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SUMMARY Immunoglobulin G (IgG) prepared from sera containing the long-acting thyroid stimulator (LATS) inhibited the receptor binding of 125 I-labelled thyrotrophin (TSH) in a particulate fraction of guinea-pig thyroid homogenate. This inhibition was shown to involve a binding interaction between IgG containing LATS and the receptor with a diminution in the number, but not affinity, of sites available for binding of TSH. Studies of dissociation kinetics and gel filtration of receptor—TSH complexes indicated that IgG containing LATS did not combine with receptors occupied by TSH. The data provide evidence that LATS and TSH bind to the same receptor site.
Manley et al. (Sat,) studied this question.