Goats milk xanthine oxidoreductase is grossly deficient in molybdenum

Xanthine oxidoreductase (XOR) was purified from goatsʹ milk. The u.v.-visible absorption spectrum was essentially identical to those of the corresponding bovine and human milk enzymes and showed an A280/A450 ratio of 5u20+-0u12, indicating a high degree of purity. Like bovine and human milk XORs, enzyme purified from goatsʹ milk showed a single band on SDS-PAGE corresponding to a subunit with approximate Mr 150000. On Western blotting, mouse monoclonal anti-human XOR antibody cross-reacted with purified caprine and bovine XORs. The specific xanthine oxidase activity of goatsʹ milk XOR, however, was very much lower than that of bovine XOR, although NADH oxidase activities of XOR from the two sources were similar. In these respects, the caprine milk XOR mirrors the human milk enzyme, in which case the kinetic effects have previously been attributed to relatively low molybdenum content. The molybdenum content of goatsʹ milk XOR also was shown to be relatively low, with 0u09 atoms Mo per subunit, compared with 0u55 atoms Mo per subunit for the bovine enzyme. A parallel purification of human milk XOR showed 0u03 atoms Mo per subunit. The possible physiological significance of the low molybdenum content of the caprine milk enzyme and of its correspondingly low enzymic activity is discussed.