Large Molecular Weight TSH-β: The Sole Immunoactive Form of TSH-β in Certain Human Sera*

Abstract

The β subunit of TSH (TSH-β) usually cannot be detected (4 to patient B, serum TSH-β did not decrease, although TSH and α decreased. Gel chromatography and rechromatography of the patients' sera on a Sephadex G-100 column showed elution of all TSH-β immunoactivity in or near the void volume (V₀; >150,000 mol wt), whereas sera of hypothyroid patients demonstrated 0. By chromatography on a Sephadex G- 200 column, the TSH-β immunoactivity had a 160,000 mol wt in patient A and 200,000 mol wt in patient B. Incubation of labeled or unlabeled TSH-β with serum or γ-globulin fractions from both patients resulted in no significant increase in the binding of TSH-β to serum components, as determined by both gel chromatography and precipitation with antihuman γ-globulin. Large TSH-β was stable after incubation with 6 M guanidine. Ribonuclease failed to affect the large TSH-β. Interchain disulfide bonding was not demonstrated in large TSH-β after treatment with three different reducing agents (mercaptoethanol, sodium sulfite, and dithioerythritol). Treatment with trypsin did not convert the large TSH-β immunoactivity to standard TSH-β. These experiments demonstrated that the large TSH-β immunoactivity was not caused by binding of TSH-β to an immunoglobulin or other serum protein or by aggregation of TSH-β molecules. The significance of these apparently covalently bonded large forms of TSH-β immunoactivity is not yet known; the presence of small amounts of a large molecular weight form in the serum of hypothyroid patients and normal pituitary extracts raises the possibility that they may be components of normal TSH biosynthesis or represent posttranslational modifications.