Corticosteroid binding in plasma of Xenopus laevis. Modifications during metamorphosis and growth.

The binding of corticosteroids has been studied by equilibrium dialysis at 4 degrees C and electrophoresis on polyacrylamide gel. Aldosterone was not bound specifically. Large amounts of corticosterone (B) were bound. A high affinity (Ka = 2.8 X 10(8) M-1) low capacity (70.1 nM) component was found in tadpoles. Its affinity and its electrophoretic mobility were not significantly modified during metamorphosis and growth until adult. It differs from mammalian CBG with respect to affinity, electrophoretic mobility and specificity. During growth, the capacity of the high affinity component increased significantly (173 nM in adults) and a low affinity (Ka = 2.0 X 10(5) M-1) high capacity (18.26 microM) component was detected. This last component has the same electrophoretic mobility as bovine serum albumin. These modifications can be related to the large increase in the level of plasma proteins (especially albumins). The concentrations of bound and free B deduced from our data indicate that in tadpoles the increase in the total concentration in the climax is not a good reflection of modifications of these two fractions since the change of free B is larger than that of bound B. This information is an important consideration in interpreting the physiological role of interrenal secretion during metamorphosis.