From Big Medical Encyclopedia

PIRIMIDINOVY BASES — derivatives of a pyrimidine, the six-membered heterocyclic compound containing two nitrogen atoms in the cycle. Among P. there is the greatest lake biol, matter tsitozin (2-oxy-6-aminopyrimidine), uracil (2, 6-dioxypyrimidine) and thymine (5-methyl-2,4-dioxypyrimidine), to-rye are a component of a molecule nucleic acids (see), and also free nucleotides. Tsitozin is a part both DNA, and RNA, uracil meets only in molecules RNA, and thymine — only in molecules DNA. Except the main P. of the lake, so-called minor bases are known, to-rye also are a part nucleinic to - t, but meet much less often. 5 methylcytosine (it is found as a part of DNA of the higher plants and some bacteria) and 5 oxymethylcytosine concern to them (it is found in some strains of phages of colibacillus). At live organisms there is one more pirimidinovy basis which is not a part nucleinic to - t — orotovy acid (see). This connection is an intermediate product of biosynthesis of pirimidinovy nucleotides, its potassium salt — potassium orotat — is used as medicine. Uracil is a part uridinephosphates (see) — the connections playing an important role in exchange of carbohydrates, and tsitozin is a component of the cytidinephosphates participating in lipid metabolism.

Pirimidinovy bases: tsitozin, uracil and thymine

Artificially received P.'s analogs the lake, such as 2 thiouracil, 4 azauracil are important, 5-ftoruratsit also 5 bromouracil. They are of interest as chemotherapeutic agents and mutagens; 4 azauracil also 5-ftoruratsit have carcinostatic effect; 5-ftoruratsit (see. Ftoruratsil ) apply at treatment of inoperable forms, a recurrence and metastasises of a carcinoma of the stomach and a large intestine, at treatment of tumors of a pancreas and acute pancreatitis.

Items of the lake represent crystal substances of white color. Tsitozin crystallizes in the form of monohydrate which during the heating to 100 ° loses water. At a temperature apprx. 320 ° P. lakes melt with decomposition. In a cold water of P. of the lake badly rastvorima, but are well dissolved in alkalis. It is the most convenient to item of the lake to receive chemical synthesis from urea and connections of type acetoacetic and low-new ethers.

All P.' molecules of the lake have almost flat configuration. Existence of system of conjugated double bonds and deputies (OH-and NH 2 - groups) at the second and fourth carbon atoms causes their ability to tautomeric transformations: a lactam-laktimnomu for oksiproiz-water and amine iminenoma for amin derivatives. For uracil and thymine existence of four tautomeric forms is possible, but the form of a lactam is preferential.

Amin-iminnaya the tautomerism is presented amine and imino-forms of a tsitozin though this connection exists generally in the form of amine. P.'s ability of the lake to tautomeric reorganizations (see. Tautomerism ) plays an important role in some biol, processes, in particular at mutagenesis (see).

In total P. of the lake (except for orotovy to - you) are mild bases. Orotovy to - that has properties as to - you, and the bases.

Items of the lake determine by characteristic maxima of absorption in an ultra-violet part of a range (tsitozin — at 275 nanometers, thymine — at 264 nanometers, uracil — at 260 nanometers). For identification and quantitative definition of P. of the lake sometimes use ion-exchange chromatography (see. Chromatography ).

Bibliography: Davidson J. N. Biochemistry of nucleic acids, the lane with English, M., 1976; Mick of e of l s it is A. M. Himiya of nucleosides and nucleotides, the lane with English, M., 1966; Organic chemistry of nucleic acids, under the editorship of N. K. Kochetkov and E. I. Budovsky, M., 1970; Handbook of biochemistry and molecular biology, nucleic acids, ed. by G. D. Fas-man, y. 1 — 2, Cleveland, 1975 — 1976.

O. D. Lopina.