Lipid peroxidation

Lipid peroxidation refers to the oxidative degradation of lipids. It is the process whereby free radicals "steal" electrons from the lipids in cell membranes, resulting in cell damage. This process proceeds by a free radical chain reaction mechanism. It most often affects polyunsaturated fatty acids, because they contain multiple double bonds in between which lie methylene -CH2- groups that possess especially reactive hydrogens. As with any radical reaction the reaction consists of three major steps: initiation, propagation and termination.

Initiation
Initiation is the step whereby a fatty acid radical is produced. The initiators in living cells are most notably reactive oxygen species (or ROS), such as OH°, which combines with a hydrogen atom to make water and a fatty acid radical.

Propagation
The fatty acid radical is not a very stable molecule, so it reacts readily with molecular oxygen, thereby creating a peroxyl-fatty acid radical. This too is an unstable species that reacts with another free fatty acid producing a different fatty acid radical and a hydrogen peroxide or a cyclic peroxide if it had reacted with itself. This cycle continues as the new fatty acid radical reacts in the same way.

Termination
When a radical reacts it always produces another radical, which is why the process is called a "chain reaction mechanism." The radical reaction stops when two radicals react and produce a non-radical species. This happens only when the concentration of radical species is high enough for there to be a high probability of two radicals actually colliding. Living organisms have evolved different molecules to catch free radicals and protect the cell membrane. One important such antioxidant is alpha-tocopherol, also known as vitamin E. Other anti-oxidants made within the body include the enzymes superoxide dismutase, catalase, and peroxidase.

Tests
Certain diagnostic tests are available for the quantification of the end products of lipid peroxidation, specifically Malondialdehyde (MDA) The most commonly used test is called a TBARS Assay.