Abstract
Previous studies have indicated that crude preparations of hCG (hCG-c) are much more active than purified preparations (hCG-p) in displacing 125I-labeled bovine TSH ([l25I]iodo-bTSH) from receptors in human and bovine thyroid membranes when the two are compared on the basis of their gonadotropic activity. In an attempt to elucidate the nature of the TSH-displacing factor in hCG-c and in view of the past reports indicating that such preparations contain a spectrum of desialylated hCG molecules, we have desialylated both hCG-c and hCG-p by treatment with neuraminidase and have studied their interactions with human thyroid membranes. Exposure of hCG-c to neuraminidase for increasing lengths of time yielded preparations with progressively greater ability to inhibit the binding of [125I]iodo-bTSH to the membranes, and the increase in inhibitory potency was directly related to the quantity of sialic acid released into the medium. On an equal weight basis, desialylated hCG-c was approximately 5 times more active...
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