Adenine ribosides

Adenosine. 9 0-J>-Ribofuranosyt-9H-puriti 6 amine 6-ainino-9 P-D-ribofurano yl 9//-purine 9-/3-D-ribofuranosidoadenine adenine riboside Adenocard. C f-Hi3N 04 mol wt 267.24. 0 44.94%, H 4-90%, N 26.21%, O 23.95 %, Nucleoside widely distributed in nature. From yeast nucleic acid Levene and Bass. Nucleic Acids (New... 

Example adenine riboside = adenosine adenine deoxyriboside = deoxyadenosine... 

Fig. 11.1.3. Reversed phase HPLC of nucleic acid bases and nucleosides. Chromatographic conditions column, Spherisorb ODS-2, 5 pm (250 x 4.6 mm) mobile phase, 0.05 M monobasic ammonium phosphate, pH 3.5 flow rate, 1.5 ml/min temperature, ambient detection, UV at 260 nm. Peaks C, cytosine U, uracil FU, fluorouracil CR, cytosine riboside A, adenine CdR, cytosine deoxyriboside UR, uracil riboside T, thymine FUR, fluorouracil riboside UdR, uracil deoxyriboside, FUdR, fluorouracil deoxyriboside GR, guanine riboside GdR, guanosine deoxyriboside TdR, thymine deoxyriboside AR, adenine riboside. Reproduced from Miller et al. (1982), with...

Bios5mthetic pathways of naturally occurring cytokinins are illustrated in Fig. 29.5. The first step of cytokinin biosynthesis is the formation of A -(A -isopentenyl) adenine nucleotides catalyzed by adenylate isopentenyltransferase (EC 2.5.1.27). In higher plants, A -(A -isopentenyl)adenine riboside 5 -triphosphate or A -(A -isopentenyl)adenine riboside 5 -diphosphate are formed preferentially. In Arabidopsis, A -(A -isopentenyl)adenine nucleotides are converted into fraws-zeatin nucleotides by cytochrome P450 monooxygenases. Bioactive cytokinins are base forms. Cytokinin nucleotides are converted to nucleobases by 5 -nucleotidase and nucleosidase as shown in the conventional purine nucleotide catabolism pathway. However, a novel enzyme, cytokinin nucleoside 5 -monophosphate phosphoribo-hydrolase, named LOG, has recently been identified. Therefore, it is likely that at least two pathways convert inactive nucleotide forms of cytokinin to the active freebase forms that occur in plants [27, 42]. The reverse reactions, the conversion of the active to inactive structures, seem to be catalyzed by adenine phosphoiibosyl-transferase [43] and/or adenosine kinase [44]. In addition, biosynthesis of c/s-zeatin from tRNAs in plants has been demonstrated using Arabidopsis mutants with defective tRNA isopentenyltransferases [45]. 

If adenosine (adenine riboside) is formed by either of the two mechanisms discussed above, the formation of adenylic acid (AMP) and the pyrophosphates depends upon the introduction of a phosphate group. Kinases for the formation of riboside 5 -phosphates from the riboside and ATP are known, Another reaction sequence (reaction b) has also been found for the formation of nucleotides (IV). Individual enzymes have been found for handling orotic acid (a pyrimidine precursor) and adenine... 

Lactic acid, glycolic acid, glyceric acid, mahc acid, erythritol, isothreonic acid, threonic acid, arabinoic acid, y-lactone, ribonic acid, y-lactone, quinic acid, urea, adenosine (adenine riboside), and guanosine. 

The isomerism existing between the pairs of nucleotides was attributed to the different locations of the phosphoryl residues in the carbohydrate part of the parent nucleoside,49 63 since, for instance, the isomeric adenylic acids are both hydrolyzed by acids to adenine, and by alkalis or kidney phosphatase to adenosine. Neither is identical with adenosine 5-phosphate since they are not deaminated by adenylic-acid deaminase,68 60 and are both more labile to acids than is muscle adenylic acid. An alternative explanation of the isomerism was put forward by Doherty.61 He was able, by a process of transglycosidation, to convert adenylic acids a" and 6 to benzyl D-riboside phosphates which were then hydrogenated to optically inactive ribitol phosphates. He concluded from this that both isomers are 3-phosphates and that the isomerism is due to different configurations at the anomeric position. This evidence is, however, open to the same criticism detailed above in connection with the work of Levene and coworkers. Further work has amply justified the original conclusion regarding the nature of the isomerism, since it has been found that, in all four cases, a and 6 isomers give rise to the same nucleoside on enzymic hydrolysis.62 62 63 It was therefore evident that the isomeric nucleotides are 2- and 3-phosphates, since they are demonstrably different from the known 5-phosphates. The decision as to which of the pair is the 2- and which the 3-phosphate proved to be a difficult one. The problem is complicated by the fact that the a and b" nucleotides are readily interconvertible.64,64... 

