Nuclear specific thyroxine binding in human mononuclear blood cells

A method for measuring specific, high affinity, low capacity nuclear binding sites for thyroxine in human mononuclear blood cells is described. The binding is temperature and pH dependent and saturable. The cross-reactivity of triiodothyronine for the thyroxine binding sites was only 5% at the 50% inhibition level. Chromatographic and electrophoretic studies suggested two nuclear thyroxine binding proteins with a molecular weight of 66,000 and 140,000 daltons. Maximal specific nuclear binding capacity for thyroxine in nuclei from 10 healthy subjects was 1.8 +/- 0.4 fmol T4/100 micrograms DNA (mean +/- SD), and the equilibrium association constant for the high affinity sites was Ka = 2.0 +/- 0.6 X 10(9)l/mol.