Mammary function during the nonlactating period: enzyme, lactose, protein concentrations, and pH of mammary secretions.

Composition of the aqueous phase of mammary secretions during the nonlactating and postpartum periods was determined in nine cows. Protein concentrations increased until several days before parturition and then declined precipitously. Lactose declined rapidly in early involution, remained low during the middle of the nonlactating period, and increased rapidly prepartum. The pH of secretions followed an inverse pattern to lactose and was negatively correlated with lactose during the nonlactating period but not the postpartum period. Peroxidase activity initially increased in secretions in early involution, then declined until parturition when peroxidase activity again increased. Activities of the glycosidic enzymes N-acetyl-beta-D-glucosaminidase, beta-glucuronidase, and alpha-mannosidase increased through the nonlactating period until 2 to 3 wk prepartum, from which time all three enzyme activities declined through the postpartum period. The magnitude of increase in the glycosidases was not the same; peak activity of N-acetyl-beta-D-glucosaminidase increased 20-fold over the activity at d 1 of involution, whereas beta-glucuronidase and alpha-mannosidase increased 4 to 5-fold over the same period.