In fact, kinetin was not the natural cell division hormone, although as an adenine derivative it is chemically closely related to the native kinins that have been isolated from plant sources. It was D. S. Letham in New Zealand (1967) who first succeeded with a crystalline cytokinin isolated from the milky stage of immature maize (Zea mays) kernels. It was 6-substituted iso-pentenylamino purine (N6-(A2-isopentenyl)) adenosine (Figure 2d). He called the substance zeatin, and zeatin riboside was, in fact, the major active constituent of coconut milk (Letham, 1974). 

The natural occurrence of a-n-ribosides as well as of 7-glycosylpurines is rare. However, 7-(a-D-ribofuranosyl)adenine and -guanine, as well as several other purine derivatives, can be isolated from pseudovitamin B,2 -... 

The reaction of adenine, as well as of its riboside, with 4-chlorobut-2-ynoic acid esters, C1CH2C = CC0,R, has been studied in neutral and slightly acidic media (phi 4-5). Adenosine reacts at pH 4 5 with formation of the lactam 15 while at neutral pH the isomeric, slightly fluorescent compound 16 is obtained. 

Zeatin (frans-iV -[4-hydroxy-3-methylbut-2-en-l-yI]adenine) [1637-39-4] M 219.3, m 207-208, 209-209.5 , pKi 4.4 (basic), pK2 9.8 (acidic). Purified by recrystn from EtOH or H2O. The UV has A-max at 207 and 275nm (e 1400 and 14650) in O.IN aqueous HCl 212 and 270nm (e 17050 and 16150) in aqueous buffer pH 7.2 220 and 276nm (e 15900 and 14650) in O.IN aq NaOH. Thepicrate has m 192-194 (from H2O) from which zeatin can be recovered by treatment with Dowex 1 x8 (200-400 mesh, OH form). [Letham et al. Aust J Chem 22 205 1969 Proc Chem Soc (London) 230 7964 Shaw and Wilson Proc Chem Soc (London) 231 7964.] It is a cell division factor (plant growth regulator) [Letham and Palni Ann Rev Plant Physiol 34 163 1983] and inhibits mitochondrial function [Miller Plant Physiol 69 1274 7982]. Its 9-riboside is a cytokine [McDonald and Morris Methods Enzymol 100 347 7985]. 

Since no specific enzymes interconverting cytokinin bases to ribosides and nucleotides and vice versa have been found, it is generally thought that these conversions are mediated by the enzymes of purine metabolism. However, a low affinity of these enzymes toward cytokinins may be insufficient due to physiological concentrations of cytokinins in plant tissues that are up to six orders lower than those of adenine derivatives. The only cytokinin-specific enzymes have been found for glycosylation of cytokinins at N and atoms of the adenine ring or at the hydroxyl group of zeatin. The main supposed cytokinin-specific metabolic pathways are displayed in Fig. (3). 

Biosynthetically, purines are built up via formation of the imidazole ring first, from glycine and formate, and thence to hypoxanthine and then the other natural purines. In the laboratory, most imidazole-based purine syntheses start with 5-aminoimidazole-4-carboxylic acid, particularly its amide (known by the acronym AICA), which as well as its riboside, is commercially available from biological sources. The use of 5-aminoimidazole-4-carbonitrile in this approach results in the formation of 6-amino-purines, as in a synthesis of adenine itself." ... 

Nitrogen can also be used as a remote label the exocyclic amino group of adenine, which is readily inserted into adenosine by reaction of chloropurine riboside with ammonia and easily removed by adenosine deaminase, is being used in our laboratory for measurement of 0 isotope effects in the y-phosphate of ATP. In this case, 0.36% of the N-, 0-labeled species is mixed with 99.6% of the " N-labeled material to restore the natural abundance of in the amino group of adenine. Depleted C and N are commercially available. 

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