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Amine hormones include adrenaline, noradrenaline (also called epinephrine and norepinephrine respectively) and thyroid hormones. Their synthesis involves a series of enzyme-catalyzed reactions in the cytoplasm of endocrine cells. For example, thyroid hormones are synthesized by iodination of tyrosine residues in the protein thyroglobulin found in the thyroid. Most peptide hormones are synthesized as large inactive prohormones, which are subsequently cleaved by enzymes to produce the active hormone. Sometimes a number of hormones may be derived from the same prohormone. Steroid hormones are synthesized by a sequence of enzymatic reactions using cholesterol as a common precursor. The enzymes responsible for conversion of cholesterol to hormone are located in the smooth endoplasmic reticulum and mitochondria of cells. The presence or absence of particular enzymes determines the type of steroid hormone synthesized by that specific cell.
Amine hormones, such as adrenaline and noradrenaline, are stored in secretory granules within the cytoplasm, but thyroid hormones are stored within the thyroid follicles as components of thyroglobulin. Peptide hormones are usually stored in membrane-bound vesicles within the cytoplasm of the endocrine cell. Steroid hormones are not usually stored but are released upon synthesis. However, lipid droplets may be found in the cytoplasm containing precursor material for these hormones.
Hormones are released in response to nervous, hormonal or metabolic stimuli. Hormones stored in granules are released when the granules move and fuse with the plasma membrane. Some hormones, for example thyroxine, are released continuously whereas others show diurnal variation and their release varies during the day. For example, cortisol shows diurnal variation with levels being high in the morning but low at night. The concentrations of hormones in the plasma must be kept within narrow ranges for optimum function.
A number of factors control hormone production by the endocrine glands. The secretion of pituitary hormones is under the influence of peptides released from the hypothalamus and, this in turn, is influenced by signals from the central nervous system (CNS). Most hormones released from endocrine glands are controlled by a negative feedback effect, such as for thyroid hormones and cortisol. Finally, changes in the amounts of materials regulated by hormones themselves may influence the release of that hormone, as is the case for insulin. Target cells and the liver contain enzymes that degrade hormones. Hormones of low Mr are removed from the circulation by the kidneys and excreted in urine.
The half-lives of hormones vary from a few seconds to weeks. Many small or water insoluble hormones form complexes with large plasma transport proteins. The kidneys cannot filter out these large complexes and so their rapid loss is prevented. In addition, these complexes protect the hormone from degradation by enzymes and release the hormone slowly. The bound and free hormones are in equilibrium and it is only the free fraction that is biologically active.
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