The word hormone is derived from the Greek word hormon, which means to set into motion. Hormones regulate almost every physi-ological process in our body.
Chemical Nature Of Hormones
Hormones fit into one of two chemical categories: lipid-soluble hormones and water-soluble hormones, a distinction based on their chemical composition, which influences their chemical behavior. Recall that the plasma membrane is a selectively permeable phospholipid bilayer that excludes most water-soluble molecules but allows lipid-soluble molecules to pass through. Therefore, the entire basis of a hormone’s metabolism—its trans-port in the blood, its interaction with its target, and its removal from the body—is dependent on the hormone’s chemical nature.
Within the two chemical categories, hormones can be sub-divided into groups based on their chemical structures. Steroid hormones are those derived from cholesterol, thyroid hormones are derived from the amino acid tyrosine, and other hormones are categorized as amino acid derivatives, peptides, or proteins.
Lipid-soluble hormones are nonpolar, and include steroid hormones, thyroid hormones, and fatty acid derivative hormones, such as certain eicosanoids.
Transport of Lipid-Soluble Hormones
Because of their small size and low solubility in aqueous fluids, lipid-soluble hormones travel in the bloodstream attached to binding proteins, proteins that transport the hormones. As a result, the rate at which lipid-soluble hormones are degraded or eliminated from the circulation is greatly reduced and their lifespans range from a few days to as long as several weeks.
Without the binding proteins, the lipid-soluble hormones would quickly diffuse out of capillaries and be degraded by enzymes of the liver and lungs or be removed from the body by the kidneys. Circulating hydrolytic enzymes can also metabolize free lipid-soluble hormones. The breakdown products are then excreted in the urine or the bile.
Water-soluble hormones are polar molecules; they include protein hormones, peptide hormones, and most amino acid derivative hormones.
Transport of Water-Soluble Hormones
Because water-soluble hormones can dissolve in blood, many circulate as free hormones, meaning that most of them dissolve directly into the blood and are delivered to their target tissue with-out attaching to a binding protein. Because many water-soluble hormones are quite large, they do not readily diffuse through the walls of all capillaries; therefore, they tend to diffuse from the blood into tissue spaces more slowly. The capillaries of organs that are regulated by protein hormones are usually very porous, or fenestrated . On the other hand, other water-soluble hormones are quite small and require attachment to a larger protein to avoid being filtered out of the blood.
All hormones are destroyed either in the circulation or at their target cells. The destruction and elimination of hormones limit the length of time they are active. When hormones are secreted that remain functional for only short periods, the body processes regu-lated by them tend to change quickly.
Water-soluble hormones have relatively short half-lives because they are rapidly degraded by enzymes, called proteases, within the bloodstream. The kidneys then remove the hormone breakdown products from the blood. Target cells also destroy water-soluble hormones when the hormones are internalized via endocytosis. Once the hormones are inside the target cell, lyso-somal enzymes degrade them. Often, the target cell recycles the amino acids of peptide and protein hormones and uses them to synthesize new proteins. Hormones with short half-lives normally have concentrations that change rapidly within the blood and tend to regulate activities that have a rapid onset and short duration.
However, some water-soluble hormones are more stable in the circulation than others. In many instances, protein and peptide hormones have a carbohydrate attached to them, or their terminal ends are modified. These modifications protect them from protease activity to a greater extent than water-soluble hormones lacking such modifications. In addition, some water-soluble hormones also attach to binding proteins and therefore circulate in the plasma longer than free water-soluble hormones do.
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