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

Abstract

Seventy rats were fasted for 20h and used in two experiments. In the first experiment fasted rats were intubated with 5ml 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 1.5h the recovered activities were 62 and 45%, and in some rats activity persisted up to 8h. In the second experiment, fasted rats received 2ml marketed milk. Enzyme activity dropped faster than in the first experiment. Typical gastric transit times for ingested milk were 30 to 40min. 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 37C for 0, .5, 8, and 24h, 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 .5h and followed by pancreatin solution for 0, .5, 3.5, and 7.5h, 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.