Lactose synthetase: progesterone inhibition of the induction of alpha-lactalbumin.

Lactose synthesis in the mammary gland is dependent on the hormonally controlled synthesis of the two protein components of lactose synthetase, alpha-lactalbumin and a galactosyltransferase. Prolactin induces the synthesis of both proteins in mammary gland explants treated with insulin and hydrocortisone, but the induction kinetics cannot account for the asynchronous synthesis of the two proteins that are observed in vivo. Progesterone appears to take part in the control of lactose synthesis and acts to repress the formation of alpha-lactalbumin throughout pregnancy. At parturition, when the concentration of progesterone in the plasma decreases, the rate of alpha-lactalbumin synthesis increases.