Characterization of insulin-like growth factor receptors in human thyroid tissue

We have characterized the binding of 125I-IGF-I and 125I-IGF-II to plasma membranes purified from human thyroid tissue. IGF binding was time- and temperature-dependent. At 4 degrees C, maximal specific binding of 125I-IGF-I was 17.3 +/- 2.5% and of 125I-IGF-II was 8.8 +/- 2.0% (mean +/- SD/60 micrograms membrane protein). 125I-IGF-I binding was inhibited completely by unlabeled IGF-I, IGF-II, insulin, and the type-I IGF receptor monoclonal antibody, alpha IR-3. 125I-IGF-II was inhibited completely by unlabeled IGF-II and nearly completely by IGF-I. 125I-IGF-II binding also was inhibited significantly by insulin, suggesting that much or all of the IGF-II was bound to the type-I IGF receptor. Scatchard analysis revealed a single class of binding sites with a Kd of 6.0 +/- 4.2 x 10(-10) M for IGF-I binding and 5.7 +/- 1.3 x 10(-10) M for IGF-II binding. IGF-I binding was inhibited by a variety of salts in a dose-dependent manner, calcium and magnesium salts being more effective than sodium or potassium salts. Affinity crosslinking of 125I-IGF-I and -II showed clear evidence only for type-I IGF receptors. Thus, a crude plasma membrane fraction of human thyroid tissue expresses predominantly type-I IGF receptors.