Some Properties of a Steroid-Binding Protein in the Plasma of an Ovoviviparous Dogfish, Squalus acanthias, at Different Stages of the Life Cycle1

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Abstract
The steroid-binding properties of plasma from mature or embryonic dogfish, Squalus acanthias, were studied. A single protein from the plasma of both sexes binds estrogens, androgens, and progesterone with high affinity (107 -108 M-1) and limited capacity (10-7 M). The protein has a molecular weight of 150-180 x 103 dalton and a sedimentation coefficient of 8-9 S. Testosterone and 5α-dihydrotestosterone bind with higher affinity than estradiol-17β, but progesterone binds with an affinity similar to that of estradiol-17β. Estriol, estrone, and corticosterone compete poorly for estradiol-17β binding sites. No sex difference in either the association constant or the binding capacity was observed, and the binding capacity in female fish did not change during pregnancy. The binding specificity of Stage C embryonic plasma was shown to be identical to that of mature fish. However, the capacity of embryonic plasma for [3 H] -estradiol binding was half that of mature fish, with a slightly lower capacity being found in male embryos.