Interaction of Purified Long-Acting Thyroid Stimulator (LATS) and Thyroid Microsomesin Vitro1

Abstract

LATS was purified by adsorption to and subsequent elution from thyroid microsomes. Eluted LATS retained activity in the McKenzie mouse bioassay and exhibited electrophoretic and immunologic properties of IgG. Eluted LATS, labeled with ¹²⁵I, had a high degree of affinity and specificity for thyroid microsomes. Serum and IgG preparations with LATS activity inhibited the binding of the eluted LATS ¹²⁵I to thyroid microsomes, whereas normal serum and normal IgG did not. The degree of such inhibition was related to the dose and LATS response index of the serum or IgG used. Experiments in which whole IgG was isolated from a serum with high LATS activity and labeled with ¹²⁵I prior to adsorption with thyroid microsomes indicated that LATS is a rather small proportion of the IgG population and that the source of the material reacting with thyroid microsomes was indeed the IgG and not derived from thyroid microsomes. These experiments provide direct evidence that the mechanism of neutralization of LATS by thyroid microsomes is mediated by binding of LATS to thyroid microsomes and suggest that an in vitro detection system for LATS may be feasible.