Xanthine dehydrogenase (XDH) is a key enzyme in purine metabolism that catalyzes the oxidation of hypoxanthine and xanthine to uric acid, primarily using NAD⁺ as an electron acceptor to produce NADH. Traditionally considered less harmful than its interconvertible form, xanthine oxidase (XO), which generates reactive oxygen species (ROS), recent research reveals that XDH itself can contribute to oxidative stress by producing ROS under certain conditions. 

Biochemical Properties and ROS Generation by XDH

XDH catalyzes the oxidation of hypoxanthine and xanthine, transferring electrons to NAD⁺, forming NADH without directly producing ROS under normal conditions. However, under pathological states such as ischemia or increased NADH concentrations, XDH can oxidize NADH, leading to superoxide radical generation and lipid peroxidation, as demonstrated in rat liver models. This NADH oxidase activity of XDH suggests it is an intrinsic source of ROS, capable of inducing oxidative damage without needing conversion to XO.

Structural studies reveal that the difference between XDH and XO lies in the conformation around the flavin adenine dinucleotide (FAD) cofactor, affecting electron transfer and reactivity with oxygen and NAD+. The flavin semiquinone intermediate in XDH is thermodynamically stable, but can react with oxygen to form superoxide, although NAD⁺ presence inhibits this ROS production significantly.

Xanthine dehydrogenase assay kits, often measuring total xanthine oxidoreductase activity, are indispensable tools in oxidative stress research. By enabling sensitive, rapid, and reproducible quantification of enzyme activity in diverse biological samples, these assays advance our understanding of XDH’s dual role in metabolism and ROS generation. Continued refinement of assay specificity and sensitivity will further clarify XDH’s involvement in oxidative stress and support therapeutic development.