Xanthine oxidoreductase (XOR) is a widely distributed housekeeping enzyme in mammals that catalyzes the last two steps in human purine catabolism to produce uric acid. The enzyme exists as a homodimer with independent electron transfer in each monomer. This has been studied extensively as a major constituent of the milk fat globule membrane (MFGM) which surrounds fat globules in cow's milk even though purine catabolism is the most accepted function of XOR. A huge number of literature highlights on the different catalytic forms of XOR and their importance in the generation of reactive oxygen species/reactive nitrogen species (ROS/RNS) and synthesis of uric acid which are involved in many physiological and pathological processes. However, a slight ambiguity resides in their biochemical functions. The aim of this article was to review the literature published on the structural, catalytical, physiological and pathological role of XOR and to resolve the ambiguity in biochemical processes and to firm up various natural inhibitors of XOR collectively. Uric acid, the product of purine catabolism shows antioxidant activity, and XOR-derived ROS and RNS play a role in innate immunity, milk secretion and also be involved in signaling and metabolism of xenobiotics. Furthermore, XOR is likely to be engaged in pathology because of excessive production of uric acid and ROS/RNS. This review also reports natural XOR inhibitors in plants which inhibit the enzyme to treat XOR associated pathology
