Azetidine-2-carboxylic acid is a non proteinogenic amino acid homologue of proline found in common beets. Azetidine-2-carboxylic acid can be misincorporated into proteins in place of proline in many species including humans. It is a toxic and teratogenic agent.

Endogenous central calcitonin (CT) receptor agonist that stimulates cAMP formation at a potency 350-fold greater than CT (ED50 values are 0.2 and 71 nM respectively). Displays no activity at calcitonin-gene related peptide (CGRP) and adrenomedullin receptors. Inhibits formation of multinuclear osteoclasts with similar efficacy to CT in vitro. Suppresses food intake and increases body temperature in free-feeding rats, and significantly decreases plasma calcium levels in vivo.

Copper peptide GHK-Cu is a naturally occurring copper complex of the tripeptide glycyl-L-histidyl-L-lysine. The tripeptide has strong affinity for copper(II) and was first isolated from human plasma. It can be found also in saliva and urine.