Human Biochemistry, Volume 2 - Murray R. 1993

Biochemistry of Intracellular and Intercellular Communications
Mechanism of Hormone Action
Classification of Hormones

Hormones can be classified by their chemical composition, solubility, receptor localization, and The Nature of the signal mediating the intracellular hormonal effect. A Classification based on the latter two properties is illustrated in Table 44.1, and the General characteristics of each group are summarized in Table 44.2.

Hormones of the first group are lipophilic and, with the exception of T₃ and T₄, are Cholesterol derivatives. Following secretion, they bind to carrier Proteins, which solves the solubility issue and simultaneously prolongs their half-life in Blood Plasma. Free hormone easily crosses Cell/30.html">The Plasma Membrane of any cell and, upon entering the target cell, binds to receptors located in the Cytoplasm or Nucleus. The Ligand–receptor complex is considered the intracellular mediator of hormone action for this group.

The second major group consists of Water-soluble hormones that attach to the plasma membrane of target Cells. The Effect of surface-bound hormones on intracellular metabolic processes is mediated by intermediate compounds called second messengers (the first messenger being the hormone itself); the latter are formed As a result of the ligand–receptor interaction. The second messenger concept emerged from the work of Sutherland, who demonstrated that epinephrine binds to pigeon erythrocyte Plasma Membranes and increases the intracellular concentration of сAMP. Subsequent series of studies revealed that сAMP mediates the metabolic effects of many hormones. Hormones for which this MECHANISM OF ACTION has been proven constitute group II.A. Some hormones utilize calcium or complex phosphoinositide metabolites (or both) as an intracellular signal, although it was initially assumed that they act via сAMP. These hormones are included in group II.B. For the large and very interesting group II.C, the intracellular mediator has not been definitively established. Various compounds have been considered as potential candidates for this role in Insulin action: cAMP, cGMP, Н₂О₂, calcium, several short Peptides, phospholipid, insulin itself, and the Insulin Receptor, but as yet none has been found that meets all the necessary criteria. It may turn out that the Action of Hormones in this group is mediated by diverse mechanisms and multiple mediators. Individual hormones are difficult to assign to a single category, and as new information accumulates, their placement in the classification may change.

Class="center">Table 44.1. Classification of hormones by mechanism of action

Table 44.2. General characteristics of hormone classes