From Big Medical Encyclopedia

NICOTINAMIDE ADENINE DINUCLEOTIDE [OVER, NAD + , NADH; synonym: diphosphopyridine nucleotide, DPN, ustar. — coenzyme of I, KoI, cozymase, kodegidr (ogen) aza of I] — the coenzyme which is present at all living cells and entering along with nicotinamide adenine dinucleotide-phosphate [NADF, NADP+, NADPH; synonym: trifosfopiridinnukleotid, TPN, ustar. — a coenzyme of II (KoII), phosphocozymase, kodegidr-(ogen) an aza of II], in composition of enzymes of group of the dehydrogenases catalyzing the most important redoxreactions of energy and plastic metabolism. The most important biol, function nicotine - amide coenzymes, i.e. Over and NADF, consists in their ability to transfer electrons and protons from the oxidized substrates to oxygen in the course of cellular respiration (see. biological oxidation ). Definition of a recovery rate or oxidation of niko-tinamidny coenzymes is of great importance in biochemistry and medicine since allows to define activity of many of NAD(F) - dependent dehydrogenases, and also other major enzymes in conjugated systems with use of dehydrogenases that at a number of diseases has essential diagnostic value.

OVER it was opened in 1905 in yeast juice by English biochemists A. Harden and W. Young, and NADF is found in 1931 in blood of a horse. For the first time NAD and NADF were allocated and identified by O. Varburg and X. Euler-Heljpin; the structure of NADF is established in 1934, and OVER — in 1936.

Molecule OVER represents the peculiar dinucleotide constructed of an adenineribonucleotide and a nikotinamidribonukleotid — catalystically active group. Both nucleotides are connected by the fosfo-angidridny bridge. NADF differs from OVER existence of the third rest phosphoric to - you in situation 2' riboses of an adenylic nucleotide.

Have catalytic activity only this way naz. (3 isomers of both dinucleotides, in to-rykh purine and nikotinamidny kernels spatially are pulled together.

The oxidized forms niacinamide-nykh of coenzymes designated according to NAD+ and NADF + are quite steady in acid and are unstable in alkaline solutions (especially during the heating), and got into condition (OVER * H and NADF-N) are rather steady in alkaline and quickly collapse in acid solutions. Various stability of coenzymes in acid and alkaline environments is used at separate definition their oxidized and got into condition.

The main source for preparative receiving OVER are pressed yeasts, and NADF — a liver of sheep or pigs. Allocation of NAD+ and NADF + consists of extraction by warm water of suspension of yeast or homogenate of animal fabrics, sedimentation by silver salts or mercury and a chromatography on ion-exchange resins. Got into condition coenzymes receive by chemical or enzymatic recovery of their oxidized forms.

Contents oxidized and got into condition by NAD and NADF and a ratio between them in different tissues of mammals very variously. In the majority of NAD fabrics is present at much large numbers, than NADF. In normally functioning cell the oxidized NAD form always prevails over recovered whereas NADF is present at the basic in got into condition. Nucleotide coenzymes are found in all cellular fractions; in a liver their contents is especially high in soluble fraction (cytoashes). The maintenance of nikotinamidny coenzymes, ratio between their oxidized and got into condition, and also a ratio between the maintenance of NAD and NADF are the indicators of activity of processes of metabolism in fabrics to a certain extent characterizing them funkts, a state. In an embryonal muscle contents OVER much lower, and NADF — above, than in muscles of a mature animal. At an atrophy of muscles after denervatsip decrease in contents OVER and increase in maintenance of NADF is observed. At oxygen insufficiency decrease in maintenance of the oxidized forms of NAD and NADF is noted. In the trained muscles the size of the relation of NAD+ / OVER» is higher than H after 10-minute loading, than in unexercised. The factors causing neurogenic dystrophy lead to decrease in quantity of NAD+ and increase in quantity of NADF-N in a myocardium and a brain of experimental animals.

Okislitelno - the recovery reactions catalyzed by dehydrogenases and proceeding with participation of NAD or NADF can be represented in the form of the equation: AH2 - f-+ OVER (F)+ ^А + NAD(F) • H + - j-H+ (where AH2 — recovered, and And — the oxidized form of substrate) also come down to reversible transfer of two recovery equivalents from substrate to the oxidized form of a nikotinamidny coenzyme. At the same time one equivalent is present at the recovered coenzyme in the form of hydrogen atom, and another — in the form of an electron (the cation of the second hydrogen atom passes on Wednesday in the form of free H+ - an ion). As a result of it at recovery of NAD(F) reaction of solution is displaced in the acid party, and at oxidation — in alkaline. At recovery of a coenzyme hydrogen and an electron (in a look hydride ion;: N“) are had to the fourth provision of a pyridinic ring; enzymatic accession, and also hydrogen abstraction happens stereospetsifich-but — over or under the plane of this ring, depending on specificity of the corresponding dehydrogenase.

