Nucleosides, cytokinin-active

Many 6-alkylaminopurine nucleosides are important adenosine receptor antagonists, and acyclic nucleotide analogues derived from 6-dialkylaminopurines are strong antivirals, anti-neoplastic agents, and immunomodulators. Recently, several 6-(arylalkynyl)-, 6-(arylalkenyl)-, and 6-(arylalkyl)purines have been reported to exhibit cytokinine activity. Suzuki crosscoupling reactions of 9-benzyl-6-chloropurine with boronic acids have recently been reported to provide 6-substituted purines in moderate to excellent yields (Eq. (46)) [77]. 

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]. 

Fig. 29.5 The two cytokiiim biosynthesis and activation pathways. DMAPP dimethylallyl diphosphate, iP A/ -(A -isopenteaiyl)adenme, iPRMP iP riboside S -mraiophosphate, tZ trans-zeatin, tZRTP tZ riboside 5 -triphosphate, tZRDP tZ riboside 5 -diphosphate, tZRMP tZ riboside 5 -monophosphate, cZ cis-zeatin, cZRMP cZ riboside 5 -monophosphate, cZR cZ riboside. Enzymes iPT isopentenyltransfoase, CYP735A cytochrome P450 CYP735A, tRNA-iPT transfer RNA isopentenyltransferase, LOG lonely guy, cytokinin nucleoside 5 -monophosphate phosphor-ibohydrolase (After Hirose et al. [26])...

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