Steroid-protein interactions in nonmammalian vertebrates. II. Steroids binding proteins in the serum of amphibians; a physiological approach.

Abstract

Steroid-protein interactions were studied in various urodelan and anuran amphibians: Pleurodeles waltlii, Salamandra salamandra, Amby stoma mexicanurn, Rana temp oraria and Discoglossus pictus. The detection of specific steroids binding serum proteins was determined by equilibrium dialysis and sucrose gradient centrifugation experiments. Binding constants were determined by using an equilibrium dialysis-scatchard plot method. Amphibians possess proteins with high steroid affinity and limited capacity. These proteins may be classified in two groups according to their specificity: a) the proteins binding specifically C21 steroids or “Amphibian corticosteroid binding protein” (A.CBP). The binding constants of corticosterone were determined in four amphibian species; in Rana tern poraria serum, the association constant (Ka) at 4 C was 8.2 X 108 M-1, and the binding capacity was 1.3 X 107M. b) the proteins binding specifically C18 and C19 sex steroids or “Amphibian sex steroid binding protein” (A.SBP). These proteins were found in all amphibian species studied. In Rana temporaria serum, Ka values for estradiol-17�3 and testosterone were respectively 5.1 x 108 M-1 and 1.3 X io