Enzymes Indigenous to Milk | Xanthine Oxidoreductase

Abstract

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 indigenous enzymes of the intestinal epithelium. XOR is also involved in the secretion of milk fat globules in a process dependent on the enzyme protein rather than on its enzymatic activity, which is known to vary with time after parturition. Xanthine oxidase activity of milk XOR also varies greatly between species as a consequence of varying molybdenum content of the enzyme. Some potential hazards and benefits of ingestion of cows’ milk XOR are briefly discussed.