Nicotinamide ribosides

Animals and yeasts can synthesize nicotinamide from tryptophan via hydroxyanthranilic acid (52) and quinolinic acid (53, Fig. 6A) (31), but the biosynthetic capacity of humans is limited. On a diet that is low in tryptophan, the combined contributions of endogenous synthesis and nutritional supply of precursors, such as nicotinic acid, nicotinamide, and nicotinamide riboside, may be insufficient, which results in cutaneous manifestation of niacin deficiency under the clinical picture of pellagra. Exogenous supply of nicotinamide riboside was shown to promote NAD+-dependent Sir2-function and to extend life-span in yeast without calorie restriction (32). 

Belenky P, Racette EG, Bogan KL, McClure JM, Smith JS, Brenner C. Nicotinamide riboside promotes Sir2 silencing and extends... 

The separation of phosphohydrox5dmino acids by h.p.l.c. has been described. A very sensitive method for the determination of inorganic phosphate has been described which is based on the formation of a Rhodamine B-phosphomolybdate complex, and the continuous enzsmiic removal of inorganic phosphate from reactions has been achieved by means of the nucleoside phosphorylase-catalysed phosphorolysis of nicotinamide riboside, when the product is ribose 1 -phosphate. ... 

Figure 2. NAD biosynthesis and its role for regulatory pathways in mammalian cells. Note added in proof A recent study demonstrates an alternative biosynthetic route of NAD, not indicated in Figure 2 a specific kinase has been found which converts nicotinamide riboside to NMN (Bieganowski P, Brenner C Cell 2004 117 495-502). Nicotinamide riboside could be an important nutritional factor. Also, nicotinamide riboside kinase could be involved in the conversion of the anticancer drugtiazofiirin to its active form.

In the oxidized form of NAD, the pyridinium cation of nicotinamide is bound by an N-glycosidic linkage to Cl of D-ribose. This nicotinamide riboside moiety is linked to adenosine via a pyrophosphate group. NAD therefore has the structure of a dinucleotide (Fig. 1). Af, of the oxidized form (NAD ) = 663.4. 

Macrocychc bisphosphonates 165 and 166 were prepared from 2, 3 -0-isopropyhdene-5 -tosyladenosine 158 or -mesyladenosine 159 precursors, and the bis(tetrabutylammonium) salt of nicotinamide riboside 5 -methylbis (phosphonate) 160 (05JMC4177). Nucleophilic substitution of the precursors afforded the acyclic pyrophosphate 161 or 162 in modest yields. After deprotection with an aqueous hydrochloric acid solution, nicotinamide... 

Figure 7.3 NAD recycling. Humans have two metabolic pathways that are able to recycle nicotinamide. NAD-consuming enzymes (ARTs, PARPs, sirtuins) break down NAD to nicotinamide and ADP-ribosyl product. Nicotinamide by the enzymatic action of nicotinamide phosphoribosyltransferase (NAMP/PBEF) and nicotinamide/nicotinate-mononucleotide-adenyltransferases isoenzymes (NMATl-3) is then retransformed to NAD. In a second pathway, nicotinamide riboside is phosphorylated by nicotinamide riboside kinase (NRK 1,2) to nicotinamide mononucleotide. Subsequently, nicotinamide mononucleotide is converted to NAD by the catalytic action of NMNATs.

Bieganowski, P., and Brenner, C., 2004. Discoveries of nicotinamide riboside as a nutrient and conserved NRK genes establish a Preiss-Handler independent route to NAD+ in fungi and humans. Cell. 117 495-502. 

Pyridine Nucleotides. Nucleoside phosphorylase is capable of using the base nicotinamide. The product of the reaction of this base with ribose-l-phosphate is nicotinamide riboside. The formation of the corresponding nicotinamide mononucleotide is catalyzed by a typical kinase, using ATP. A specific enzyme purified from human erythrocytes has been shown to form nicotinamide mononucleotide by a second mechanism in which PRPP and nicotinamide react to form inorganic pyrophosphate and... 

Another compound which undergoes phosphorolysis is nicotinamide riboside, a component part of the pyridino-coenzymes. It is apparently split by the same enzyme which acts on inosine. Thus, the ratio of enzymatic activities against inosine and nicotinamide riboside has been found to remain constant during the course of purification of hog liver fractions, and inosine inhibits the splitting of nicotinamide riboside competitively. The phosphorolysis can be represented as follows ... 

