Use of 125I-triiodothyroacetic acid to measure nuclear thyroid hormone receptor

125I-Triac was employed to measure hepatic thyroid hormone nuclear receptor (RT) in the rat. The binding properties of 125I-Triac and 125I-T3 were compared in a 0.4 M KCl extract of a liver nuclear preparation. The order in which the stable compounds, Triac, T3, T4 and rT3, competed for 125I-Triac and 125I-T3 binding in liver nuclear extract was similar (Triac greater than T3 greater than T4 greater than rT3), suggesting association of both radioligands with RT. Scatchard plot analysis of specific 125I-Triac and 125I-T3 binding in nuclear extract gave approximately equal estimates of the maximum binding capacity (MBC). However, the binding affinity, as represented by the equilibrium association constant (KA), was higher for 125I-Triac than for 125I-T3 (7-10 X 10(9)M-1 vs 1-3 X 10(9)M-1). To determine the effect of contaminating serum proteins on estimates of MBC and KA, a small amount of dilute rat serum was added to the same nuclear extract preparation. Addition of serum decreased the KA value and markedly increased the MBC values estimated by analysis of 125I-T3 binding data. In contrast, KA and MBC values derived from 125I-Triac binding data were not influenced appreciably by the addition of serum. These data indicate that: 1) both 125I-Triac and 125I-T3 bound to RT in rat liver nuclear extract, 2) the affinity of RT for 125I-Triac is appreciably greater than for 125I-T3, and 3) estimates of RT concentration (MBC) made with 125I-Triac are less sensitive to serum protein contamination than those made with 125I-T3. These properties of 125I-Triac may be useful in efforts to demonstrate RT in tissues that have low RT levels and/or when serum contamination is present.