The Presence of a Common Type of Subunit in Bovine Thyroid-stimulating and Luteinizing Hormones

Abstract Bovine thyroid-stimulating hormone (thyrotropin, TSH) has been separated into two different subunits by gel filtration in ammonium bicarbonate solution after prior treatment with 1 m propionic acid. The two subunits, which have been designated TSH-α and TSH-β, resemble each other more closely in amino acid composition than do the two subunits of luteinizing hormone (LH, interstitial cell-stimulating hormone), CI and CII, but there are differences in the contents of aspartic acid, isoleucine, leucine, and tyrosine. Maps of the tryptic peptides and the biological properties of TSH-α and -β differ markedly. TSH-α and the CI chain of LH are closely related both in amino acid and carbohydrate composition; their peptide maps are also very similar. By themselves each subunit of TSH has little or no TSH activity, but activity is restored to about 50% of that of undissociated TSH by combining the two in equal amounts. The CI chain of LH can substitute fully for TSH-α in reconstitution of TSH activity, and other experiments indicate that TSH-α and the CII chain of LH will combine to form a hybrid active LH. TSH-β and CII differ markedly in amino acid composition and immunological properties, and do not appear to interact with each other. The data obtained, thus far, show that the TSH-α chain and the LH-CI chain have very similar or nearly identical primary structures. Thus, the different hormonal activities of TSH and LH have originated from the combination of this type of chain with one of two other peptide chains, TSH-β and LH-CII, respectively, whose amino acid sequences most probably differ considerably.