A specific L-tri-iodothyronine-binding protein in the cytosol fraction of human breast adipose tissue

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Abstract
Binding of L-[125I]iodothyronine to the cytosol fraction of normal human female breast adipose tissue was investigated by the charcoal adsorption method. Equilibrium of binding was reached after 120 s at 25.degree. C. The L-tri-[125I]iodothyronine-binding component is a protein; this was confirmed by experiments in which binding was totally lost after heating the cytosol fraction for 10 min at 100.degree. C and in which binding was diminished after treatment with proteolytic enzymes and with thiol-group-blocking reagents. The binding protein was stable at -38.degree. C for several months. It displayed saturability, high affinity (apparent Kd 3.28 nM) and a single class of binding sites. High specificity for L-triodothyronine and L-3,5-di-iodo-3''-isopropylthyronine was observed, whereas other iodothyronines were less effective in displacing L-tri-[125I]-iodothyronine from its binding site. The binding of the hormone by the cytosol fraction did not show a pH optimum. When cytosol fractions of adipose tissue from different females were subjected to radioimmunoassay for the determination of of thyroxine-binding globulin a value of 0.304 .+-. 0.11 .mu.g/mg of cytosol protein (mean .+-. SD, 4) was obtained; the mean concentration in plasma was 0.309 .+-. 0.07 .mu.g/mg of plasma protein (mean .+-. SD, 3). The Ka value of 6.3 .times. 108 M-1 of L-tri-[125I]iodothyronine for binding to plasma, the similar thermal-inactivation profiles of binding and the reactivity to thiol-group-blocking reagents were some properties common between the binding components from the cytosol fraction and plasma. The cytosol fraction of human female breast adipose tissue contains thyroxine-binding globulin; the protein that binds L-tri-[125I]iodothyronine with high affinity and specificity appears to be similar to thyroxine-binding globulin.