High-affinity, limited-capacity triiodothyronine-binding sites in nuclei from various tissues of the rainbow trout (Salmo gairdneri).

High-affinity, limited-capacity 3,5,3'-triiodo-L-thyronine (T3)-binding sites were detected by in vitro saturation analysis in cell nuclei from liver, gill, kidney, brain, and erythrocytes (RBC), but not spleen. The sites were extracted from the purified nuclei using 0.4 M NaCl and incubated with [125]T3 in the presence of 0.2 M NaCl. In all tissues T3 binding approached equilibrium after 18 to 48 hr of incubation at 4 degrees and was reversible upon addition of excess unlabeled T3. The T3 association and dissociation rate constants (k+ and k-) were measured from the initial (4 hr) [125I]T3 association and dissociation rates for liver (k+ = 8.9 x 10(9) liters.mol-1.hr-1; k- = 0.067 hr-1) and for RBC (k+ = 1.9 x 10(8) liters.mol-1.hr-1; k- = 0.11 hr-1). The association constants (Ka) determined by saturation analysis were similar in all tissues investigated (average Ka = 2.8 x 10(9) liters.mol-1), except in RBC (Ka = 1.2 x 10(10) liters.mol-1). The Ka values calculated from the k+/k- ratio (1.4 x 10(11) liters.mol-1 and 1.8 x 10(9) liters.mol-1 for liver and RBC, respectively) differed substantially from those determined by saturation analysis. This discrepancy is likely due to nonsaturable T3 binding by coextracted nuclear proteins in the assay medium, altering the estimated k+. The maximal binding capacity of the nuclear sites varied widely between tissues (liver, 250; gill, 130; kidney, 63; brain, 30; and RBC, 10 fmol.(mg DNA)-1; spleen, below detection).(ABSTRACT TRUNCATED AT 250 WORDS)