Polypeptide hormones, or simply peptide hormones, are hormones made up of amino acids that are secreted by the endocrine system and distributed to end organs through the circulating blood. Endocrine organs that secrete peptide hormones include the hypothalamus, pituitary gland, thyroid gland, adrenals, ovaries, endocrine pancreas, and adipose tissues. Organs not generally traditionally considered part of the endocrine system, such as the heart and gastrointestinal tract, can also secrete peptide hormones.
The process of manufacturing these hormones is the same as the process of producing proteins. An organism’s deoxyribonucleic acid (DNA) is first translated into messenger ribonucleic acid (mRNA) in the cell’s nucleus, after which the mRNA template is translated to amino acid chains or peptide hormone precursors in the ribosomes. These amino acid chains, also called pre-prohormones, are then sent to the endoplasmic reticulum for the removal of the signal or leading sequences, which are about 15 to 30 amino acids long and are located at the N-terminal of the amino acid chain. Cleavage of the signal sequences results in processed peptides called prohormones. Prohormones are either packaged into secretory vesicles or cleaved by enzymes called endopeptidases to form the mature hormone before being released into the circulation.
Peptide hormones secreted by the hypothalamus are generally named releasing factors, and include corticotropin-, gonadotropin-, somatotropin-, and thyrotropin-releasing factor. Those secreted by the anterior pituitary include melanocyte-stimulating hormone, follicle-stimulating hormone, luteinizing hormone, adrenocorticotropic hormone (ACTH), thyrotropic hormone, and growth hormone or somatotropin. The anterior pituitary also secretes prolactin and mammatrophic hormone. The posterior pituitary secretes vasopressin and oxytocin. Other peptide hormones include thyroxine from the thyroid gland, cortisol from the adrenals, and insulin from the pancreas.
Specific extracellular stimuli induce the secretion of the polypeptide hormones. For instance, when there is a change in the homeostatic balance, they are secreted in order to reestablish equilibrium. The endocrine system typically works by negative and positive feedback or closed-loop feedback mechanisms. For instance, the anterior pituitary secretes ACTH, which stimulates the secretion of cortisol from the adrenal cortex. When the pituitary gland detects that the cortisol levels in the blood are elevated, it reduces its production of ACTH.
To stimulate an organ, a peptide hormone has to have a receptor in that organ. Receptors for peptide hormones are situated in the plasma membrane, except for the thyroid hormone receptor, which is located in the nucleus. When a peptide hormone binds to its receptor, signal transduction occurs, and substances called second messengers are released to activate specific proteins in order to increase or inhibit the production of certain substances. The main second messengers include calcium, cyclic adenosine monophosphate (cAMP), inositol triphosphate, and diacylglycerol (DAG).