Biological Chemistry - Berezov T. T., Korovkin B. F. 1998

Hormones
Nomenclature and Classification of Hormones

The Chemical Nature of almost all known Hormones has been elucidated in detail (including the Introduction/19.html">Primary Structure of protein and Peptide Hormones); however, General Principles for their nomenclature have not yet been established. The chemical names of many hormones accurately reflect their chemical structure and are very cumbersome. Therefore, trivial names are more commonly used. The adopted nomenclature indicates either the source of the hormone (e.g., Insulin, from the Latin insula, meaning islet) or reflects its function (e.g., prolactin, vasopressin). For some Pituitary Hormones (such as luteinizing and follicle-stimulating hormones) as well as all Hypothalamic hormones, new working names have been developed.

A similar situation exists regarding the Classification of hormones. Hormones are classified according to their site of natural synthesis, distinguishing between Hormones of the Hypothalamus, Pituitary Gland, Thyroid Gland, Adrenal Glands, Pancreas, Gonads, Thymus, and others. However, this anatomical classification is imperfect because some hormones are either synthesized outside the Endocrine glands from which they are secreted into the Blood (e.g., the Posterior Pituitary Hormones Vasopressin and Oxytocin are synthesized in the hypothalamus and transported to the posterior pituitary) or are synthesized in multiple glands (e.g., partial synthesis of Sex Hormones occurs in the adrenal cortex, and Prostaglandins are produced not only in the Prostate Gland but also in other Organs), and so on. In light of these circumstances, attempts have been made to create a modern classification of hormones based on their chemical nature. According to this classification, there are three groups of true hormones: 1) Peptide and Protein hormones, 2) amino acid-derived hormones, and 3) Steroid Hormones. The fourth group consists of Eicosanoids, which are hormone-like substances with local action.

Peptide and protein hormones contain from 3 to 250 or more amino acid residues. These include hormones of the Hypothalamus and Pituitary gland (thyrotropin-releasing hormone, Growth Hormone-releasing hormone, Somatostatin, growth hormone, corticotropin, thyrotropin, etc. — see below), as well as Pancreatic Hormones (insulin, Glucagon). Amino acid-derived hormones are primarily represented by derivatives of The amino acid Tyrosine. These include low-molecular-weight compounds such as epinephrine and norepinephrine, synthesized in The adrenal medulla, and THYROID HORMONES (thyroxine and its derivatives). Hormones of the 1st and 2nd groups are readily soluble in Water.

Steroid hormones are represented by fat-soluble hormones of the adrenal cortex (corticosteroids), sex hormones (estrogens and androgens), and the hormonal form of vitamin D.

Eicosanoids, which are derivatives of the polyunsaturated fatty acid (arachidonic acid), are represented by three subclasses of compounds: prostaglandins, thromboxanes, and Leukotrienes. These water-insoluble and unstable compounds exert their effects on Cells located close to their site of synthesis.

Next, we will examine the chemical structure, Functions, and pathways of Biosynthesis AND DEGRADATION of the Major Classes of hormones, subdivided into distinct groups according to a classification based on their chemical nature.