A plasma steroid hormone binding protein in the viviparous water snake, Nerodia.

A plasma steroid binding protein (SHBP) with medium-high affinity and limited capacity was characterized in the viviparous water snake, Nerodia. This SHBP shows similarity to SHBPs previously described in some other nonmammalian species. A single binding component was detected by Scatchard analyses with a medium-high affinity for testosterone (T), estradiol (E), progesterone (P), and corticosterone (B). Equilibrium dissociation constants (Kd) for these steroids are as follows: T, 3.6 x 10(-8) M; E, 3.7 x 10(-8) M; P, 5.9 x 10(-8) M; B, 12.1 x 10(-8) M. In competition studies (at saturation) the relative binding affinities (RBA) for E (1.0) and T (1.0) were higher than those for P (0.8) and B (0.59). Further analysis of binding specificity for [3H]estradiol at 100-fold excess competitor concentrations revealed that dihydrotestosterone also competes; however, estrone and estriol were relatively poor competitors. Displacement of 3H-E by antiestrogen clomiphene derivatives and synthetic estrogen varied; enclomiphene citrate (67.8%), clomiphene citrate (42.2%), diethylstilbestrol (37.3%), and zuclomiphene citrate (15.2%). The SHBP has a relatively high binding capacity (Bmax = 0.09-0.7 M), which may be correlated with the relatively high circulating plasma steriod levels in this species. Scatchard analysis, disc gel electrophoresis, sucrose gradient centrifugation, and competition studies indicate the presence of a single moiety binding estradiol, testosterone, progesterone, and corticosterone. The estradiol-SHBP complex is unstable, exhibiting very short times of association (t less than 1.5 min) and dissociation (Kd = 0.0165 sec-1, t 1/2 = 18.3 sec). Measurement of SHBP levels throughout the seasonal reproductive cycle revealed high levels of binding in previtellogenic, vitellogenic, early pregnant, and postpartum animals. A significantly lower level of SHBP was detected in mid-late pregnancy.

Identification of a putative progesterone receptor in the oviduct of a viviparous watersnake (Nerodia).

The steroid hormone progesterone is of particular interest in reproductive tract adaptations associated with the processing of embryonic materials in the evolution of viviparity. To further understand the mechanisms of action of this steroid, a progesterone receptor-like moiety was partially characterized in the oviduct of the viviparous snake Nerodia. Using DNA-cellulose and LH-20 column chromatography, a binding component was identified which exhibits high affinity (KA = 1.5 X 10(9) M-1) and low capacity (approximately 10(-11) M) binding of the synthetic progestin, R5020. In addition, the binding exhibited physiological steroid specificity, and was only identified in reproductive structures considered targets of progesterone action. These characteristics are consistent with the primary characteristics associated with steroid-receptor binding and are similar in both cytosolic and nuclear extract preparations.