Observations Concerning the Binding of L-Triiodothyronine in the Human Polymorphonuclear Leukocyte

Abstract

To examine the subcellular locus of L-triiodothyronine (T3) binding in the human polymorphonuclear leukocyte, intact leukocytes (greater than 90% polymorphonuclear) were incubated with small tracer concentrations of [125I]T3 with or without an excess of non-radioactive T3. Measurement of the [125I]T3 content of several subcellular fractions revealed that the non-radioactive T3 had led to significant displacement of [125I]T3 from the nuclear fraction alone. After correction for degradation and non-specific binding of the added T3 in intact leukocytes from hypothyroid patients in which the endogenous T3 concentration would be minimal, the T3-nuclear binding interaction was found to have an equilibrium dissociation constant of 1.1 X 10(-10)M and a binding capacity of 4.3 fmol/1 X 10(7) cells. Pre-incubation of intact cells with non-radioactive T3 did not increase the subsequent specific binding of [125I]T3 by the nuclei isolated therefrom. The data indicate that: 1) the human polymorphonuclear leukocyte possesses saturable nuclear binding sites for T3, and 2) the sites appear to bind T3 without the intermediation of a cytosol receptor.