OVER - dependent dehydrogenases participate preferential in catabolic processes (e.g., in the course of cellular respiration), and NADF-zavisimye in anabolic processes (e.g., in recovery biosynthetic reactions). It is revealed apprx. 349 NAD(F) - the zavisymy oxidoreductases usually specific or in the relation OVER [a mitochondrial izotsitratdegidroge-naza, a glitseraldegidfosfatdegidro-genaza, a digpdrolipoildegidrogenaza, lactate dehydrogenase (see), alcohol dehydrogenase (see), malate dehydrogenase (see), etc.], or concerning NADF (a cytoplasmatic isocitratedehydrogenase, glyukozo-@-phosphate — a dehydrogenase, etc.), and only the few enzymes (e.g., nek-ry glutamatdegidrogenaza) can use both NAD+, and NADF+. Communication of nikotinamidny coenzymes with a proteinaceous part of enzyme often (but not always) is easily dissociating; in such cases of NAD and NADF carry out a role of the mobile intermediate hydrogen carriers combining and connecting among themselves various oxidation-reduction acts.

Accepting hydrogen directly from substrates (carbohydrates, amino acids, fat to - t etc.) and transferring them to carriers with higher redox potential (in many cases to yellow enzymes), OVER is the first link in a chain of hydrogen carriers in process biol, oxidations. Standard redox potential of E\of couple of NAD (F)+/NAD (F)-N is equal — 0,32 century. Oxidation of the molecule NAD-N in a respiratory chain allows to save up in the course of respiratory phosphorylations (see) three molecules ATP; at the same time hydrogen is oxidized molecular oxygen to water: OVER • H - + +

+ N + + 1/2 02->OVER + + H20.

OVER carries out also hydrogen transfer between various oxidized and recovered substrates (e.g., in the course of glycolysis, at transformation of testosterone into androsterone etc.).

Oxidation of NADF-N happens hl. obr. in processes of recovery biosintez: at synthesis fat to - t, carbohydrates (e.g., in dark reactions of photosynthesis), at recovery amination and - ke-toglutarovoy to - you etc. Direct oxidation of NADF * is possible H in a respiratory chain at action specific NADF-tsitokhromredukta-zy. Indirect oxidation of NADF-II in a respiratory chain can be carried out after hydrogen transfer on NAD+ by the transdehydrogenases catalyzing a reversible test OVER. H + NADF + ^НАД + + - f-NADF-N.

The recovered molecules NADF are formed by hl. obr. at oxidation in cytoplasm glyukozo - 6 - phospha - that in a pentozny cycle, at oxidation of malate to pyruvate and C02 a malate dehydrogenase, and also at photo recovery in light reactions of photosynthesis.

Biosynthesis of NAD and NADF is carried out by the multifermental system spatially separated in cellular structures. In the majority of NAD fabrics it is synthesized both from niacinamide, and from niacin (see). The liver and in kidneys contains the fermental systems capable to carry out synthesis of niacinamide from tryptophane and further stages of synthesis OVER. Formation of NADF comes by phosphorylation OVER due to direct transfer of the phosphatic rest from the ATP catalyzed OVER - a kinase (KF Disturbance of biosynthesis OVER happens at insufficiency of RR vitamin (Niacinum), being the predecessor of this coenzyme. Splitting of nikotinamidny coenzymes is carried out by several fermental systems including NAD-glikogidrolazu (KF 3.2. 2.5), pyrophosphatase (KF, NAD-pirofosforilazu (KF, adenosinedeaminase (KF, alkaline phosphatase (KF, etc.

For quantitative definition of nikotinamidny coenzymes use several methods. The spectrophotometric method is most widespread (see. Spektrofotometriya ), based that are characteristic an intensive strip of absorption at 260 nanometers of the oxidized forms of these coenzymes and lack of absorption in more long-wave spectral range, recovery of NAD and NADF is followed by emergence of a strip of absorption with a maximum at 340 nanometers. Definition of NAD and NADF by a flyuo-rimetrichesky method (see. Flyuorimetriya ) is the most sensitive (allows to define these coenzymes in concentration 10 ~ 8 M); it is based that the recovered nikotinamidny coenzymes, unlike their oxidized forms, fluoresce, giving at excitement them light with the wavelength of 340 nanometers at most to fluorescence at 480 nanometers. A method of a rn-metriya (see. Potentiometric titration ) comes down to measurement of the size pH of the environment, edges changes at recovery or oxidation of coenzymes.

See also Coenzymes .

Bibliography: Dickson M. Pi Webb E. I. Enzymes, the lane with English, M., 1966; Severin S. E., Telepneva V. I. and Tseitlin L. A. Ways and localization of processes of exchange of nicotinamide adenine dinucleotide in a cell, in book: Chemical factors of regulation of activity and biosynthesis of enzymes, under the editorship of V. N. Orekhovich, page 118, M., 1969; Telepneva V. I. Ways of synthesis and disintegration of nikotinamidny coenzymes (NAD+ and NADF +), in book: Private enzimol., under the editorship of S.E. Severin, page 79, M., 1978; Tseitlin JI. A. Nikotinamidnye coenzymes, Usp. biol, chemical, century 8, page 249, 1967; With h and at k i n S. Nicotinamide coenzymes, Ann. Rev. Biochem., v. 36, p. 149, 1967; K a plan N.O. The pyridine coenzymes, in book: The enzymes, ed. by P. D. Boyer a. o., v. 3, p. 105, N. Y., 1960; Metabolic pathways, ed. by D. M. Greenberg, v. 2, p. 627, N. Y. — L., 1961; Methods in enzymology, ed. by S. P. Colowick a. N. O. Kaplan, v. 4, p. 840, N.Y., 1957.

H. V. Gulyaeva.