Binding of Endogenous Iodothyronines to Isolated Liver Cell Nuclei*

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Abstract
The metabolic role of a number of the metabolites of T4 is unknown. Hence, these iodothyronines, now known to be present in human serum, were tested for their ability to displace [125I]T3 from specific binding sites in isolated pig liver nuclei. Compared with T3 (1.0), the molar inhibition ratios of the analogs tested were: triiodothyroacetic acid, 4.4; T4, 6.2; 3.3′- diiodothyronine, 56; 3,5-diiodothyronine, 245; rT3, 264; and 3′,5′- diiodothyronine, 60,000. In isolated pig liver nuclei, the Ka for T3 was 1.73 ± 0.21 × 109 M−1 and that for T4 was 0.17 ± 0.06 × 109 M−1. Nuclei stored in liquid nitrogen for up to 8 weeks leaked bound [125I]T3 into the supernatant during the incubation period. No loss of bound [125I]T3 was observed with freshly prepared nuclei. The data indicate that, with the exception of T3 and T4, iodothyronines derived from T4 are unlikely to modulate the interaction of T3 with its receptor unless their perireceptor concentration is significantly greater than their serum concentration. (Endocrinology106: 1133, 1980)