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Relationship of Thyrotropin to Exophthalmos-producing Substance

FORMATION OF AN EXOPHTHALMOS-PRODUCING SUBSTANCE BY PEPSIN DIGESTION OF PITUITARY GLYCOPROTEINS CONTAINING BOTH THYROTROPIC AND EXOPHTHALMOGENIC ACTIVITY

From the ¹ From the Laboratory of Biochemical Pharmacology, National Institute of Arthritis and Metabolic Diseases, National Institutes of Health, Bethesda, Maryland 20014, and The Institute of Medicine, Department of Clinical Medicine and Semiology, University of Liège, Belgium

Homogeneous bovine pituitary glycoproteins with both thyrotropic and exophthalmogenic activity have been subjected to limited proteolysis by pepsin. One fraction lost 90% of its thyrotropin action, but only 15% of its exophthalmogenic activity, when incubated with a 1% weight ratio of pepsin at pH 2.2 and at 37°. Analytical disc gel analyses showed that the decrease in thyrotropin activity could be correlated with the destruction of the glycoprotein originally in this fraction and with the appearance of several new protein-staining bands. One of the digestion products was an oligosaccharide-containing derivative with terminal galactose on at least one of its carbohydrate moieties. After isolation by electrofocusing, this derivative was shown to have exophthalmogenic activity but to be without significant thyrotropin action; the residual thyrotropin activity of limited digests remained associated with nondigested glycoproteins containing both activities. The exophthalmogenic derivative had an isoelectric point at pH 6.9 and exhibited a single component on analytical disc gels. It had a molecular weight of 20,000 to 22,000 when evaluated by electrophoresis on gels containing sodium dodecyl sulfate, but appeared to be composed of two polypeptide units, one with a molecular weight of approximately 14,000 and the other with a molecular weight of approximately 6,000. The results indicate that limited proteolysis of a homogeneous pituitary glycoprotein with both thyrotropic and exophthalmogenic activity can yield a derivative uniquely active as an exophthalmos-producing substance.

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