Pyridine nucleotide cycle

Figure 6. Proposed pyridine nucleotide cycle. Abbreviations NaMN, nicotinic acid mononucleotide NaAD, nicotinic acid adenine dinucleotide and NAD, oxidized form of nicotinamide adenine dinucleotide.

Ricinine.—Quinolinic acid (29) is known to be a precursor of ricinine (32). The results of other studies have indicated that the pyridine nucleotide cycle (Scheme 6)... 

Azaserine, a glutamine antagonist," is known to inhibit the NAD synthetase reaction in which nicotinic acid adenine dinucleotide is converted into NAD with glutamine or ammonia as the nitrogen donor. ° When azaserine or azaleucine was fed to Ricinus communis plants followed by [6- C]quinolinic acid, a marked decrease in incorporation of radioactivity into ricinine was observed with azaleucine, less so with azaserine." Both azaserine and azaleucine were found also to inhibit the incorporation of [6- " C]quinolinic acid into pyridine nucleotide cycle intermediates [in the case of azaserine the conversion of nicotinic acid dinucleotide into nicotinamide adenine dinucleotide (NAD ) was apparently inhibited]. 

Ricinine and ethionine were found to inhibit ricinine biosynthesis with shunting of radioactivity from [6- C]quinolinic acid through the pyridine nucleotide cycle into N-methylnicotinic acid (30) and N-methylnicotinamide (31) (c/. Scheme 6), and quinolinic acid consumption was reduced. 

These results strongly indicate a dependency between ricinine biosynthesis and the pyridine nucleotide cycle. The similarity in incorporation of members of this cycle into ricinine" is explained by allowing each member to be diverted directly into a pathway leading to ricinine this explanation covers the observation that excess exogenous NAD increases the incorporation of quinolinic acid into ricinine" since this precursor can simply be shunted along one of these diversions if the formation of NAD is blocked by its presence in large excess. 

Ricinine.—Ricinine (49), the alkaloid of castor bean plants, is derived from nicotinic acid (28) and quinolinic acid (48), and its formation is intimately associated with the pyridine nucleotide cycle cf. ref. 6. Quinolinic acid is built from a C3 fragment that is formed from glycerol via glyceraldehyde and a C4 unit that is related to succinic or aspartic acids. A recent investigation has confirmed this pathway for ricinine (49) and indicated that dihydroxyacetone phosphate lies between glycerol and glyceraldehyde (loss of tritium from C-2 of labelled glycerol). ... 

Physiology biosynthesis Like the tropane alkaloids, T. a. are formed in the roots and transported to the aboveground parts for storage by the plant s phloem system. In some sorts of tobacco plants a part of the nicotine is demethylated to nomicotine during transport to the shoot. Nomicotine and anabasine are often the main alkaloids in the so-called nicotine-poor tobacco plant types. The T. a. are formed biogenetically from nicotinic acid, made available via the pyridine nucleotide cycle (see nicotinamide), and a pyrrolidine or piperidine building block (figure). In the case of nicotine, like for the tropane alkaloids, Al-methylpyr-roline is an intermediate, in the biosynthesis of anabasine the intermediate is a piperidine derived from the amino acid lysine (see piperidine alkaloids). 

Gholson (73) proposed in 1966 a "pyridine nucleotide cycle to describe the continual synthesis and breakdown of these nucleotides in the cell. This turnover as described by Rechsteiner et al. 175) is both rapid and extensive. The half-life for NAD in HeLa cells was approximately 1 hour (10 molecules of NAD turning over/second/cell 175). Further work by Hillyard et al. demonstrated that the main biochemical event leading to turnover of NAD in mammalian cells is ADP-ribosylation 85). To quote Purnell et al. 171), "The magnitude of the turnover of NAD can best be described by the fact that more adenine leaves NAD than enters DNA. ... 

Ibe nicotinamide moiety of NAD is biosynthesized from quinolinic acid (see Pyridine nucleotide cycle. Tryptophan). NAD is degraded by a pyrophosphatase, and by nucleosidases that cleave the glycosidic bonds to nicotinamide and adenine. 

Pyridine nucleotide cycle a salvage pathway in which lucotinamide produced by degradation of... 

The pyridine nucleotide cycle. For formulas of intermediates, see L-Tryptophan and Nicotinamide adenine dinucleotide. 

Ribose phosphates phosphorylated derivatives of ribose. Ribose is phosphorylated in position 5 by the action of ribokinase (EC 2.7.1.15) and ATP ribose 5-phosphate is also produced in the Pentose phosphate cycle (see), and in the Calvin c cle (see) of photosynthesis. Phosphoribomutase cat yses the interconversion of ribose 5-phospbate and ribose 1-phosphate, and the cosubstrate of this reaction is ribose l,5-f>isphosphate. 5-Phosphoribosyl 1-pyrophos-phate donates a ribose 5-phosphate moiety in the de novo biosynthesis of purine and pyrimidine nucleotides (see Purine biosynthesis. Pyrimidine biosynthesis), in the Salvage pathway (see) of purine and pyrimidine utilization, in the biosynthesis of L-Histi-dine (see) and L-Tryptophan (see) and in the conversion of nicotinic acid into nicotinic acid ribotide (see Pyridine nucleotide cycle). Ribose 1-phosphate can also take part in nucleotide synthesis (see Salvage pathway). 

