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Binding of endogenous iodothyronines to isolated liver cell nuclei
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 X 10(9) M-1 and that for T4 was 0.17 +/- 0.06 X 10(9) 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.
Insights
Thyroxine (T4) metabolites were tested for their ability to bind to thyroid hormone receptors in pig liver nuclei. Most T4 metabolites showed low binding affinity, suggesting they unlikely affect T3 receptor interactions.
Area of Science:
- Endocrinology
- Molecular Biology
- Biochemistry
Background:
- The metabolic fate and physiological roles of many thyroxine (T4) metabolites remain largely uncharacterized.
- Several T4 metabolites are present in human serum, but their biological significance is unclear.
- Understanding the interaction of these iodothyronines with cellular receptors is crucial for comprehending thyroid hormone regulation.
Purpose of the Study:
- To investigate the binding affinities of various T4-derived iodothyronines to specific binding sites in isolated pig liver nuclei.
- To compare the relative abilities of these metabolites to displace triiodothyronine (T3) from nuclear receptors.
Main Methods:
- Competitive binding assays using [125I]T3 and isolated pig liver nuclei.
- Quantification of molar inhibition ratios for different iodothyronine analogs.
- Determination of the dissociation constant (Ka) for T3 and T4 in pig liver nuclei.
- Assessment of nuclear integrity and [125I]T3 retention during incubation.
Main Results:
- Most tested T4 metabolites exhibited significantly lower binding affinities compared to T3.
- Molar inhibition ratios varied widely, with 3',5'-diiodothyronine showing the highest (60,000) and triiodothyroacetic acid the lowest (4.4).
- The dissociation constant (Ka) for T3 was 1.73 x 10^9 M-1, while for T4 it was 0.17 x 10^9 M-1.
- Stored nuclei showed leakage of bound [125I]T3, whereas freshly prepared nuclei maintained integrity.
Conclusions:
- Except for T3 and T4, other T4-derived iodothyronines are unlikely to significantly modulate T3's interaction with its nuclear receptor.
- This effect is improbable unless the perireceptor concentrations of these metabolites substantially exceed their serum concentrations.
- Ensuring the integrity of isolated nuclei is critical for accurate assessment of thyroid hormone receptor binding.