The LPO assay Kit measures the MDA and HNE concentrations as an index of lipid peroxidation. Reactions between indoles and aldehydes (MDA and HNE) gives a diindolylalkane (chromophore) whose maximal absorbance is in the 580-620 nm region.

Lipid peroxidation is a well-known example of oxidative damage in cell membranes, lipoproteins, and other lipid-containing structures. Peroxidative modification of unsaturated phospholipids, glycolipids, and cholesterol can occur in different reactions. They can be triggered by i) free radical species such as oxyl radicals, peroxyl radicals, and hydroxyl radicals derived from iron-mediated reduction of hydrogen peroxide or ii) non-radical species such as singlet oxygen, ozone, and peroxynitrite generated by the reaction of superoxide with nitric oxide.

Malondialdehyde (MDA) and 4-hydroxyalkenals are important toxic byproducts of lipid peroxidation. So, the measurement of the amounts of such aldehydes corresponds to an index of lipid peroxidation in vitro and in vivo. 4-Hydroxynonenal (4-HNE) is a major product of the peroxidative decomposition of ω-6 polyunsaturated fatty acids (PUFA). It possesses cytotoxic, hepatotoxic, mutagenic, and genotoxic properties.
Furthermore, increased levels of HNE were found in plasma and various organs under oxidative stress conditions. In fact, MDA is in many instances the most abundant individual aldehyde resulting from lipid peroxidation. In vitro MDA can alter proteins, DNA, RNA, and many other biomolecules.