Design of a long-lived thyrotropin antagonist from derivatives of human chorionic gonadotropin

Previous studies have revealed that desialylated forms of hCG have a high potency to inhibit both the binding of bovine TSH (bTSH) to human thyroid membranes and the bTSH-stimulated adenylate cyclase activity therein. The biological activities of these desialylated forms, however, are greatly reduced in vivo due to the high affinity of asialo-glycoproteins for hepatic receptors and their consequent rapid clearance from the circulation. Intact purified hCG (hCGp), on the other hand, has little affinity for hepatic receptors and has a relatively long half-life in vivo, but displays only negligible inhibitory potency in the human thyroid. In the present studies, we have sought to obtain one or more compounds derived from hCG that are able to inhibit binding to TSH to the high affinity receptor in human thyroid membranes, and, consequently, could inhibit the stimulatory effect of TSH and Graves' immunoglobulin G and which also display a relatively low affinity for the hepatic asialoglycoprotein receptor and, as a consequence, have a reasonable survival in the circulation. Two hCG forms isolated by sequential chromatography of crude hCG on DEAE-52 and Sephadex G-100 were of interest, since they displayed some degree of selectivity in binding to thyroid and liver receptors. The first form (hCGv), whose isolation we described previously, was comprised of partially desialylated variant forms of intact hCG. The second material, as judged from its immunoreactivity, elution behavior on Bio-Gel P-100, and migration during sodium dodecyl sulfate-polyacrylamide gel electrophoresis, contained a fragment of the beta-subunit (BF), similar to the beta-core fragment described by others. However, BF differed from the beta-core fragment in having a higher sialic acid content. Interestingly, we found that BF as well as the enzymatically desialylated form of BF displayed a much lower affinity for mouse liver receptors than did asialo-hCGp or the free asialo-beta- and asialo-alpha-subunits. Further, the activity of BF to inhibit the binding of [125I]bTSH to human thyroid membranes exceeded that of the desialylated subunits of hCGp as well as that of intact hCGp, but was only exerted at the low affinity binding site and was not accompanied by inhibition of TSH-stimulated adenylate cyclase. In an attempt to shift the locus of BF action from the low to the high affinity TSH receptor, we recombined BF with either an intact alpha-subunit of hCG or an asialo-alpha-subunit. This led to the creation of two novel forms of hCG with properties of the type that we were seeking.(ABSTRACT TRUNCATED AT 400 WORDS)