Gastric Proteinase Digestion of Caseins in Newborn Pups of the Mouse

¹ Department of Animal Breeding, Graduate School of Agriculture and Life Sciences, The University of Tokyo, Tokyo 113-8657, JAPAN

Casein micelles of mouse milk consist of -, ß-, -, and -caseins. By digestion with alkaline phosphatase, they were separated as an independent band by sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis (PAGE). The compositions of -, ß-, -, and -caseins were 24.3, 25.1, 9.4, and 41.2% in colostrum, and 36.8, 15.6, 11.9, and 35.7% in mature milk, respectively. Zero-day-old pups were allowed to access either colostrum or mature milk, and the aggregated milk in the stomach was analyzed by SDS-PAGE. Caseins in colostrum were digested more rapidly and efficiently than those in mature milk. Among the seven peptides present in the aggregated caseins, four peptides were colostrum-specific and derived from - and -caseins. It was expected that colostrum-specific and soluble peptides were generated from - and -caseins through gastric proteinase digestion. Amino acid sequence analysis and the pH of the aggregated milk suggested that caseins in the stomach were digested by a chymotrypsin-like proteinase. Caseins in colostrum were different from those in mature milk, with respects to the casein composition as well as the gastric proteinase sensitivity. It is concluded that the lactating mice on the day of parturition supply particular caseins to their young.

Key Words: casein • gastric digestion • colostrum • mature milk

Submitted on September 21, 2000
Accepted on April 14, 2001

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