Binding of steroids to blastokinin.

The authors studied the binding of steroids with Blastokinin (BKN). They confirm that progesterone is the steroid that has a greater affinity for BKN than estradiol or testosterone; the latter is hardly bound to the protein at all. They also show that the presence of estradiol interferes with the formation phase of the BKN-progesterone complex, while testosterone does not. When the BKN-progesterone complex is already formed, a release of the progesterone occurs only when there is a double concentration of estradiol. The biological importance of this phenomenon is briefly discussed.