Textbook - BIOLOGICAL CHEMISTRY - Gubsky Yu.I. - 2000

Section V. HORMONES IN THE SYSTEM OF INTERCELLULAR INTEGRATION OF BODY FUNCTIONS

CHAPTER 23. HORMONAL REGULATION OF METABOLISM AND CELLULAR BIOLOGICAL FUNCTIONS. I. BIOCHEMICAL SYSTEMS OF INTRACELLULAR TRANSDUCTION OF HORMONAL SIGNALS

A defining feature of Multicellular Organisms is the differentiation of Cells into tissue structures and Organs that perform specialized physiological Functions necessary for the survival of the Organism as a whole. The coordination of cellular and tissue responses to changes in internal and external environmental conditions is achieved through Intercellular Communication mechanisms. During the evolution of higher animals, two PHYSIOLOGICAL AND BIOCHEMICAL systems emerged to establish communication and coordination between distinct Cell groups: the Nervous system, which operates via electrochemical signal conduction, and the Endocrine System, whose action is realized through the secretion and transport of specific long-range humoral factors known as Hormones.

23.1. HORMONES AND BIOREGULATORS: DEFINITION, CLASSIFICATION

Hormones are physiologically active compounds (PACs), BIOREGULATORS produced by Endocrine glands (ductless glands) or other specialized cells that act as regulators of metabolic processes and physiological functions within the organism. The BIOLOGICAL EFFECTS OF hormones are exerted at extremely low concentrations — 10^-11-10^-6 mol/L.

The term hormone (from the Greek hormao meaning to excite, stimulate) was first proposed in 1905 by E. Starling to designate secretin, a humoral factor produced by the duodenum that stimulates the exocrine secretion of the Pancreas. Subsequently, the term was extended to encompass physiologically active compounds produced in specialized endocrine glands.

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Fig. 23.1. Starling, Ernest H. (1866–1927), English physiologist and biochemist, Professor of Physiology at the University of London. One of the founders of endocrinology.

Classes of Hormones and Other Bioregulators

Hormones synthesized in endocrine glands ("true" hormones) are secreted into the bloodstream and, after being carried by specialized transport Proteins, typically exert their biological effects at a distance, meaning they act on a remote target organ or organs.

"True" hormones include:

- Hypothalamic and pituitary hormones;

- THYROID HORMONES;

- parathyroid hormones;

- Hormones of the endocrine pancreatic cells;

- Adrenocortical Hormones;

- male and Female Sex Hormones;

- pineal hormones.

Physiologically active compounds that are humoral regulatory factors of non-endocrine origin are functionally similar to hormones. These bioregulators are not produced in endocrine glands, but rather in specialized cells located within other tissue elements, such as the lymphoid system, leukocytes, Connective Tissue, Stomach, intestines, nervous system, Kidneys, myocardium, etc., and are referred to as hormone-like compounds (hormonoids) or tissue hormones (histohormones). Unlike "true" hormones, which are characterized by distant action, histohormones can exert their regulatory influence on target cells responsive to them right at the site of their production ("isocrine", local action).

A common feature of bioregulators of various origins is their informational function, aimed at controlling, regulating, and modulating the metabolic and physiological functions of sensitive biostructures (informones, informopheres).

The most thoroughly studied classes of bioregulators to date include:

- hormones ("true" hormones);

- Neurotransmitters and opioid Peptides;

- PHYSIOLOGICALLY ACTIVE Eicosanoids;

- hormones and mediators of The Immune System;

- peptide growth factors (cytomedins, interleumins);

- Gastrointestinal Hormones;

- kinin system peptides;

- cardiac and Brain natriuretic peptides.

Chemical Structure of Hormones

According to their chemical structure, all hormones are classified into the following groups:

(1) protein-Peptide Hormones (simple proteins; Glycoproteins; peptides): hypothalamic-Pituitary Hormones; parathyroid hormones; hormones of the endocrine pancreas (islet hormones); gastrointestinal hormones; Neuropeptides; numerous peptide-based tissue bioregulators;

(2) amino acid-derived hormones: thyroid hormones; adrenal medullary hormonesoids (catecholamines); other neurotransmitters with histohormone properties (serotonin, dopamine, histamine); the pineal hormone — melatonin;

(3) Steroid Hormones: glucocorticoids and mineralocorticoids of the adrenal cortex; male and female sex hormones; vitamin D derivatives;

(4) bioregulators — arachidonic acid derivatives (eicosanoids): Prostaglandins, prostacyclins, thromboxanes, Leukotrienes.

Hormone synthesis takes place in specialized cells («true» hormones are produced in endocrine glands). Many hormones are initially synthesized as biological precursors — preprohormones and prohormones, which are subsequently transformed into biologically active hormones. In the case of protein-peptide hormones, this activation occurs via post-translational modification (Processing) through The Mechanism of Limited proteolysis of primary ribosomal Translation products.

Targets of Hormonal Action

The Physiological effects of hormones and other bioregulators are realized in target cells, Tissues, and organs.

Targets (cells, tissues, organs), or hormone-competent structures, are hormone-sensitive biostructures that selectively respond to the interaction with a hormone through a specific physiological and biochemical reaction. Depending on the extent of the hormone's influence on their biological properties, hormone-dependent and hormone-responsive cells are distinguished (Fig. 23.2).

Fig. 23.2. Types of cellular responses to hormonal action (V.B. Rosen, 1984): 1 — hormone-dependent target cells; 2 — hormone-responsive target cells; 3 — non-target cells.

Examples of hormone-dependent structures include the tissues of peripheral endocrine glands (Thyroid Gland, adrenal cortex) in response to pituitary trophic hormones (TSH and ACTH, respectively), or Cells of the male and female reproductive systems regarding the presence and effects of corresponding sex hormones. Hormone-responsive cells include those in organs that react to Insulin, which regulates glucose, lipid, and Amino acid METABOLISM within them (Muscle cells, adipose tissue, lymphoid system).

The ability of target Cells and Tissues to respond specifically to a certain hormone is determined by the presence of receptor molecules that bind with the hormone or chemically closely related compounds. On the other hand, the interaction with the receptor occurs via a specific domain of the hormone molecule — the «active center», which, in its Molecular structure and conformation, is complementary to the corresponding site on the receptor.