Specific binding of thyroid-stimulating hormone by human serum globulins

Abstract

Globulin preparations (41) from patients with Graves's disease (positive to thyroid stimulating immunoglobulins; TSI) and 12 from healthy persons (TSI-negative) were tested for their specific thyrotrophin (TSH)-binding properties. Globulins from both groups possessed binding sites for 131I-labelled TSH. The mean dissociation constant (Kd) was 6.8 pmol/1 per mg globulin and the maximum specific binding (Bmax) was 3.0 pmol/mg globulin per 1 for the TSI-negative control group. Twenty-four (58.5%) globulin preparations from the TSI-positive group had similar TSH-binding characteristics with mean Kd of 7.2 pmol/1 per mg globulin and Bmax of 3.6 pmol/mg globulin per 1 (A-type binding) but the remaining 17 (41.5%) bound TSH in a different fashion with Kd of 71.5 pmol/1 per mg globulin and Bmax of 13.6 pmol/mg globulin per 1 (B-type binding). Both types of specific TSH binding reached the maximal level within 1 h of incubation and had an optimum pH of 7--8. There was a linear correlation between the amount of bound TSH and the globulin content of the samples. Both types of binding were reversible by the addition of an excess of TSH and gonadotrophins, ACTH, prolactin and insulin competed with TSH for the binding sites only when in relatively high concentrations. The binding sites were associated with macromolecules; they emerged with the void volume after chromatography on Sephadex G-200 and migrated with immunoglobulin G (IgG) on paper electrophoresis. The binding capacity of the globulin preparations could be decreased by preincubation with antiserum to human IgG or with human thyroid membranes.