Amino acid-derived hormones

4 AMINO ACID-DERIVED HORMONES 16.4.1 Thyroxine and Triiodothyronine 

Several types of effects are ascribed to thyroid hormones. T3 and T4 control heat production in the organism by affecting the electron transport pathway (see Chapter 17). Apparently these hormones stimulate electron transport, thereby producing more ATP. The latter is used to increase the activity of Na+/K+ pumps throughout the organism, in the process of which ATP is hydrolyzed and heat 

Thyroid disorders may be divided into over- and underproduction of the thyroid hormones. These may be caused by thyroid gland disorders or disorders of the pituitary gland (TSH production) or hypothalamus (thyrotropin-releasing hormone release). Thyroid hormone deficiency in infancy may cause mental retardation if it is not corrected immediately after birth. For this reason, many states require thyroid function tests in all newborns. In adults, thyroid deficiency may be caused by Hashimoto s thyroiditis, an immune disorder, or dietary iodine deficiency, in which case it is called simple goiter. The term myxedema has been used to refer to hypothyroidism of whatever cause. Myxedemas may 

There are numerous thyroid gland function tests, each designed to determine the etiology of thyroid dysfunction. In general, though, when hypothyroidism is present, circulating T3 and T4 levels are down and TSH is up. The opposite is true of hyperthyroidism. In addition, free (non-protein-bound) T4 and TBG may be determined to clarify inconclusive results. In hyperthyroidism, free T4 is increased but total T4 may be normal. It is the free serum T4 that has been correlated with clinical symptoms rather than total T4. 

This occurs when the quantity of aromatic amino acids entering the brain is larger than normal (as in hepatic encephalopathy), so that octopamine becomes a major tyrosine metabolite. The accumulation of octopamine induces coma. 

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Amino acid-derived hormones include the catecholamines, epinephrine and norepinephrine (qv), and the thyroid hormones, thyroxine and triiodothyronine (see Thyroid AND ANTITHYROID PREPARATIONS). Catecholamines are synthesized from the amino acid tyrosine by a series of enzymatic reactions that include hydroxylations, decarboxylations, and methylations. Thyroid hormones also are derived from tyrosine iodination of the tyrosine residues on a large protein backbone results in the production of active hormone. 

Ambulatory ECG, 24-6-24-7 AMD (age-related macular degeneration), 35-1 Amino acid derivatives hormones, 2-3 Amir, R., 22-13 AMP (adenosine... 

Classification of Hormones. Depending upon their chemical constitutions and their patterns of biosynthesis, three groups of hormones can be distinguished (1) Steroid hormones, (2) Amino acid-derived hormones, and (3) Peptide and protein hormones. Table XVIII gives a survey of known hormones and their effects. 

In mammals, peptide hormones typically contain only the a-amino acids of proteins finked by standard peptide bonds. Other peptides may, however, contain nonprotein amino acids, derivatives of the protein amino acids, or amino acids finked by an atypical peptide bond. For example, the amino terminal glutamate of glutathione, which participates in protein folding and in the metabolism of xenobiotics (Chapter 53), is finked to cysteine by a non-a peptide bond (Figure 3—3). The amino terminal glutamate of thyrotropin-... 

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... 

Hormonal actions on target neurons are classified in terms of cellular mechanisms of action. Hormones act either via cell-surface or intracellular receptors. Peptide hormones and amino-acid derivatives, such as epinephrine, act on cell-surface receptors that do such things as open ion-channels, cause rapid electrical responses and facilitate exocytosis of hormones or neurotransmitters. Alternatively, they activate second-messenger systems at the cell membrane, such as those involving cAMP, Ca2+/ calmodulin or phosphoinositides (see Chs 20 and 24), which leads to phosphorylation of proteins inside various parts of the target cell (Fig. 52-2A). Steroid hormones and thyroid hormone, on the other hand, act on intracellular receptors in cell nuclei to regulate gene expression and protein synthesis (Fig. 52-2B). Steroid hormones can also affect cell-surface events via receptors at or near the cell surface. 

Amino acid derivatives include the thyroid hormones, catecholamines (e.g. adrenaline (epinephrine)) and dopamine, neurotransmitters such as y-aminobutyric acid (GABA) and noradrenaline (norepinephrine). All of these signalling molecules retain... 

