| Abstract: | Thyroglobulin binds to isolated thyroid plasma membrane preparations. Binding is pH- and temperature-dependent with 10-fold better binding at pH 5.0 and 37 degrees C than at 0 degrees C and pH 6.0 through pH 7.5. Binding is, however, maximal in 90 min at all pH values and temperatures examined. Although salts can inhibit or enhance thyroglobulin binding depending on the temperature or pH, conditions approaching those of the physiological state are not inhibitory; physiological conditions do inhibit thyrotropin binding to the same membrane preparations. 125I-Labeled thyroglobulin binding is poorly reversed by unlabeled thyroglobulin at all pH values and temperatures studied; excess unlabeled thyroglobulin can, however, readily prevent binding. At pH values greater than 6.0 and at 0 degrees C, the iodine content of thyroglobulin can affect binding, and the 27 S thyroid iodoprotein is relatively ineffective in preventing the binding of the 19 S species. At pH 5.0 and 37 degrees C, there is no difference in binding of highly and less iodinated thyroglobulin, and the 27 S thyroglobulin iodoprotein is effective in preventing 19 S thyroglobulin binding. The complex nature of these results is interpreted in the light of additional data which show (i) that the thyroid membrane recognizes asialothyroglobulin and (ii) that at pH 5.0 and 37 degrees C a membrane-associated neuraminidase is activated which removes sialic acid from thyroglobulin. Vibrio cholerae neuraminidase can substitute for the endogenous neuraminidase. The receptor on thyroid membranes for asialothyroglobulin is similar to the asialoglycoprotein receptor on liver membranes (Morell, A.G., Gregoriadis, G., Scheinberg, I.H., Hickman, J., and Ashwell, G. (1971) J. Biol. Chem. 246, 1461-1467) in that sialic acid on the receptor is critical for receptor expression. It is distinct from the liver asialoglycoprotein receptor in its binding specificity and in its sensitivity to different bacterial and mammalian neuraminidase preparations. Relationships between thyroglobulin and thyrotropin receptors on thyroid membranes are explored, and the functional role of the thyroglobulin receptor is discussed. |
