Principles of Biochemistry Volume 3 - A. Lehninger 1985

Selected Aspects of Human Biochemistry
Hormones
Prostaglandins and thromboxanes influence the action of a number of hormones

Prostaglandins (Fig. 25–28) are a family of lipid-soluble organic acids containing a five-carbon ring. They are synthesized from Essential Fatty acids (Section 21.6) via arachidonic acid (an unsaturated fatty acid). These compounds act as regulators of hormone action and were named prostaglandins because they were first isolated from Prostate Gland secretions. Although initially thought to regulate only male reproductive Tissues, prostaglandins were later found to be produced and to function in virtually all Organs. They exert diverse physiological effects, and some are used therapeutically. A number of prostaglandins stimulate and enhance adenylate cyclase activity. Unstable transformation products of prostaglandins include thromboxanes (Fig. 25–28), which are believed to be involved in regulating The activity of Blood Platelets and other Cells. Notably, aspirin—an effective analgesic for Arthritis—serves as a potent inhibitor of prostaglandin synthase, the enzyme responsible for The Biosynthesis of prostaglandins from arachidonic acid. It appears that certain prostaglandins also play a role in regulating pain perception.

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Fig. 25–28. A typical prostaglandin (E₂) and thromboxane (B₂) derived from arachidonic acid (Section 21.6).

It is also hypothesized that Ca²⁺ ions and the cytoplasmic Ca²⁺-binding protein calmodulin function as modulators and even intracellular second messengers for the Introduction/43.html">Action of Certain Hormones. Free Ca²⁺ ions are present in the Cytosol at very low concentrations—less than 10^-6 M. Upon stimulation by specific hormones, the concentration of Ca²⁺ ions increases. Ca²⁺ ions influence many types of intracellular activity, likely by binding to calmodulin, which in turn stimulates phosphodiesterase and various protein Kinases involved in Carbohydrate METABOLISM, Muscle contraction, and intracellular membrane transport mechanisms.