Secretin is the first peptide hormone discovered and described as a gastrointestinal peptide hormone at the beginning of the last century. Since then, others have been added that have been studied in more detail, such as insulin, which is essential in sugar breakdown.
What is a peptide hormone?
Peptide hormones are characterized by their amino acid composition and have water-soluble properties. They consist of one or more peptide chain(s), each consisting of a small number (between 10 to 100) of amino acids that are enzymatically linked by peptide bonds. In this process, the carboxyl group of one amino acid reacts with the amino group of another amino acid with elimination of water. Peptide hormones can also induce the release of other hormones as signaling molecules.
Function, effects, and roles
Peptide hormones either act locally at their site of production, or arrive at their site of action in the bloodstream. Here, they cannot pass through cell membranes and instead dock to membrane-bound cell receptors according to the lock-and-key principle. As a result of the binding, the structures of the receptors change so that they bind and activate intracellularly, e.g. the enzyme adenylyl cyclase. This activation mediates a conversion of ATP into cAMP (cyclic adenosine monophosphate) inside the cell. Peptide hormones are therefore also referred to as first messengers and cAMP as second messenger. The cAMP has different functions depending on where it was produced, cannot leave the cell practically and is therefore converted to non-cyclic AMP over time by another enzyme and inactivated. The most important peptide hormones are insulin and its counterpart glucagon. The latter prevents hypoglycemia in the body. Insulin, on the other hand, enters insulin-dependent tissues such as muscle cells, liver cells and fat cells via the bloodstream in order to induce glycogen build-up (depot form of sugar) here. Other relevant peptide hormones are the sex hormones FSH (follicle-stimulating hormone) and LH (luteinizing hormone). In women, they regulate the maturation of follicles in the ovaries and ovulation. In men, they control spermatogenesis, where LH is called ICSH (interstitial cell stimulating hormone). The peptide hormone GH (growth hormone in English) primarily influences the metabolism of cells and, as a growth hormone, is thus jointly responsible for the differentiation of cells and organs. It indirectly controls postnatal body growth by stimulating other messengers in the liver.
Formation, occurrence, properties, and optimal levels
In most cases, longer precursor peptides are formed in organs and are stored inactively in the body. When needed, they are then proteolyzed enzymatically, which means that the inactivating portions are removed from the precursor peptide hormone, thereby activating it. In the case of insulin, which consists of two peptide chains (A and B chains) and is formed in the pancreas, the two chains are linked by a C peptide and are inactive as proinsulin. As soon as the C-peptide is cleaved, the two chains are activated. Glucagon is also produced in the pancreas. The production cells for insulin are the beta cells and for glucagon the alpha cells of the islets of Langerhans. These are named after their first describer, the German pathologist Paul Langerhans (1847-1888). Even small amounts of the peptide hormones are sufficient for their effect to become visible in the body. For example, a healthy person needs only 0.13-0.7 ng/ml of insulin for sugar breakdown to occur successfully. According to the hormone-producing glands, a distinction is made between thyroid, adrenal medullary, hypothalamic or pituitary hormones. FSH and LH, for example, are produced in the pituitary gland and transported to the reproductive organs via the bloodstream. In childbearing women, sex-specific normal values for FSH vary between 3.5-21.5 mlE/ml depending on the menstrual cycle phase, while after menopause values between 26-135 mlE/ml are normal. In men, FSH levels differ according to age (under 40 years: <6 mlE/ml FSH; over 40 years: <13 mlE/ml FSH). For LH, levels also vary in a sex-specific manner (in men ≥ 25 years: 1.7 -8.6 mlE/ml) or in a cycle-specific manner in women (1-95 mlE/ml, post-menopausal: 7.7-58.5 mlE/ml). For GH, the concentration varies between adults (0-8 ng/ml) and children up to puberty (1-10 ng/ml).When determining the concentration of hormones, the blood should always be taken in the morning, as hormone release is subject to a daily rhythm.
Diseases and disorders
During the production or at the site of action of peptide hormones, disorders can occur that lead to a wide variety of diseases. In this context, insulin has gained sad notoriety due to the widespread disease diabetes mellitus (diabetes). If, for example, the beta cells can no longer produce insulin, the body must be supplied with it from outside. In insulin resistance, many of the specific cell surface receptors that normally interact with insulin can no longer accomplish this interaction and the second messenger remains inactive. Increased glucagen production is caused by glucagonomas. These are neuroendocrine tumors that primarily affect the alpha cells of the pancreas. Glucagonomas account for approximately 1% of all pancreatic tumors and are thus very rare. In contrast, hypoglycemia is usually due to glucagon deficiency. In the case of an unwanted pregnancy, the concentrations of FSH or LH may be severely below normal, causing ovarian hypofunction. Disorders in pubertal development may also be due to a deficiency or malformation of FSH and/or LH. FSH disorders may inhibit pubertal development in boys and may be the cause of inadequate sperm maturation in men.
