Physiologic properties of steroid hormone-binding proteins in avian blood.

The physiologic properties of steroid hormone-binding proteins have been partially characterized in plasmas of 23 avian species (8 orders and 12 families). A specific sex hormone-binding protein (SBP) with high affinity and low capacity, as found in amphibians and some reptiles and mammals, could not be identified in any of the avian species investigated. In addition SBP appeared to be totally absent in the blood of posthatching and juvenile stages of two species, and in the embryonic blood of three species. For birds it is concluded that sex steroid hormones circulate bound to plasma albumins with low affinity (Kd = 10(-5) mol/liter) and very high capacity. There are two protein-binding systems for corticosterone in the blood of all species investigated, including samples collected from embryos. One is saturable and specific with low capacity (10(-8) -10(-9) mol/liter) and high affinity (Kd = 10(-7) -10(-9) mol/liter), whereas the other is a nonspecific component with very high capacity and low affinity (Kd = 10(-5) mol/liter). Specificity studies show that the high-affinity binding system for corticosterone also binds progesterone with virtually identical affinity, whereas testosterone and estradiol are bound with lower affinity. These data suggest that the high-affinity binding protein in avian blood has physiologic properties similar to mammalian corticosteroid-binding globulin (CBP).