Saturable 3,5,3′-triiodo-l-thyronine-binding sites in liver nuclei of rainbow trout (Salmo gairdneri Richardson)

Saturable binding of 3,5,3′-triiodo-l-thyronine (T3) was demonstrated in liver nuclei of rainbow trout using an in vivo isotope displacement method; saturable T3 sites were not found in mitochondrial, microsomal, or cytosol fractions. Equilibrium constants ranged from 0.9 to 1.2 × 108 kg liver/mol T3, indicating an affinity comparable to that of mammals. Binding capacities ranged from 0.43 to 0.62 × 10−12 mol T3/g liver. Approximately 50% of the sites were occupied at endogenous T3 levels and a 6- to 11-fold increase in plasma T3 levels was required to achieve saturation. A low food ration followed by starvation did not alter the equilibrium constant but reduced the capacity of the nuclear sites. The sites were intranuclear and represented in a macromolecular fraction extracted with 0.4 N KCl. The macromolecule was identified as a heat-labile protein, probably nonhistone in nature. Further study is necessary to determine the significance of these sites in initiating T3 effects at the cellular level.