Peroxidation of lipids
How peroxidation of lipids happens
One of the chemical and biochemical processes that takes place in a living organism is the peroxidation of lipids (it can also be encountered quite frequently in industry). Usually, the type of oxidation of fats arises as a consequence of free radicals acting on lipid molecules – particles which contain in unpaired electron in their structure. Chemically and biochemically, free radicals are quite aggressive substances and cause the destruction of cells and molecules. Sometimes free radicals may prevent necessary biochemical reactions from taking place. Free radicals arise as a consequence of the biological oxidation of molecules.
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Signs of peroxidation
As this type of oxidation is often encountered in industry (especially the food industry), it is often detected by special signs – for example an unpleasant smell and rancidity. These signs manifest themselves when an excess of free radicals form, which are capable of interacting with fat molecules.
To slow down the free radical oxidation process, antioxidants are added to products – substances that prevent the formation of free radicals. Peroxidation starts in dual bonds of unsaturated fatty acids. In cell membranes they mainly take the form of phospholipids and glycolipids.
Stages of peroxidation
In general, the stages of this reaction (they are equivalent to a mechanism) are typical – they are characteristic for any reaction in which free radicals take part:
These processes may be examined from the example of a fatty acid or lipid, which has several conjugated double bonds (when double bonds alternate with single ones).
At the first stage, the hydroxyl radical is attacked by the methylene group, located between the short bonds. The radical takes an electron from the group and is then reduced to water. The short bond, like the radical group, shifts in the molecule. At the stage of reaction initiation, a lipoperoxyl radical is formed.
In the reaction of the obtained radical with neighboring molecules, its neutralization takes place with the simultaneous formation of a new radical structure (thus making the free radical reaction a chain reaction).
In irradiation or when hydroperoxide takes electrons from other elements, the branching of the initial structure is possible, which increases the diversity of products of reaction.
Termination of the chain is possible in two cases – in the reaction of two radicals between each other (both of them are neutralized by the pairing off of unpaired electrons) or in the action of antioxidants (substances hindering oxidation). For example, tocopherol (vitamin E) has this effect. The substance is capable of giving a free radical an electron that it lacks, thus moving independently to a more stable state.
In simplified form, the reaction process is the following:
ROOH + ·OH = ROO· + H₂O;
*2. Development: *
R· + O₂ = ROO·;
ROO· + RH = ROOH + R·;
ROO· + ROO· = ROOR + O₂;
ROO· + R· = ROOR;
R· + R· = RR.
Products of peroxidation
At the end of oxidation of fatty acids, 2 main types of products form – unstable diene conjugates and peroxides. These are the initial products of peroxidation. As one of the products has an unstable structure, secondary and tertiary products may form – aldehydes, dialdehydes and Schiff’s bases respectively.
Hydroperoxides can also decompose further, although they are rather stable – among products of secondary oxidation we may note alcohols, aldehydes, dialdehydes, ketones and epoxides.
The most active among products of reaction is malonic aldehyde, as it is capable of reacting with amino groups of many compounds which have great biochemical significance for the body.
The role of peroxidation
In the peroxidation of lipids, malonic aldehyde often forms. It is capable of reacting with amino groups of lysine, phospholipids and glucosamines, and also N-terminal amino acids of proteins. Malonic aldehyde “forms” bridges inside according molecules or between them, bonding individual parts of molecules or different molecules together. These bonds change the properties of initial substances – for example, the activity of enzymes change, the structural capabilities of proteins drop, the penetrability of membranes grows from the deformation of release canals, and thus the structure or cell deforms or even perishes.