Triiodothyronine binding to putative solubilized nuclear thyroid hormone receptor in liver and gill of the brown trout (Salmo trutta) and the European eel (Anguilla anguilla).

The binding of 3,5,3'-triiodothyronine (T3) to salt-extracted nuclear protein of liver and gill from trout and eel was studied in vitro. [125I]T3 binding depended on the temperature and protein concentration. Binding equilibria were achieved in the two tissues of each species between 15 and 24 hr at 4 degrees. The binding was reversible in the presence of excess unlabeled T3. Scatchard analysis showed a single class of high affinity and low capacity T3 binding species considered, 2.71 x 10(-10) M for eel liver and gill. Association (k + 1) and dissociation (k-1) rate constants, calculated from the kinetics of hormone association, were respectively 8.3 x 10(8) M.hr-1 and 0.216 hr-1 for trout liver and 8.3 x 10(8) M.hr-1 and 0.257 hr-1 for trout gill. Their ratio, the equilibrium dissociation constant for liver (2.60 x 10(-10)M) and for gill 3.10 x 10(-10) M), was in good agreement with the apparent Kd from the Scatchard plot. Half-times (t1/2) of dissociation of T3 calculated from association curves, liver (3.2 hr), and gill (2.7 hr) were in reasonable agreement with corresponding values determined directly, liver (7.7 hr) and gill (11.5 hr). These data are consistent with a reversible bimolecular process to describe the binding of T3 to nuclear extracts. The maximal binding capacity (MBC) was lower in gill than in liver. MBC values were similar in liver of trout, 163, and eel, 234, but were different in gill, 82 for trout and 29 fmol/mg protein for eel.(ABSTRACT TRUNCATED AT 250 WORDS)