title: Kinetics of T4 and T3 binding to plasma sites in salmonid teleost fish.
abstract: The relative affinities and maximum capacities of the classes of L-thyroxine (T4)- and 3,5,3'-triiodo-L-thyronine (T3)-binding sites in plasma of three species of salmonid teleost fish were determined by saturation analysis on miniature Sephadex G-25 columns at 20-21 degrees and kinetic data were analyzed by the multiligand, multisite LIGAND computer program. In plasma of rainbow trout (Salmo gairdneri) homologous ligand displacement indicated that both thyroid hormones (TH) bound to a minimum of two classes of saturable sites and at least one nonsaturable site. For T4 (n = 13) the relative site affinities were 0.61 +/- 0.08 (SEM) X 10(7) M-1 and 0.86 +/- 0.11 X 10(5) M-1 and the site capacities were 8.3 +/- 1.16 X 10(-7) M and 5.15 +/- 1.34 X 10(-5) M, respectively; for T3 (n = 14) the relative site affinities were 1.8 +/- 0.16 X 10(7) M-1 and 1.7 +/- 0.17 X 10(5) M-1 and the site capacities were 7.8 +/- 1.3 X 10(-7) M, respectively. The greater affinities of T3 than T4 for plasma binding sites would explain the lower proportion of free T3 than free T4 in trout plasma. Two-site models with comparable values for TH-binding parameters were determined for brook trout (Salvelinus fontinalis) and arctic charr (Salvelinus alpinus). The TH-binding parameters were uninfluenced by severe hemoglobin contamination of plasma, bleeding of fish 24 hr previously, or 2 weeks starvation. Heterologous ligand displacement (T4 displaced by T3 or T3 displaced by T4) on rainbow trout plasma suggested two low-affinity, high-capacity sites, one binding predominantly T4 and one binding predominantly T3: a high-affinity, low-capacity site binding T3 exclusively: and a high-affinity, low-capacity site binding T4 predominantly but also binding T3 with low affinity.
mesh_terms: D000818:Animals; D001665:Binding Sites; D005260:Female; D007700:Kinetics; D008297:Male; D008954:Models, Biological; D012487:Salmonidae / Q000097:blood*; D013045:Species Specificity; D013974:Thyroxine / Q000097:blood*; D014284:Triiodothyronine / Q000097:blood*; D014337:Trout / Q000097:blood*
chemical_list: D014284:Triiodothyronine; D013974:Thyroxine