Hormones can be classified in several ways
المؤلف:
Peter J. Kennelly, Kathleen M. Botham, Owen P. McGuinness, Victor W. Rodwell, P. Anthony Weil
المصدر:
Harpers Illustrated Biochemistry
الجزء والصفحة:
32nd edition.p490-491
2025-11-09
58
Hormones can be classified according to chemical composition, solubility properties, location of receptors, and the nature of the signal used to mediate hormonal action within the cell. A classification based on the last two properties is illustrated in Table 1, and general features of each group are illustrated in Table 2.
Table1. Classification of Hormones by Mechanism of Action
Table2. General Features of Hormone Classes
The hormones in group I are lipophilic. After secretion, these hormones associate with plasma transport or carrier proteins, a process that circumvents the problem of solubility while prolonging the plasma half-life of the hormone. The relative percentages of bound and free hormone are determined by the amount, binding affinity, and binding capacity of the transport protein. The free hormone, which is the biologically active form, readily traverses the lipophilic plasma membrane of all cells and encounters receptors in either the cytosol or nucleus of target cells. The ligand-receptor complex is the intracellular messenger in this group.
The second major group consists of water-soluble hormones that bind to specific receptors spanning the plasma membrane of the target cell. Hormones that bind to these surface receptors of cells communicate with intracellular metabolic processes through intermediary molecules called second messengers (the hormone itself is the first messenger), which are generated as a consequence of the ligand-receptor interaction. The second messenger concept arose from an observation that epinephrine binds to the plasma membrane of certain cells and increases intracellular cAMP. This was followed by a series of experiments in which cAMP was found to mediate the effects of many hormones. Hormones that employ this mechanism are shown in group II.A of Table 1. Atrial natriuretic factor (ANF) uses cGMP as its second messenger (group II.B). Several hormones, many of which were previously thought to affect cAMP, appear to use ionic calcium (Ca2+) or metabolites of complex phosphoinositides (or both) as the intracellular second messenger signal, and are shown in group II.C of the table. The intracellular messenger for group II.D is a protein kinase–phosphatase cascade; several have been identified, and a given hormone may use more than one kinase cascade. A few hormones fit into more than one category, and assignments change as new information is discovered.
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