SPECIFICITY OF MEDROXYPROGESTERONE ACETATE BINDING IN HUMAN-ENDOMETRIUM - INTERACTION WITH TESTOSTERONE AND PROGESTERONE BINDING-SITES

Human endometrial cytosol contains high affinity, low capacity binder(s) for testosterone as well as for progesterone and medroxyprogesterone acetate (MPA). The equilibrium constant of dissociation is the same for all three compounds. In a series of individual samples the concentration of binding sites for [3H]-MPA did not exhibit the cyclic variations of the progesterone binder and was consistently more than 3× higher. The latter observation was confirmed when [3H]-progesterone and [3H]-MPA binding were directly compared using several different pools of endometrial cytosol. Competition studies employing cytosol pools demonstrated that, unlike [3H]-progesterone, [3H]-MPA is displaced from its binding sites by an excess of unlabeled testosterone. Furthermore, [3H]-MPA and [3H]-testosterone binding were demonstrable in a cytosol pool that contained no detectable binder for [3H]-progesterone. It was concluded that while testosterone and progesterone each interact chiefly with their specific binders, MPA has broader specificity and interacts with binders for both of the other steroids. Unless this property of dual binding is recognized and measured, values for progesterone-specific" binding sites determined using MPA as the test progestin will be greatly overestimated. © 1979."