Milk xanthine oxidase: Properties and physiological roles

Xanthine oxidoreductase (XOR) is a complex molybdoflavoenzyme that occurs as a major protein component of the milk fat globule membrane surrounding lipid droplets in milk. The enzyme has broad substrate specificity and is capable of reducing oxygen to generate the reactive oxygen species (ROS), superoxide and hydrogen peroxide. It can also reduce nitrite, yielding reactive nitrogen species (RNS), such as nitric oxide and peroxynitrite. By virtue of its capacity to generate ROS and RNS, milk XOR may play an antimicrobial defensive role in the neonatal gut, complementing endogenous enzyme of the intestinal epithelium. XOR is also involved in secretion of milk fat globules in a process dependent on the enzyme protein rather than on its enzymic activity, which is known to vary greatly with time after parturition and also between species. Some potential hazards and benefits of ingestion of cows milk XOR are briefly discussed.