LPO assay kit measures MDA and HNE concentrations as an index of lipid peroxidation. Firstly, an acid-catalyzed attack at the 3-position of the indole ring initiates the reactions between indoles and aldehydes (MDA and HNE). As a result, this reaction gives a diindolylalkane (chromophore) with maximum absorbance in the region of 580-620 nm.
Malondialdehyde (MDA) and 4-Hydroxynonenal (4-HNE) 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-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.