méthodique, Physiologie végétale, t.1, Paris 1791). He observed that olive oil kept in air lost its fluidity and went rancid, while another sample not exposed to air remains unmodified.
Extract from the Encyclopédie méthodique, 1791
The first rigorous study of this lipid transformation was that 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).