The specific binding of the thyroid hormones to matrix isolated from rat liver nuclei

Specific binding sites for the thyroid hormones have been demonstrated in the liver nuclear matrix, a structural framework of the nucleus. When labelled 3,5,3'-tri-iodo-L-thyronine ([125I]T3) is injected into rats, 5% of the total nucleus bound T3 is bound to the matrix after 1 hour. However, when either isolated nuclei or isolated nuclear matrices were incubated with [125I]T3 in vitro, a 3- to 7-fold greater number of specific T3 binding sites were revealed in the nuclear matrix. The properties of the matrix-associated thyroid hormone binding sites were investigated in vitro. These binding sites showed limited capacity and high affinity for T3; the equilibrium association constant (Ka) was 1,3 x 10(9) M-1 and the binding capacity was 20,2 fmol T3 per 100 microgram matrix protein.