A peptide hormone is a signalling peptide or protein released by cells to influence other cells. Insulin, glucagon and somatostatin are examples. Biosynthesis commonly includes a precursor and regulated processing before secretion. Because a peptide generally does not freely cross a lipid bilayer, its signal is commonly received by a cell-surface receptor and relayed through intracellular pathways. [ref]
Molecular Biology of the Cell, 4th edition by Bruce Alberts and others. Chapter 15, General Principles of Cell Communication: endocrine signals and surface receptors
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