Binding of steroids by plasma of a teleost: The rainbow trout, Salmo gairdnerii

Abstract

Steroid binding in female trout plasma has been studied. Two thermolabile binding systems are detected by saturation analysis and competitive assays. One is a “sex binding protein” with high affinity for testosterone ( K 4° C = 3.8 × 10 8 M −1) and oestradiol-17β ( K 4° C = 1.8 × 10 8 M −1). The second is a “transcortin type” system binding testosterone ( K 4° C = 3.7 × 10 7 M −1), progesterone ( K 4° c = 2.2 × 10 7 M −1), oestradiol-17β ( K 4 C = 1.4 × 10 7 M −1) and corticosteroids. Association constants and capacities determined by gel filtration or by equilibrium dialysis are compared. As in other fish, measured capacities are higher than for mammalian binding systems. Whether the two kinds of sites are on a single or two proteins is discussed. Competitive assays performed with fractions of a DEAE-cellulose chromatography of plasma show a partial separation. But only one peak of radioactivity sediments in the 5–6 S zone when plasma is ultracentrifuged with tritiated steroids in a sucrose gradient. To test the steroid inactivation by binding, the induction of maturation trout oocytes in vitro by 17α-hydroxy-20β-dihydroprogesterone has been used as biological assay. 17α-Hydroxy-20β-dihydroprogesterone activity is suppressed by plasma binding. Biochemical and biological results are compared and discussed.