BIOCHEMISTRY - Textbook - Ostapchenko L. I. - 2012

Chapter 15. INTEGRATION OF METABOLIC PATHWAYS. HORMONES

15.2. Classification of hormones

BIOLOGICALLY ACTIVE SUBSTANCES synthesized in the Glands of Internal Secretion (Endocrine glands) are called Hormones. Hormones are integrative regulators; they link diverse regulatory mechanisms and METABOLISM across various Organs, functioning as chemical messengers that transmit signals originating in different organs and the Central Nervous system. Hormones are secreted from the endocrine gland into the bloodstream and reach a target organ located at a considerable distance.

The cellular response to a hormone's action is highly diverse and is determined by both the Chemical Structure of the hormone and the type of target Cell. The concentration of hormones in the Blood is very low (10^-6 mol/L and lower) and can change rapidly. This is ensured, firstly, by The regulation of their Synthesis and Secretion and, secondly, by the high rate of inactivation of circulating hormones.

In addition to classical endocrine action, paracrine and autocrine pathways also exist. In paracrine signaling, the hormone enters the intercellular fluid and acts on neighboring Cells without entering the bloodstream. The autocrine mechanism involves the hormone acting on the same cell in which it was synthesized.

The same hormone can act via multiple pathways. For instance, the hormone Insulin exerts a classical endocrine effect on Muscles and adipose tissue, regulating numerous metabolic processes such as Glycogen and fat synthesis. In addition, it has an autocrine effect: it is secreted by β-cells into the intercellular fluid and acts back on the same cell to inhibit its own synthesis. Insulin also exerts a paracrine effect by influencing neighboring α-cells, thereby suppressing Glucagon secretion.

Based on their chemical structure, hormones are classified into Peptide Hormones, Amino Acid Derivatives, and Steroid Hormones. Peptide hormones include those of the hypothalamo-pituitary system, parathyroid hormone, Calcitonin, insulin, and glucagon (Table 15.3). Amino acid derivatives include catecholamines and THYROID HORMONES (Table 15.4). Steroids include Sex Hormones and corticosteroids (Table 15.5).

Class="center">Table 15.3

Peptide Hormones

Note: a. r. - amino acid residue.

Table 15.4

Amino Acid-Derived Hormones

Table 15.5

Steroid Hormones