Posttranslational ruling of xanthine oxidase activity in bovine milk by its substrates.

The aims of this study were to test the hypothesis that the substrates of xanthine oxidase (XO), xanthine and hypoxanthine, are consumed while the milk is stored in the gland between milkings, and to explore how XO activity responds to bacteria commonly associated with subclinical infections in the mammary gland. Freshly secreted milk was obtained following complete evacuation of the gland and induction of milk ejection with oxytocin. In bacteria-free fresh milk xanthine and hypoxanthine were converted to uric acid within 30 min (T1/2 approximately 10 min), which in turn provides electrons for formation of hydrogen peroxide and endows the alveolar lumen with passive protection against invading bacteria. On the other hand, the longer residence time of milk in the cistern compartment was not associated with oxidative stress as a result of XO idleness caused by exhaustion of its physiological fuels. The specific response of XO to bacteria species and the resulting bacteria-dependent nitrosative stress further demonstrates that it is part of the gland immune system.