Nicotinamide riboside contains a quaternary nitrogen with a strongly ionized positive charge at neutral pH. Phosphorolysis yields nicotinamide, which is a weak base, and hydrogen ion. Consequently, the reaction proceeds practically to completion in buffered solutions at neutral pH. 

Nicotinamide mononucleotide does not have any known coenzyme function but is an intermediate in the formation of DPN. It can be synthesized by the direct phosphorylation of nicotinamide riboside. ... 

FAD is similar in structure to DPN. It differs in that riboflavin replaces the nicotinamide riboside moiety. It is formed when red blood cells are incubated with riboflavin.A purified enzyme system from yeast catalyzes the synthesis of this coenzyme from ATP and flavin mononucleotide. ... 

The fact that nuclei do contain enzymes likely to be involved in nucleoside metabolism (nucleoside phosphorylase, adenosine deaminase, and guanase), together with the fact that nucleoside phosphorylase catalyzes the formation of nicotinamide ribosides, leads the authors to point out the possible role of the nucleus in controlling the formation of di- and triphos-phopyridine nucleotide coenzymes. 

Several nucleoside hydrolases have been described. A hydrolase purified from baker s yeast (79) has been found which specifically degrades uridine to uracil and n-ribose. Another nucleoside hydrolase also purified from yeast splits guanoane, adenosine, inosine, xanthosine, nicotinamide riboside, and a group of synthetic unnatural riborides. A highly specific uridine hydrolase is found in yeast, and a nucleoride hydrolase has been described in Lactobacillus pentosus which degrades both purine and pyrimidine nucleosides (74)- A nonspecific hydrolase as well as a i cific inosine hydrolase have been purified from fish muscle (76). The spores of BaciUus eereus contain a heat-stable hydrolase which can cleave adenosine and inosine (76, 77). Finally, a riboside hydrolase of broad spedfidty which attacks only 9-ribofuranosides has been purified from extracts of Ladobacil-lus delbrueckii (72, 78). 

The enzyme purified from bull semen dephosphorylates the 5 -phosphate esters of adenosine, uridine, cytidine, guanosine, nicotinamide riboside, and the 5 -phosphate esters of deoxynucleosides, but is inactive with adenosine pyrophosphates and substrates which bear other substituents on the ribose, e.g., adenosine 2, 5 -diphosphate (124). However 5 -nucleo-tidase from potato can hydrolyze the 5 -phosphate of this diphosphate... 

However, as pointed out by Zatman et oZ., this value is erroneous, and the K for the sjmthesis at pH 7 is 10 116). The nicotinamide riboside linkage appears to be a high energy bond, and the free energy of hydrolysis has been calculated to be 8300 cal. Hence, the direction of the reaction is greatly toward phosphorolysis of the riboside, and at physiological pH s, one would expect little synthesis of the riboside. 

Perhaps the most important enzyme in animal tissues which destroys DPN is the DPNase attacking the coenzyme at the nicotinamide riboside linkage. This enzyme induces the hydrolysis of DPN and TPN according to the schematic reactions given in Eq. (5a) and (5b), where N represents nicotinamide, R = ribose, P = phosphate, and A = adenine. 

Mann and Quastel (167) noted, in 1941, that nicotinamide prevented the inactivation of DPN in beef brain preparation. The work of Handler and Klein (168, 159) clearly demonstrated that nicotinamide inhibited the cleavage of DPN at the nicotinamide riboside linkage. Studies on the mechanism of this reaction by Zatman et al. (116) have shown that the... 

A pyridine ribosidase from Xanthomonas pruni has been described which splits nicotmamide riboside in preference to purine ribosides (18 ). The relative activity towards nicotinamide riboside and inosine is about 30 to 1. In contrast to this, extracts of SaccJiaromyces cereviaiae promote the hydroljrtic cleavage of nicotinamide riboside and inosine at approximately the same rate (183). A hydrolytic nucleoadase from a strain of Lactobacillus delbruckii has been purified and also shown to have equal activity towards the two nucleosides (184). Xanthomonas pyridine nucleosidase appears to be unusual for bacteria its significance is not clear as yet. 

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