Actually pasteurizing of the milk destroys the xanthine oxidase. Depending on the temperature used the amount of xanthine oxidase destruction will vary. All of the xanthine oxidase is destroyed with pasteurization at 183F. But pasteurization temperatures can vary from 160F to ultra-pasteurization that uses temperatures of 282F.
This is going to get even more confusing as the liver also contains xanthine oxidase. It actually functions to reduce purines in to uric acid. But the xanthine oxidase activity is generally confined to the liver except in rare cases. For example where elevated calcium increases xanthine oxidase assoicated with reperfusion injuries of heart attacks.
The good news is that mercury inhibits the activity of xanthine oxidase:
Inhibition of bovine xanthine oxidase activity by Hg2+ and other metal ions. J Inorg Biochem 1996;62(4): 271-9
I just thought I would throw that in for fun considering all the mercury amalgam claims on CZ.
Then there is this study which refutes the claims about raw milk and homogenization:
J Nutr 1976 Nov;106(11):1600-9
Digestion and absorption of bovine milk xanthine oxidase and its role as an aldehyde oxidase. Ho CY, Clifford AJ The effects of acidic and intestinal proteolytic environments on bovine milk xanthine oxidase (XO) activity were determined in order to evaluate the extent to which this enzyme was absorbed in biologically active form. The inhibition of XO by folic acid and the relative affinities of XO for the oxidation of palmitaldehyde, stearaldehyde, and xanthine were compared. The effects of acid and gastric juice on XO activity were measured by incubating purified enzyme, and non-purified enzyme (milk), in buffers ranging in pH from 2 to 9. Fresh gastric juice was also incubated with milk. Increasing amounts of the enzyme were inactivated as the pH of the incubation mixture was reduced below pH 6.5. Below pH 3.5, the enzyme was completely inactivated. Gastric juice, pH juice incubated with milk. Milk XO activity was reduced 36% when mild was incubated with an equal volume of gastric juice. Homogenized milk had 59% less XO activity compared with raw milk.Fresh raw milk XO, homogenized milk XO, and purified XO were equally susceptible to inactivation by acid or gastric juice. After incubation of milk with gastric juice, or gastric juice followed by pancreatin, XO activity was associated with a macromolecule of 300,000 daltons molecular weight and subunits containing activity were not found. It was estimated that 0.00008% of the XO in the intestine was absorbed. Both folic acid and allopurinol inhibited XO activity in vitro. Allopurinol was 3.5 times more potent an inhibitor than folic acid. A large excess of dietary folic acid did not reduce rat liver or intestinal XO activity in vivo. XO had a much greater affinity for xanthine than for palmitaldehyde or stearaldehyde substrates. It was estimated that of 100 mg of XO in fresh raw milk, 41 mg remained after homogenization, 27 mg entered the intestine and only 20ng were absorbed as intact enzyme. PMID: 10360
I have not been able to find if any xanthine oxidase is detroyed through fermentation or by the acids formed, but the bottom line is that if you do not go overboard with the dairy then it should not present a problem. Remember the saying the dosage makes the poison? Anything can be harmful in excessive amounts.