Role of Xanthine Oxidoreductase as an Antimicrobial Agent

Xanthine oxidoreductase (XOR) is widely distributed in mammalian tissues and has long been known to be a major constituent of the milk fat globule membrane (MFGM), which surrounds fat globules in cow's milk (36,50).This source has been exploited for isolation and characterization of the bovine enzyme for many decades (44).XOR is a complex enzyme comprising two identical 147,000-M r subunits, each of which contains one molybdenum, one flavin adenine dinucleotide, and two nonidentical iron-sulfur redox centers (8,32,33).While the enzymology of XOR is well documented, its physiological role is unclear.The enzyme occurs in most mammalian tissues, and although it has a broad specificity for reducing substrates, its conventionally accepted role is in purine catabolism, catalyzing the oxidation of hypoxanthine to xanthine and the oxidation of xanthine to uric acid.Despite its wide tissue distribution, the enzyme is believed to be largely concentrated in endothelial and epithelial cells.Such a specific cellular location implies a physiological role apart from that of a simple housekeeping enzyme, and other functions have been sought.Indeed, the role of XOR in milk has long been a puzzle.During the last 20 years, attention has been focused on the ability of the enzyme to generate reactive oxygen species (ROS).Mammalian XOR exists in two interconvertible forms, xanthine dehydrogenase (XDH) (EC 1.1.1.204),which predominates in vivo, and xanthine oxidase (XO) (EC 1.1.3.22).Both forms of the enzyme reduce molecular oxygen, although only XDH can reduce NAD ϩ , which is its preferred electron acceptor.Reduction of oxygen generates the ROS, superoxide anion, and hydrogen peroxide, and because of this, XOR has been implicated as a destructive agent, particularly in many forms of ischemia-reperfusion (IR) injury (27,32,46).Apart from a pathological context, ROS are increasingly recognized as key components in normal signal transduction pathways (20,21,29), and XOR is often a potential source of such species.The cytotoxic properties of ROS can also be beneficial, and an antimicrobial role for bovine milk XOR has been considered for many years.As early as 1943, two groups independently reported the antibacterial activity of purified milk enzyme and attributed this activity to XOR-catalyzed production of hydrogen peroxide (28,42).Subsequently, the enzyme has been linked to infection in many diverse instances, and increased