Biochemical classification of hormones

Hormones are classified into three biochemical categories (see Table 10.1):

• Proteins/peptides

Steroid hormones are produced by the adrenal cortex, testes, ovaries, and placenta. Synthesized from cholesterol, these hormones are lipid soluble; therefore, they cross cell membranes readily and bind to receptors found intracellularly. However, because their lipid solubility renders them insoluble in blood, these hormones are transported in the blood bound to proteins. Furthermore, steroid hormones are not typically preformed and stored for future use within the endocrine gland. Because they are lipid soluble, they could diffuse out of the cells and physiological regulation of their release would not be possible. Finally, steroid hormones are absorbed easily by the gastrointestinal tract and therefore may be administered orally.

Protein/peptide hormones are derived from amino acids. These hormones are preformed and stored for future use in membrane-bound secretory granules. When needed, they are released by exocytosis. Protein/peptide hormones are water soluble, circulate in the blood predominantly in an unbound form, and thus tend to have short half-lives. Because these hormones are unable to cross the cell membranes of their target tissues, they bind to receptors

Table 10.1 Distinguishing Features of Steroid, Protein/Peptide, and Amine Hormones

Amine hormones

Feature

Steroid hormones

Protein/peptide hormones

Thyroid hormones

Catecholamines

Synthesis

Synthesized on demand; derived from cholesterol

Synthesized in advance; derived from amino acids

Synthesized inadvance; stored as part of thyroglobulin

Synthesized in advance; derived from tyrosine

Transport in blood

Bound to carrier proteins

Soluble in plasma

Bound to carrier proteins

Soluble in plasma

Half-life

Long

Short

Long

Short

Location of receptor

Within cell nucleus

Membrane surface

Within cell nucleus

Membrane surface

Mechanism of action

Gene activation

Second messenger systems

Gene activation

Second messenger systems

Target tissue response

Synthesis of new enzymes

Modification of existing enzymes

Synthesis of new enzymes

Modification of existing enzymes

Sites of production

Adrenal cortex, testes, ovaries, placenta

Hypothalamus, pituitary gland, thyroid gland, parathyroid glands, pancreas

Thyroid gland

Adrenal medulla

on the membrane surface. Protein/peptide hormones cannot be administered orally because they would be digested in the gastrointestinal tract. Instead, they are usually administered by injection (e.g., insulin). Because small pep-tides are able to cross through mucus membranes, they may be given sublin-gually or intranasally. For example, Miacalcin®, the synthetic form of the hormone calcitonin, is prepared in the form of a nasal spray.

Amine hormones include the thyroid hormones and the catecholamines. The thyroid hormones tend to be biologically similar to the steroid hormones. They are mainly insoluble in the blood and are transported predominantly (>99%) bound to proteins. As such, these hormones have longer half-lives (triiodothyronine, T3, = 24 h; thyroxine, T4, = 7 days). Furthermore, thyroid hormones cross cell membranes to bind with intracellular receptors and may be administered orally (e.g., synthryoid). In contrast to steroid hormones, however, thyroid hormones have the unique property of being stored extra-cellularly in the thyroid gland as part of the thyroglobulin molecule.

The catecholamines are biologically similar to protein/peptide hormones. These hormones are soluble in the blood and are transported in an unbound form. Therefore, the catecholamines have a relatively short half-life. Because these hormones do not cross cell membranes, they bind to receptors on the membrane surface. Finally, the catecholamines are stored intracellu-larly in secretory granules for future use.

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