27 - Chapter 27. THE NATURE AND SYNTHESIS OF HORMONES The...

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Unformatted text preview: Chapter 27. THE NATURE AND SYNTHESIS OF HORMONES The hormones represent a diverse array of chemical classes of molecules, ranging in size from about 100 daltons to large proteins of more than 30,000 daltons. The size and other physicochemical properties of the molecules are important in relation to the methods of synthesis, secretion, transport, site of action, and method of elimination, all bearing on hormonal regulation. The largest group of hormonally active molecules includes several chemical forms of a size between 100 and 1,000 daltons. They include biogenic amines; peptides (small polymers of amino acids); thyronines, the iodine-containing hormones of the thyroid; steroids; and prostanoids, derivatives of long chain polyunsaturated fatty acids, especially arachidonic acid. Larger polypeptides and proteins will be described later (in Section F). A. Biogenic amines The simplest group of molecules are the biogenic amines (Figure 27-1), some of which are paracrines, some of which were discovered as neurotransmitters, and some are classic hormones. Paracrines serve as local chemical mediators between cells but their actions share much with hormones in how they convey information and exert control. Biogenic amines are 100 to 200 daltons in size and are distinguished from each other by the number, type, and location of functional groups on the molecule. Of this group, the catecholamines - dopamine, norepinephrine, and epinephrine - are derivatives of catechol, a dihydroxy- substituted benzene. The synthesis of epinephrine from norepinephrine is shown in Figure 27-2. This synthesis occurs predominantly in the medullary portion of the adrenal gland, a key part of the sympathetic nervous system. Interestingly, the ability to transform norepinephrine into epinephrine depends on an action of one of the steroid hormone products of the overlying adrenal cortex. 2007 version page 207 Figure 27-2. Synthesis of epinephrine. Epinephrine is synthesized by the methylation of the primary amine group on norepinephrine. The enzyme responsible, phenylethanolamine-N-methyl transferase (PNMT), is found in highest concentrations in the adrenal medulla. The control of PNMT by cortisol, a steroid hormone of the adrenal cortex, will be described later as an example of one hormone controlling the synthesis of another. Figure 27-1. Biogenic amines. All of these molecules serve as endocrines, paracrines or neurotransmitters. They represent the simplest forms of information-carrying molecules in mammalian systems. 2007 version page 208 B. Peptides Peptides are the smaller members of a wide range of polymers of amino acids that extend through to the massive proteins consisting of hundreds of amino acids. The smallest peptide hormone is thyrotropin-releasing hormone (TRH), an unusual tripeptide with just three amino acids that mediates hypothalamic control over the secretion of thyrotropin (thyroid-stimulating hormone, TSH) from the anterior pituitary. Thyrotropin also contributes to the control of...
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27 - Chapter 27. THE NATURE AND SYNTHESIS OF HORMONES The...

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