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Physicochemical and Kinetic Properties of Purified Sheep’s Milk Xanthine Oxidoreductase

¹ Laboratory of Applied Biochemistry, Department of Biology, Faculty of Sciences, University Ferhat Abbas of Setif, Algeria
² Laboratory of Biochemistry, Faculty of Life and Nature Sciences, University of Bejaia, Algeria
³ Department of Biology and Biochemistry, University of Bath, UK

Corresponding author: M. Benboubetra; e-mail: benboubetra{at}yahoo.co.uk.

Xanthine oxidoreductase (XOR) was purified for the first time from sheep’s milk. The ultraviolet-visible absorption spectrum was essentially identical to those of the corresponding bovine, human, and goats’ milk enzymes and showed an A₂₈₀/A₄₅₀ ratio of 5.35 ± 0.24, indicating a high degree of purity. Like milk XOR from other species, sheep’s milk enzyme showed a single band on SDS-PAGE corresponding to a subunit with approximate M_r 150,000. Xanthine oxidase activity of purified sheep’s milk XOR (0.69 ± 0.04 µmole urate min^–1 mg^–1) was low relative to that of the bovine milk enzyme (1.83 ± 0.02 µmole urate min^–1 mg^–1), but higher than those of human or goats’ milk XOR. As in the latter 2 cases, the low activity of sheep’s milk XOR can be attributed to its relatively low molybdenum content (0.18 atoms per subunit), compared with that of the bovine milk enzyme (0.56 atoms Mo per subunit). Consistent with this, NADH oxidase activity of sheep’s milk XOR was similar to that of enzymes purified from bovine, human, or goats’ milk. The presence of desulpho-enzyme in sheep’s milk XOR was demonstrated by resulfuration experiments, whereby xanthine oxidase activity was increased by approximately 75%.

Key Words: molybdenum • NADH oxidase • sheep’s milk • xanthine oxidoreductase

Abbreviation key: MFGM = milk fat globule membrane, NAD⁺(H) = nicotinamide adenine dinucleotide (reduced), PFR = protein/flavine ratio, ROS = reactive oxygen species, XDH = xanthine dehydrogenase, XO = xanthine oxidase, XOR = xanthine oxidoreductase

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