The N-terminal amino-acid sequence of bovine proparathyroid hormone.

Proparathyroid hormone (calcemic fraction-A) is a biosynthetic precursor of parathyroid hormone in bovine glands. Limited amounts of the isolated prohormone have been obtained for the purpose of initial structural studies. The results of automated sequence analysis on two separate preparations indicate that the N-terminal region of the prohormone consists of the sequence Lys-Ser-Val-Lys-Lys-Arg followed by a sequence exactly corresponding to residues 1 to 34 of bovine parathyroid hormone. Also observed was a minor sequence (about 12% of the total) in which the N-terminal lysine was absent. These data suggest that more than one species of prohormone may exist. Due to the small quantities of sample available, the analyses were restricted to the N-terminal portion of the prohormone only. However, because the amino-acid composition of the prohormone indicates it to be a molecule containing more than 100 amino acids, the possibility remains that additional residues occur at the C-terminus. Thus, the prohormone structure based on these data is believed to consist of the hexapeptide sequence above, followed by the known sequence of the 84 residues in parathyroid hormone, possibly followed by an additional sequence of 10-15 residues.