Peptide hormones arehormones composed ofpeptide molecules. These hormones influence theendocrine system ofanimals, includinghumans.[1] Most hormones are classified as either amino-acid-based hormones (amines, peptides, or proteins) orsteroid hormones. Amino-acid-based hormones are water-soluble and act on target cells viasecond messenger systems, whereas steroid hormones, being lipid-soluble, diffuse throughplasma membranes to interact directly withintracellular receptors in thecell nucleus.
Like all peptides, peptide hormones are synthesized incells fromamino acids based onmRNA transcripts, which are derived fromDNA templates inside thecell nucleus. The initial precursors, known aspreprohormones, undergo processing in theendoplasmic reticulum. This includes the removal of theN-terminalsignal peptide and, in some cases,glycosylation, yieldingprohormones. These prohormones are then packaged into secretoryvesicles, which are stored and released viaexocytosis in response to specific stimuli, such as an increase in intracellular Ca2+ and cAMP levels.[2]
Prohormones often contain extra amino acid sequences necessary for properfolding but not for hormonal activity. Specificendopeptidases cleave the prohormone before secretion, producing the mature, biologically active hormone. Once in the bloodstream, peptide hormones travel throughout the body and bind to specificreceptors on target cell membranes.
Someneurotransmitters are secreted and released in a manner similar to peptide hormones, and certain "neuropeptides" function as both neurotransmitters in thenervous system and hormones in the bloodstream.
When a peptide hormone binds to its receptor on the cell surface, it activates asecond messenger within thecytoplasm, triggeringsignal transduction pathways that lead to specific cellular responses.[3]
Certain peptides, such asangiotensin II,basic fibroblast growth factor-2, andparathyroid hormone-related protein, can also interact withintracellular receptors in thecytoplasm or nucleus through anintracrine mechanism.[4]