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Persistence of Bovine Milk Xanthine Oxidase Activity after Gastric Digestion In Vivo and In Vitro¹

Department of Animal Science and Agricultural Biochemistry, University of Delaware

ABSTRACT

Seventy rats were fasted for 20 h and used in two experiments. In the first experiment fasted rats were intubated with 5 ml marketed milk of known xanthine oxidase activity. After dosing, food was withheld, and rats were sacrificed at intervals. The stomach of each rat was excised and its contents assayed polarographically for xanthine oxidase activity. Activity decreased with time. At .5 and 1.5 h the recovered activities were 62 and 45%, and in some rats activity persisted up to 8 h. In the second experiment, fasted rats received 2 ml marketed milk. Enzyme activity dropped faster than in the first experiment. Typical gastric transit times for ingested milk were 30 to 40 min. Enzyme activity in milk began to decrease at pH 6.70; below pH 3.90 the enzyme was inactivated completely. Inactivation was partly reversible at pH 2.50 and above by adjusting the pH to 7.3. When simulated gastric juice and milk were combined in ratios of 3:1, 2:1, 1:1, 1:2, and 1:3, pH's were 1.85, 2.08, 3.57, 5.16, and 5.56, and respective activities were 0, 0, 0, 14.2, and 23.8%. In one in vitro experiment, when juice and milk (1:2) were intubated at 37 C for 0, .5, 8, and 24 h, their pH's were 5.20, 5.26, 5.13, and 5.15 while recovered activities were 19.2, 18.7, 14.9, and 9.9%. When juice and milk (1:2) were incubated for .5 h and followed by pancreatin solution for 0, .5, 3.5, and 7.5 h, the recovered activities were 13.5, 11.9, 7.2, and 6.1%. Considerable xanthine oxidase in milk is not inactivated in the gastrointestinal tract but is available for absorption.

¹ Published with the approval of the Director of the Delaware Agricultural Experiment Station as Miscellaneous Paper No. 774, Contribution 29 of the Department of Animal Science and Agricultural Biochemistry, University of Delaware, Newark.