Why might it be desirable to coordinately lower the levels of nicotinamide deamidase The 8-fold depression in nicotinamide deamidase activity causes excretion of nicotinamide xthR mutants have shown to be "feeders" for nicotinamide auxotrophs, indicating that these strains continuously excrete nicotinamide into the medium. This is presumably a consequence of the pyridine nucleotide cycle, shown in Fig. 2. Nicotinamide deamidase is not only an enzyme for the salvage of exogenous pyridine, but it is part of a NAD recycling pathway, i.e., a "pyridine nucleotide cycle"... 

Any intracellular nicotinamide which is produced from NAD, either directly or indirectly (from NMN), must be deamidated to nicotinic acid before it can be resynthesized into NAD. When nicotinamide deamidase levels faU, intracellular nicotinamide is not efficiently recycled, and a significant fraction of nicotinamide produced by pyridine nucleotide cycles is excreted into the medium. Thus, lowering nicotinamide deamidase levels shunts the normal pyridine nucleotide cycle to cause excretion of nicotinamide. 

Fig. 2. Intracellular pyridine nucleotide cycles in enteric bacteria. The breakdown and resynthesis of NAD occurs in bacteria by the metabolic steps shown above. All abbreviations are as in Fig. 1, with the addition of NMN, nicotinamide mononucleotide. The metabolic step catalyzed by nicotinamide deamidase, the levels of which are reduced by anxt/iR mutation (see text) is shown by the bold arrow.

The exquisite sensitivity of resting lymphocytes to DNA damage and poly(ADP-ribose) polymerase activation may be due, in part, to a decreased capacity of the cells to synthesize NAD. Human lymphocytes possess an intact pyridine nucleotide cycle (10). The cells synthesize NAD from either nicotinamide or nicotinic acid, and they release nicotinamide as a by-product of ADP-ribosylation reactions. NMN pyrophosphorylase is the rate-limiting enzyme in NAD biosynthesis from nicotinamide, and the content of this enzyme in unstimulated human lymphocytes is quite low (9). We have examined the rate of NAD turnover in resting lymphocytes in order to quantitate the contribution of ADP-ribosylation to the overall consumption of NAD (10). In addition, we present here preliminary results of in vitro biochemical studies performed on malignant lymphocytes from patients with chronic lymphocytic leukemia (CLL), who were treated with 2-chlorodeoxyadenosine. 

The already mentioned occurrence of certain alkaloids or alkaloid types in unrelated plants creates some problems for the chemotaxonomist (Matveyev, 1959 Sharapov, 1962). The pyrrolizidine alkaloids, for example, are produced by plants like Crotalaria in Fabaceae, Senecio in Com-positae, Lolium in Glumiflorae, Cynoglossum in Boraginaceae, and Secu-rinega in Euphorbiaceae (Crowley and Culvenor, 1962 Culvenor and Smith, 1967 Culvenor et aL, 1968a,b). The alkaloids derived from the pyridine nucleotide cycle are distributed in unrelated plants. The most common... 

The pyridine nucleotide cycle is a series of reactions that are ubiquitous in nature, differing only in the biosynthesis of quinolinic acid. It (quinolinic acid) occurs from tryptophan in animals, fowl, molds, and in certain microorganisms. It may come from either aspartate and glyceraldehyde-3-phosphate in higher plants and bacteria. In other plant systems highly specialized examples exist, such as mimosine, fusaric acid, and actinidine, where other precursors are used. 

Figure 1.17. Biosynthesis of pyridine alkaloids from compounds in the pyridine nucleotide cycle (Waller et ai, 1966, 1975). Courtesy of the J. of Biological Chemistry, edited by the American Society of Biological Chemists, Inc.

Ricinine is obviously translocated via the phloem tissue, since its movement via xylem would result in accumulation in the large older leaves (Crafts, 1961). There is a possibility that ricinine passively accompanies other metabolites from leaves to sinks or sites of utilization moving along by mass flow. However, the reason for its intense export from leaves prior to abscission remains unclear. Perhaps this alkaloid is eventually utilized for the synthesis of, or controls the synthesis of, compounds in or closely related to the pyridine nucleotide cycle (Waller et al., 1966). 

Figure 6.37. A metabolic grid proposed for the biosynthesis and control of metabolism of ricinine. A1—quinolinic acid A2—nicotinic acid mononucleotide A3—nicotinic acid adenine dinucleotide A4—nicotinamide adenine dinucleotide A5—nicotinamide A6—nicotinic acid A7—nicotinamide mononucleotide B1— AT-demethjiiicinine B2— ricinine Cl— N-methylnicotinic acid and C2— AT-methylnicotinamide. pyridine nucleotide cycle and postulated reaction sequence (Nowacki and Waller 1975a). Courtesy of Pergamon Press, Ltd., copyright 1975.

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