prostaglandin
[‚präs·tə′glan·dən]Prostaglandin
in mammals, a hormone that has a broad spectrum of physiological action. Prostaglandins were discovered in human semen by the Swedish scientist U. Euler in 1936. Initially, they were thought to be secretions of the prostate gland (hence the name). They were obtained in a pure form in 1956–65 by Swedish and American scientists.
About 20 natural prostaglandins are known, including thick liquids and low-melting crystalline substances. All prostaglandins are unsaturated hydroxy fatty acids that have a skeleton of 20 carbon atoms. According to their chemical structure, prostaglandins are divided into four groups—A, B, E, and F—E and F prostaglandins being the most important biologically. The subscripts in the formula below indicate the number of double bonds in the lateral chains of the molecule.
Prostaglandins are found in low concentrations (about 1 μg/g) in almost all organs, tissues, and biological fluids of higher animals. The most important physiological effect that is stimulated by prostaglandins is the ability to contract smooth muscles, especially the muscles of the uterus and fallopian tubes; at childbirth and during menstruation, the concentration of prostaglandins in uterine tissues increases substantially. For this reason, they are used in obstetrics and gynecology to facilitate normal labor and to artificially terminate pregnancy in its early stage.
Prostaglandins are also cardiotonics and bronchodilators. Arterial pressure is lowered by A and E prostaglandins and raised by F prostaglandin. A, E, and F prostaglandins intensify coronary and renal blood flow, inhibit gastric secretion, and affect the endocrine glands, including the thyroid gland; they also affect water-salt metabolism by altering the ratio Na+: K+ and blood coagulation by inhibiting the aggregation of thrombocytes.
The biosynthesis of prostaglandins occurs in the cells of different tissues. The precursors of prostaglandins are phospholipids; polyunsaturated fatty acids with a linear chain of 20 carbon atoms are released from phospholipids by the enzyme phospholipase. The oxidative cyclization of the carbon atoms, which occurs with the participation of prostaglandin synthetases (a special system of enzymes), results in the synthesis of E and F prostaglandins.
The classification of prostaglandins as local, or cellular, hormones is justified by their varied functions and the absence of a special organ for their biosynthesis. Their mechanism of action is still unclear. It has been established that prostaglandins affect the activity of the enzyme adenyl cyclase, which regulates the concentration of cyclic adenosine 3’: 5’-monophosphate (cyclic AMP) in the cell. Since prostaglandins influence the biosynthesis of cyclic AMP and since cyclic AMP participates in hormonal regulation, a possible mechanism of action of prostaglandins could consist in correcting (intensifying or weakening) the action of other hormones.
Clinical tests have shown prostaglandins to be promising in the treatment of such conditions as gastric ulcer, asthma, hypertonia, thromboses, arthritides, and inflammations of the nasopharynx. For medical and research purposes, prostaglandins are produced: (1) by enzymatic synthesis based on polyunsaturated fatty acids that are produced in the food-processing industry, (2) by complete chemical synthesis in nine to 13 stages chiefly based on cyclopentadiene, and (3) by partial synthesis in three to five stages based on prostaglandin A2 and E2 derivatives that are present in high concentrations (reaching 1.4 percent of the raw mass) in several varieties of the soft marine coral Plexaura homomalla.
REFERENCES
Markov, Kh. M. “Prostaglandiny.” Uspekhi fiziologicheskikh nauk, 1970, vol. 1, no. 4.Prostaglandins. New York, 1971. (Annals of the New York Academy of Sciences, vol. 180.)
The Prostaglandins: Progress in Research. Edited by M. M. Karim. Oxford-Lancaster, 1972.
O. S. RADBIL’ and E. P. SEREBRIAKOV