Distribution of xanthine oxidase and xanthine dehydrogenase activity in bovine milk: Physiological and technological implications

The aim of this study was to evaluate the distribution of xanthine oxidoreductase (XOR) and its two forms, xanthine oxidase (XO) and xanthine dehydrogenase (XD), in milk fractions. XOR associated with milk phospholipid membranes was found to be distributed among an intra-membranous pool in which it takes the form of a mixture of XO and XD, with a clear predominance of XD, and a free pool of XO, of which 33% is found in the outer surface of milk fat globule membrane, 20.5% in the outer surface of whey membrane particles, and the remaining 46.7% in apparent solution. The inner-membrane XOR may play a nonenzymatic role in fat secretion, whereas extra-membranous XO is freely available for a role in the innate gland immune system and may affect milk quality.