PARATHORMONE (grech, para about + armor. [glandula] of thyroidea thyroid gland + hormone[s]; synonym: parathyroid hormone, parathyrocrin, paratirin) — the polypeptide hormone produced by epithelial bodies and regulating exchange of calcium and phosphorus. The item increases the content of calcium and reduces the content of phosphorus (phosphates) in blood (see. Mineral metabolism ). An antagonist P. is calcitonin (see), causing decrease in concentration of calcium in blood. As target organs for P. serve the skeleton and kidneys, besides, P. exerts impact intestines where it strengthens absorption of calcium. In P.'s bones activates resorptive processes. The resorption of bone mineral — oxyapatite — is followed by receipt being its part of calcium and phosphate in blood. Increase in content of calcium in blood is connected with this action of P. (see. Hypercalcemia ). Along with dissolution of bone mineral there is a resorption and an organic matrix of a bone, the consisting hl. obr. from collagenic fibers and glikozaminoglikan. It leads, in particular, to increase in removal with urine of oxyproline, a typical component collagen (see). In P.'s kidneys significantly reduces a reabsorption of phosphate in distal departments of nephron and increases a reabsorption of calcium a little. Considerable increase of excretion of phosphate with urine causes decrease in content of phosphorus in blood. Despite a nek-swarm strengthening of a reabsorption of calcium in renal tubules under the influence of P., release of calcium with urine owing to quickly accruing hypercalcemia eventually increases. The important party of action of P. on kidneys is stimulation of education in them an active metabolite of vitamin D — 1,25 dioxycholecalciferols. This connection in much bigger degree increases absorption of calcium from intestines, than vitamin D. T. the lake, P.'s action on absorption of calcium from intestines can be not direct, but indirect.
On chemical structure of P. represents the one-chained polypeptide which is consisting of 84 amino-acid remains and having a pier. the weight (weight) apprx. 9500. The sequence of the amino-acid remains is completely deciphered for P. of cattle and a pig; in a molecule P. of the person the sequence of 37 amino acids of the N-trailer site of a polypeptide chain is established. Specific differences in a molecule P. are insignificant. Chemical synthesis of a fragment of a molecule P. of the person and animals containing 34 amino-acid rests is carried out and substantially possessing biol, native P.'s activity, thus it is proved that for manifestation biol, existence of all its molecule is not obligatory for P.'s activity.
P.'s biosynthesis begins with synthesis of his predecessor — a prepropa-ratgormon (the polypeptide consisting at cattle of 115 amino-acid remains). As a result of effect of specific proteolytic enzymes from the N-end of a molecule of the predecessor P. peptide from 25 amino acids is chipped off and the low-active product in the hormonal relation — pro-parathormone is formed, to-ry later proteolytic eliminating of N-trailer hexapeptide turns into the active P. cosecreted in blood.
P.'s secretion is regulated by concentration in blood of ionized Sa2 + by the principle of a feed-back: at decrease in ion concentration of Sa2 + emission in blood P. and vice versa increases.
The main place of a catabolism of P. are kidneys and a liver; the period of semi-life of active P. in blood makes apprx. 18 min. In blood P. it is quickly split on fragments (peptides and oligopeptida), a considerable part to-rykh has antigenic properties of hormone, but is deprived of it biol, activities.
At the initial stage of action of P., as well as others proteinaceous and peptide hormones (see), the specific receptor of a plasma membrane of target cells, enzyme adenylatecyclase take part (KF 4.6. 1.1), cyclic 3', 5 '-AMF and a protein kinase (KF 188.8.131.52). Activation of adenylatecyclase leads to education in cells cyclic 3', 5 '-AMF, to-ry activates enzyme the protein kinase which is carrying out reaction of phosphorylation of functionally important proteins and thus «starts» a row biochemical, the reactions causing eventually fiziol, effect of the Item. Increase in maintenance of P. in blood at a hyperparathyreosis of any etiology (see. Hyperparathyreosis ) causes disturbance of phosphorus-calcium exchange, there is a strengthened release of calcium from bones, its abnormally high removal to urine, the hypercalcemia of different degree is noted.
At a shortcoming or total absence to P. the picture of disturbances of phosphorus-calcium exchange is opposite to a picture of disturbances of this exchange at a hyperparathyreosis. Decrease in content of calcium in extracellular liquid leads to sharp strengthening of excitability of neuromuscular system and, as a result, can bring to tetanies (see).
P.'s maintenance in blood is determined biological and radio immuno-logical by methods.
Biol, methods of definition of P. are based on its ability to increase the content of calcium in blood at experimental animals (paratireoidektomirovanny rats, chickens, dogs), and also to increase at them excretion of phosphate and cyclic 3', 5 '-AMF with urine. Besides, biol, the test for P. is strengthening under its influence of a resorption of a bone tissue of in vitro, stimulation of activity of adenylatecyclase in cortical substance of kidneys, increase in concentration endogenous cyclic 3', 5 '-AMF in a bone tissue or suppression of formation of CO in it 2 from citrate.
Determination of content of P. in blood by a radio immunological method (see) does not show the true content in biologically active P.'s blood since nek-ry products of its catabolism do not lose the specific antigenic properties inherent in native hormone, but this method allows to judge the general level of activity of epithelial bodies.
Standardization biol, activities of drugs P. carry out by its comparison to activity of the international standard drug P. P.'s activity is expressed in conventional units of action — MVS (Medical Research Council) of PIECE.
The method of definition of P. based on its ability to activate a glyukozo-6-phosphate-dehydrogenase (KF 184.108.40.206) of distal departments of nephron of cortical substance of kidneys of Guinea pigs of in vitro differs in high sensitivity. The active P.'s maintenance determined by this method is in a blood plasma of healthy people ranging from 6•10 - 6 to 10•10 - 5 PIECE/ml.
See also Epithelial bodies .
Bibliography: Damask steels A. A. Paratgormon and a calcitonin, in book: Biochemistry of hormones and hormonal regulation, under the editorship of N. A. Yudayev, page 126, M., 1976; M and sh to about fi-sky M. D. Pharmaceuticals, p.1, page 555, M., 1977; P about m of An en to ov. D. Fiziologiya of calcic exchange, Kiev, 1975; The Guide to clinical endocrinology, under the editorship of V. G. Baranov, page 7, D., 1977; Stukkey A. JI. Parathyroids, in book: Fiziol, endocrine system, under the editorship of V. G. Baranov, page 191, D., 1979; With h a m b e of s D. J. a. o. A sensitive bioassay of parathyroid hormone in plasma, Clin. Endocr., v. 9, p. 375, 1978; Labhart A. Klinik der inneren Sekretion, B. u. a., 1978; Parsons J. A. a. P o t t s J. T. Physiology and chemistry of parathyroid hormone, Clin. Endocr. Metab., v. 1, p. 33, 1972; Schneider A. B. a. S h er w o o d L. M. Calcium homeostasis and the pathogenesis and management of hypercalcemic disorders, Metabolism, v. 23, p. 975, 1974, bibliogr.
A. A. Bulatov.