Structure-function relationship studies of PTH(1-11) analogues containing D-amino acids

Parathyroid hormone (PTH) is an 84-amino acid peptide hormone. Produced in the parathyroid glands, it acts primarily on bone and kidney to maintain extracellular calcium levels within normal limits. It has been shown that the 1-34 amino acid fragment of PTH is sufficient to bind and activate the PTH type-I receptor. Recent investigations focusing on the interaction of N-terminal fragments of PTH with PTH type-I receptor showed that certain modifications can increase signalling potency in peptides as short as 11 amino acids. To understand the role of the side chains of all the amino acid residues in PTH(1-11), we synthesized all-d PTH, three retro-inverso analogues of the most active modified PTH(1-11), H-Aib-Val-Aib-Glu-Ile-Gln-Leu-Nle-His-Gln-Har-NH2, and we substituted every l-AA of the latter with the corresponding d-AA, obtaining a library of PTH(1-11) analogues that were tested as agonists. The library was synthesized by SPPS, employing the Fmoc protocol. The biological tests showed that the activity of the d-Har11 analogue is of the same order of magnitude of that of the most active modified PTH(1-11). This behaviour is paralleled by an increase of the helical content on going from the d-Val2 to the d-Har11 analogue. This is in agreement with previous work where a correlation between activity and helical content has been demonstrated. The importance of a positively charged group in the C-terminal position is shown to be independent of the configuration of the Cα-carbon.