Identification of a novel sex steroid binding protein.

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Abstract
A cytoplasmic steroid binding protein in the chick oviduct which has intriguing characteristics is described. This protein binds [3H]estradiol with high affinity (Kd = 30 .times. 10-9 M) and limited capacity (300 femtomol/mg of cytosol protein). It sediments at 8 S in low-salt sucrose density gradients and at 4 S in high-salt gradients. Unlike the estrogen receptor, however, this protein also binds progesterone, R5020 [promegestone], testosterone and 5.alpha.-dihydrotestosterone with similar affinities and in a competitive manner. Moreover, it is not translocated to the nucleus by the in vivo administration of these sex steroids. The protein is only present in estrogen-responsive tissue, and like the sex steroid receptors, its synthesis appears to be regulated by estrogen.