Digestion and absorption of bovine milk xanthine oxidase and its role as an aldehyde oxidase.

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 molk. 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 containg 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 20 ng were absorbed as intact enzyme.