Hormones are intercellular messengers. They are typically (1) steroids (e.g., estrogens, androgens, and mineral corticoids, which control the level of water and salts excreted by the kidney), (2) polypeptides (e.g., insulin and endorphins), and (3) amino acid derivatives (e.g., epinephrine, or adrenaline, and norepinephrine, or noradrenaline). Hormones maintain homeostasis—the balance of biological activities in the body for example, insulin controls the blood glucose level, epinephrine and norepinephrine mediate the response to the external environment, and growth hormone promotes normal healthy growth and development. 

Hydrophilic hormones and other water-soluble signaling substances have a variety of biosynthetic pathways. Amino acid derivatives arise in special metabolic pathways (see p. 352) or through post-translational modification (see p. 374). Proteohormones, like all proteins, result from translation in the ribosome (see p. 250). Small peptide hormones and neuropeptides, most of which only consist of 3-30 amino acids, are released from precursor proteins by proteolytic degradation. 

Three hormones, thyroxine (3,5,3, 5 -tetraiodothy-ronine, or T4), triiodothyronine (3,5,3 -triiodothyronine, or T3), and calcitonin (see Chapter 66) are secreted by the thyroid gland. The hormones T4 and Tj are iodine-containing amino acid derivatives and are unique in that they have no discrete target tissue. Every tissue in the body is affected in some way by thyroid hormones, and almost all cells appear to require constant optimal amounts for normal operation. 

In addition to their role as the building blocks of proteins, amino acids are precursors of many specialized biomolecules, including hormones, coenzymes, nucleotides, alkaloids, cell wall polymers, porphyrins, antibiotics, pigments, and neurotransmitters. We describe here the pathways to a number of these amino acid derivatives. 

Polypeptide Hormones Are Stored in Secretory Granules after Synthesis Thyroid Hormones and Epinephrine Are Amino Acid Derivatives... 

Table 24.3 lists many of the better known hormones of vertebrates. Although many different classes of compounds are used as hormones by animals and plants, most vertebrate hormones fall into one of three classes Polypeptides, amino acid derivatives, and steroids. 

Thyroid Hormones and Epinephrine Are Amino Acid Derivatives... 

Most hormones fall into three classes Polypeptides, steroids, and amino acid derivatives. Polypeptide hormones are synthesized from large precursors. Steroid hormones are derivatives of cholesterol. Thyroid hormones and epinephrine are amino acid derivatives. 

Insulin is a storage hormone produced by the pancreas. Insulin shuttles nutrients, such as carbohydrates, fats and amino acids (derived from proteins) into cells. The main function of insulin is to maintain homeostasis of circulatory glucose, and intracellular glycogen storage. It also aids in fat storage. 

Surface Membrane Receptors. These receptors are located on the outer surface of the plasma membrane (see Fig. 28—2).6 Surface receptors tend to recognize the peptide hormones and some amino acid derivatives (e.g., pituitary hormones, catecholamines). They are typically linked to specific intracellular enzymes. When stimulated by a peptidelike hormone,... 

A therapeutic serum is a product obtained from blood by removing the clot or clot components and the blood cells. .. (4) An antitoxin is a product containing the soluble substances in serum or other body fluid of an immunized animal that specifically neutralizes the toxin against which it is immune. (5) A product is analogous...(ii) to a therapeutic serum if composed of whole blood or plasma or containing some organic constituent of product other than a hormone or an amino acid, derived from whole blood, plasma, or serum... 

As indicated in Table 9.1, the endocrine hormones can be divided into two major chemical classes (1) the peptides and amino acid derivatives and (2) the cholesterol-based steroid compounds. In general, the former are believed to interact primarily with membrane-associated receptors, while the latter are more lipophilic and are able to gain entrance into target cells. In any event, overactive or underactive endocrine... 

Hormone A chemical (usually peptide, amino acid derivative, or steroid) produced by an endocrine gland and affecting the function of target cells. 

A therapeutic serum, if composed of whole blood or plasma or containing some organic constituent or product other than a hormone or an amino acid, derived from whole blood, plasma, or a serum. 

The generic term thyroid hormones refers to the iodinated amino acid derivatives T3 (3,3, 5-triiodo-L-thyronine) and T4 (3,3, 5,5 -tetraiodo-L-thyronine), the only iodinated hormones produced endogenously. T3 is the biologically active hormone and is, for the most part, produced from T4 in extrathyroidal tissues. T4 lacks significant bioactivity and is a hormone precursor however,... 

Hie hormone made by the thyroid which is responsible for regulating BMR is called thyroxine it is an iodine-containing amino acid derived from tyrosine. Almost the whole of the body s stock of iodine is used by the thyroid in the manufacture of thyroxine, which has the structure... 

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