Temporal effect of prolactin on the activities of lactose synthetase, alpha-lactalbumin, and galactosyl transferase in mouse mammary gland explants

The onset of the prolactin (PRL) stimulation of lactose synthesis is between 4 and 8 hr after adding PRL to cultured mouse mammary tissues. The synthesis of lactose is catalyzed by the enzyme lactose synthetase, which is composed of two parts, alpha-lactalbumin and galactosyl transferase. In time-sequence studies, it was found that the activity of galactosyl transferase is enhanced by PRL in concert with the onset of the PRL stimulation of lactose synthesis. In contrast, the earliest detectable effect of PRL on alpha-lactalbumin activity occurred 24 hr after adding PRL to the cultures. It is, therefore, apparent that the rate-limiting component for the PRL stimulation of lactose synthesis in cultured mouse mammary tissues is galactosyl transferase activity.