Isopentenyl adenine

The use of the 6-amino group and N1 nitrogen of the purine base to model the pharmacophore, as seen with staurosporine, is not possible in two other adenine-type inhibitors. Both isopentenyl adenine, a nonspecific inhibitor of protein kinases, and olomoucin, a more specific inhibitor of Ser/Thr protein kinases, are modified only at the 6-amino group position. Thus, bidentate hydrogen-bond formation as seen in the ATP purine base is not possible. Furthermore, there are inhibitors that do not contain the chemical structure of adenine, for example des-chloro-flavopyridol, a potent inhibitor of cdc-2 cell cycle kinase. 

N -(AZ-isopentenyl) -adenine Agrobacterium tumefaciens TLC reverse phase-C-Q lo UV-254 nm (56)... 

Natural cytokinins are. / /-substituted adenine derivatives. The N6 side chain is either isoprenoid or aromatic (Figure 10), with the former occurring in greater abundance than the latter. The prenyl side chains vary in the presence or absence of a hydroxyl group and the stereoisomeric position. Common active isoprenoid cytokinins are isopentenyladenine (iP /V6-(A2-isopentenyl)adenine), trans-zeatin (tZ / / -(4-hydro.xyisopentenyl)adenine (A)-2-methyl-4-(1 //-purin-6-ylamino)but-2-en-l -ol), ax-zeatin (cZ (Z)-2-methyl-4-(l//-purin-6-ylamino)but-... 

The biosynthesis of sterols takes place via the protracted sterol/isoprenoid biosynthetic pathway (Chapter 1). Although the major portion of the carbon flux through this pathway is normally directed into sterols, several branches exist leading to the production of other isoprenoid compounds needed by the cell, such as ubiquinone, dolichol and isopentenyl adenine. Total carbon flux is regulated through the enzymes of the early, or common, portion of the pathway of which the most important is HMG-CoA reductase. Distribution of carbon between the various end products is regulated at later stages of the pathway. 

The identity of the multiple signals which regulate HMG-CoA reductase levels has yet to be established. One of these appears to be a sterol or sterol derivative. The second appears to be mevalonate or one of its metabolites [141]. Leading candidates for the sterol signal(s) are oxygenated derivatives of cholesterol. These are present in the circulation and many are potent suppressors of HMG-CoA reductase levels [175-177]. A candidate for the regulatory metabolite of mevalonate is isopentenyl adenine. Compactin added to baby hamster kidney-21 cells completely inhibits both DNA synthesis and cell proliferation [178]. Added mevalonolactone relieved this inhibition, but added sterols did not. Isopentenyl adenine also relieved compactin inhibition, and did so 100-200 times more effectively than did mevalonolactone [178]. 

Fig. lb. Cylokinins frans-Zeatin (t-Zeatin) Zeatin riboside Isopentenyl adenine (iPa) Kinetin Benzyladenine. 

Cytokinins are cell division promoting hormones. Highly active cytokinins include kinetin, (4-1), trans zeatin (4-2), N -isopentenyl adenine (4-3) and benzyl adenine (4-4). 

A recent review has considered structure-activity data on some 400 compounds [33]. Quantitative structure-activity regression analysis was carried out on several types of cytokinin and their competitive inhibitors, and the results were used to develop a map of the recognition site of the cytokinin receptor [34-36]. The map is shown in Fig. 6a, with the site being overlain by isopentenyl adenine (4-3). In Fig. 6b, the site is divided into a purine area, with the remaining area of the site being referred to as side chain domain. The pyiidyl urea (4-5) is fitted to the site. 

Taya et al. (1978) have reported that a cell-free system from the slime mold Dictyostelium discoideum catalyzes the condensation of DMAPP with AMP to produce isopentenyl adenine, a cytokinin. (An organic chemist would probably call this dimethylallyl adenine, but both terms are correct, and isopentenyl adenine is firmly established in the plant physiology literature.) They have emphasized that this is a direct synthesis, not involving nucleic acid breakdown, and suggest that the same system may operate in higher plants. 

Alkylations and other types of transformation may, however, also proceed with free purines. In Dictyostelium for instance, dimethylallyl pyrophosphate condenses with AMP to produce isopentenyl adenine. In mammalian tissue 3-methyladenine is formed from adenine, and in plants theobromine and caffeine... 

Z to (diH)Z have been conducted (Plant Physiol, submitted). The reaction is NADPH-dependent and does not require the presence of ATP and cations (Table 2). The enzyme has been partially purified using ammonium sulfate precipitation, anion exchange and affinity column chromatography. The affinity of the enzyme for i Ade (N -(A -isopentenyl)adenine), the unhydroxylated counterpart of Z, is negligible. Preliminary estimates of Z reductase activities varied substantially between species, with high activity in P. coccineus and P. vulgaris embryos but only marginally detectable activity in P. lunatus. 

Isopentenyl adenosine Molecular weight Isopentenyl adenine Diphenylurea O-glucosyl zeatin Dihydrozeatin Zeatin... 

Structurally, cytokinins are classified into two groups, namely, adenine-type cytokinins represented by zeatin, kinetin, and 6-benzylaminopurine, and phenylurea-type cytokinins which include diphenylurea and thidiazuron. The naturally occurring cytokinins A -(A -isopentenyl)adenine 13, frans-zeatin 14, and c s-zeatin 15 differ in the stereoisomeric hydroxylation at the isoprenoid side chain. frans-Zeatin and A -(A -isopentenyl)adenine are physiologically the most important cytokinins in plants [27]. 

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]. 

A-l-(3-D-ribofuranosylmakaluvamme I, 255 NBT-272, 3785 NCI-H460 cell lines, 3479 N -(D -isopentenyl)adenine, 958 A -(D -isopentenyl)adenine-type c34okinins, 956... 

Whether this second mode of synthesis of caffeine, theobromine, and theophyllin actually occurs in nature is rather doubtful. There are, however, other purine derivatives, the cytokinins, which show close relationships to tRNA and are of great interest to physiologists. They are a group of phytohormones, of which isopentenyl adenine (IPA) is an example. Several investigations have shown that the isopentenyl unit is built on to an adenine residue, which is already incorporated in a tRNA molecule (also cf page 209). 

Pastes made of carbonaceous materials with ionic liquids acted also as heated electrochemiluminescence sensors. A mixture of multi-wall carbon nanotubes with an ionic liquid was used in connection with lucigenine to detect ascorbic acid which influences the luminescence of the latter [68]. Another example was a carbon/ionic liquid paste electrode where Ne-isopentenyl-adenine was detected using its enhancement action on the electrochemiluminescence of ruthenium bipyridyl [69]. 

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