Diversity of the Endocrine system
المؤلف:
Peter J. Kennelly, Kathleen M. Botham, Owen P. McGuinness, Victor W. Rodwell, P. Anthony Weil
المصدر:
Harpers Illustrated Biochemistry
الجزء والصفحة:
32nd edition.p491-492
2025-11-09
58
Hormones Are Synthesized in a Variety of Cellular Arrangements Hormones are synthesized in discrete organs designed solely for this specific purpose, such as the thyroid (triiodothyronine), adrenal (glucocorticoids and mineralocorticoids), and the pituitary (TSH, FSH, LH, GH, PRL, ACTH). Some organs are designed to perform two distinct but closely related functions. For example, the ovaries produce mature oocytes and the reproductive hormones estradiol and progesterone. The testes produce mature spermatozoa and testosterone. Hormones are also produced in specialized cells within other organs such as the small intestine (glucagon-like peptide), thyroid (calcitonin), and kidney (angiotensin II). Finally, the synthesis of some hormones requires the parenchymal cells of more than one organ—for example, the skin, liver, and kidney are required for the production of 1,25(OH)2-D3 (calcitriol). Examples of this diversity in the approach to hormone synthesis, each of which has evolved to fulfill a specific purpose, are discussed later.
Hormones Are Chemically Diverse
Hormones are synthesized from a wide variety of chemical building blocks. A large series is derived from cholesterol. These include the glucocorticoids, mineralocorticoids, androgens, estrogens, progestins, and 1,25(OH)2-D3 (Figure 1). In some cases, a steroid hormone is the precursor molecule for another hormone. For example, progesterone is a hormone in its own right but is also a precursor in the formation of glucocorticoids, mineralocorticoids, testosterone, and estrogens. Testosterone is an obligatory intermediate in the biosynthesis of estradiol and in the formation of dihydrotestosterone (DHT). In these examples, described in detail in the following discussion, the final product is determined by the cell type and the associated set of enzymes in which the precursor exists.
Fig1. Chemical diversity of hormones. (A)cholesterol derivatives; (B) tyrosine derivatives; (C) peptides of various sizes; note: pyroglutamic acid (pyro) is a cyclized variant of glutamic acid in which side chain carboxyl and free amino groups cyclize to form a lactam. (D) glycoproteins (TSH, FSH, and LH) with common α subunits and unique β subunits.
The amino acid tyrosine is the starting point in the synthesis of both the catecholamines and thyroid hormones tetraiodothyronine (thyroxine; T4 ) and triiodothyronine (T3 ) (see Figure 1). T3 and T4 are unique in that they require the addition of iodine (as I−) for bioactivity. Since dietary iodine is very scarce in many parts of the world, an intricate mechanism for accumulating and retaining I− has evolved.
Many hormones are polypeptides or glycoproteins. These range in size from the small thyrotropin-releasing hormone (TRH), a tripeptide, to single-chain polypeptides like adrenocorticotropic hormone (ACTH; 39 amino acids), parathyroid hormone (PTH; 84 amino acids), and growth hormone (GH; 191 amino acids) (see Figure 1). Insulin is an A-B chain heterodimer of 21 and 30 amino acids, respectively. Follicle stimulating hormone (FSH), luteinizing hormone (LH), thyroid-stimulating hormone (TSH), and chorionic gonadotropin (CG) are glycoprotein hormones of αβ heterodimeric structure. The α chain is identical in all of these hormones, and distinct β chains impart hormone uniqueness. These hormones have a molecular mass in the range of 25 to 30 kDa depending on the degree of glycosylation and the length of the β chain.
Hormones Are Synthesized & Modified for Full Activity in a Variety of Ways
Some hormones are synthesized in final form and secreted immediately. Included in this class are hormones derived from cholesterol. Some, such as the catecholamines, are synthesized in final form and stored in the producing cells, while others, like insulin, are synthesized as precursor molecules in the producing cell, and then are processed and secreted upon a physiologic cue (plasma glucose concentrations). Finally, still others are converted to active forms from precursor molecules in the peripheral tissues (T3 and DHT). All of these examples are discussed in more detail in the following discussion.
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