NUCLEOSIDASES — the outdated name of the enzymes catalyzing splitting of N-glycosidic linkages in nucleosides with formation of free nitrogen bases and carbohydrate; it was usable during this period when considered that zymolysis of nucleosides results from direct hydrolysis of their molecules.
Kalkar's researches (H. M of Kalckar), the splitting of nucleosides which found the fosforolitichesky mechanism in 1945 and allocation by Carter (S. E. Carter) in 1951 the enzymes capable to hydrolyze glycosidic linkages of nucleosides, showed existence not of one, but two systems of enzymatic disintegration of nucleosides — hydrolytic and fos-foro lytic.
Hydrolytic nucleosidases (nukleozidgidrolaza) catalyzing splitting of a N-glycosidic linkage with the participation of a water molecule are found in many vegetable and animal fabrics, and also in microorganisms. These enzymes belong to the class of hydrolases, group of N-glikozilnykh hydrolases of connections (KF 3.2.2) and differ in strict specificity to the chemical nature of a carbohydrate component and stereochemistry of substrate: they attack only R-ri-bofuranozidnuyu communication. The reaction catalyzed by them comes to the end with full hydrolysis of a nucleoside to a free D-ribose and nitrogen base. Nukleozidgidrolaza specifically split nucleosides depending on character of nitrogen base. So, N-ribozilpurin-ribogidrolaza (KF 18.104.22.168), meeting in plants, muscles of fishes, yeast and bacteria, it is specific to purine ribonucleosides. Pirimidinovye a nucleoside-ribogidrolazy have narrower substrate specificity and catalyze hydrolysis only of a certain nucleoside, napr, uridine-ribogidrolaza (KF 22.214.171.124).
Fosforolitichesky nucleosidases (nukleozidfosforilaza) — enzymes of a class of transferases, groups pento-ziltransferaz (KF 2.4.2), catalytic activity to-rykh consists in reversible transfer of the pento-zilny rest of a nucleoside on inorganic phosphate. Nukleozidfosforilaza are eurysynusic in tissues of animals and the person, yeast, bacteria. Unlike nukleozidgidrolaz they catalyze a reversible test, participating as in the degradation of nucleosides which is followed by formation of fos-forilirovanny sugar and free nitrogen base and in their synthesis from these connections. Purine-specific nukleozidfosforilaza attack ribo-and dezoksiribonukleozida whereas pirimidilnukleo-zidfosforilaza show specificity to a carbohydrate and nitrogenous component of a nucleoside. Between the enzymes emitted from different sources considerable discrepancies concerning their substrate specificity and parameters of kinetics of enzymatic reaction are possible. In various tissues of the person several isoenzymes of a purinnukleozidfosforilaza (KF 126.96.36.199) are revealed.
The most widespread methods of measurement of activity of N. are colorimetric definition of reaction products — riboses or desoxyriboses, and also registration of the changes in the UF-spectral range caused by education or disintegration of glycosidic linkages of nucleosides (see. Colorimetry , Spektrofotometriya ).
Nukleozidfosforilaza play an important role in metabolism of a cell. By Fosforilirovannye of sugar — products of the reaction catalyzed by them are involved in various exchange processes, forming key metabolites in biosynthesis of nucleotides and other important connections.
Purinnukleozidfosforilaza show activity concerning 6 thioguanines and 6 Mercaptopurinums and their nucleosides — the agents applied in clinic for immunosuppression and therapy of cancer. Perhaps, they catalyze disintegration of the nucleosides containing antineoplastic analogs of purines or provide inclusion of similar agents in nucleotides and nucleinic to - you cells.
Bibliography: Parks R. E. a. Agarwal R. P. Purine nucleoside phosphori-lase, in book: The enzymes, ed. by P. D. Boyer, y. 7, p. 483, N. Y. — L., 1972; R a z-z e 1 1 W. E. Pyrimidine nucleoside and deoxynucleoside phosphorilases, in book: Meth. enzymol., ed. by L. Grossman a. K. Moldave, y. 12, pt A, p. 118, N. Y. — L., 1968; Tarr H. L. A. Purine nucleoside phosphorilases, ibid., p. FROM.
P. L. Ivanov.