The Primary Structure of Bovine Thyrotropin: I. ISOLATION AND PARTIAL SEQUENCES OF CYANOGEN BROMIDE AND TRYPTIC PEPTIDES

Abstract

Abstract The peptides produced from reduced S-carboxymethyl bovine thyrotropin (TSH) by hydrolysis with trypsin and by cleavage at its methionine residues with cyanogen bromide have been isolated. Partial or complete amino acid sequences of many of the tryptic peptides and of the smaller cyanogen bromide fragments have been determined. Twenty-five distinct primary tryptic peptides (of which three are glycopeptides) and free serine were recognized as well as several peptides originating from chymotryptic-like cleavages or partial cleavages of resistant bonds. Sufficient data were obtained from the studies on the composition and partial sequences of the cyanogen bromide fragments and the tryptic peptides derived from them to order 10 of the tryptic peptides plus free serine into a peptide chain of 96 residues whose composition exactly matches that of one of the recently discovered subunits of thyrotropin (TSH-α). This subunit, which is the one nearly identical with one chain of luteinizing hormone, shows heterogeneity at its NH2 terminus with some molecules lacking a Phe-Pro sequence. Seventy-six of the residues of TSH-α have been placed in sequence. Two additional large cyanogen bromide fragments, containing 49 to 53 and 51 to 53 residues, respectively, were also overlapped, and they, together with the single remaining cyanogen bromide fragment and free homoserine, accounted for the amino acid composition of TSH-β (the second of the two subunits). Only one tryptic peptide, Ser-Tyr, formerly thought to be at the COOH terminus of intact thyrotropin, could not be identified as part of a cyanogen bromide fragment. The data indicate that the two TSH subunits, while possessing quite similar amino acid compositions, have very different sequences.