Steroid C-20 oxidoreductase activity of duck intestinal mucosa: The interrelations of the enzymatic activity with steroid binding

The binding of corticosterone and aldosterone to domestic duck ( Anas platyrhynchos)-dispersed colonic mucosal cells at 37° was investigated. It was found that in contrast to experiments using cell-free intestinal preparations, corticosterone was extensively metabolized and it was the metabolite, not the native steroid that became receptor bound and all the bound ligand was in the nuclear fraction. The metabolite turned out to be identical with 4-pregnene-11β,20β,21-triol-3-one (20β-dihydrocorticosterone, 20β-DHB). Binding experiments with [ 3H]corticosterone yielded the following kinetic parameters: K d = 87.6 n M, N max = 337,900 sites/cell. When synthetic [ 3H]20β-DHB was used as the ligand a curvilinear-binding isotherm was obtained. This could be resolved into a high affinity-low capacity (HA) and a low affinity-high capacity (LA) component with the following binding parameters: K d,HA = 91 n M, N max, HA = 130,800 sites/cell; K d,LA = 5.4 × 10 −6 M, N max,LA = 3.7 × 10 6 sites/cell. Binding of the metabolite to cell-free preparations, at 0°, gave the following results: for cytosol, linear-binding isotherm, K d = 14.0 n M, N max = 26.5 fmol/mg protein; and for crude nuclei, curvilinear-binding isotherm, K d,HA = 45.0 n M, N max,HA = 5.33 pmol/mg DNA; K d,LA = 2.2 × 10 −6 M, N max,LA = 286.6 pmol/mg DNA. [ 3H]Aldosterone was also bound by the dispersed whole cells and again, this binding was only nuclear ( K d = 9.3 n M, N max = 10,042 sites/cell). The bound ligand was unchanged aldosterone. Competition experiments have shown that aldosterone did not compete with 20β-DHB for binding sites and vice versa. The intracellular 20β-hydroxysteroid oxidoreductase responsible for the transformation of corticosterone was found mostly in the cytoplasm. Kinetic studies with the enzyme yielded classical Michaelis-Menten kinetics ( K m = 15.7 μ M, V max = 2.6 nmol/min/mg protein). The enzyme had an apparent M r of 35 kDa and a R s of 25.5 Å. It is believed that our results might explain the binding of aldosterone to mineralocorticoid-binding sites in the presence of overwhelming concentrations of corticosterone and that experiments with cell-free tissue preparations, performed at 0°, do not reflect the true cellular-binding events.