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The structure-activity relationships of adenosine-3', 5'-bisphosphates as P2Y(1) receptor antagonists have been explored, revealing the potency-enhancing effects of the N(6)-methyl group and the ability to substitute the ribose moiety (Nandanan et al. J. Med. Chem. 1999, 42, 1625-1638). We have introduced constrained carbocyclic rings (to explore the role of sugar puckering), non-glycosyl bonds to the adenine moiety, and a phosphate group shift. The biological activity of each analogue at P2Y(1) receptors was characterized by measuring its capacity to stimulate phospholipase C in turkey erythrocyte membranes (agonist effect) and to inhibit its stimulation elicited by 30 nM 2-methylthioadenosine-5'-diphosphate (antagonist effect). Addition of the N(6)-methyl group in several cases converted pure agonists to antagonists. A carbocyclic N(6)-methyl-2'-deoxyadenosine bisphosphate analogue was a pure P2Y(1) receptor antagonist and equipotent to the ribose analogue (MRS 2179). In the series of ring-constrained methanocarba derivatives where a fused cyclopropane moiety constrained the pseudosugar ring of the nucleoside to either a Northern (N) or Southern (S) conformation, as defined in the pseudorotational cycle, the 6-NH(2) (N)-analogue was a pure agonist of EC(50) 155 nM and 86-fold more potent than the corresponding (S)-isomer. The 2-chloro-N(6)-methyl-(N)-methanocarba analogue was an antagonist of IC(50) 51.6 nM. Thus, the ribose ring (N)-conformation appeared to be favored in recognition at P2Y(1) receptors. A cyclobutyl analogue was an antagonist with IC(50) of 805 nM, while morpholine ring-containing analogues were nearly inactive. Anhydrohexitol ring-modified bisphosphate derivatives displayed micromolar potency as agonists (6-NH(2)) or antagonists (N(6)-methyl). A molecular model of the energy-minimized structures of the potent antagonists suggested that the two phosphate groups may occupy common regions. The (N)- and (S)-methanocarba agonist analogues were docked into the putative binding site of the previously reported P2Y(1) receptor model.

Lignin is, second to cellulose, the most abundant organic compound in the terrestrial biosphere. In different tree species, lignin content varies between 15 and 36% of the dry weight of wood (1). Lignin is a major constituent of cell walls of fibers and tracheary elements and provides these cells rigidity for structural support and impermeability for water transport. For the production of high-quality paper, lignin is considered as a negative factor because it must be extracted from the cellulose fraction by energy-requiring and polluting methods. For this reason, there is considerable interest in modifying lignin by genetic engineering to improve its extractability from wood (2-5).Lignin monomer biosynthesis starts with the deamination of phenylalanine to produce cinnamic acid (Fig. 1). Further enzymatic reactions include the hydroxylation of the aromatic ring, the methylation of selected phenolic hydroxyl groups, the activation of the cinnamic acids to cinnamoyl-CoA esters, and the reduction of these esters to cinnamaldehydes and cinnamyl alcohols. The precise order in which these reactions occur is not yet fully resolved. In dicotyledonous plants, lignin is composed mainly of guaiacyl (G) 1 and syringyl (S) units that are monomethoxylated (C-3) and dimethoxylated (C-3, C-5) and derived from coniferyl alcohol and sinapyl alcohol, respectively. The lignin monomers are transported to the cell wall and are subsequently polymerized, resulting in the deposition of a crosslinked polymer. Although most of the lignin biosynthesis enzymes have been characterized at the molecular level, their precise role in determining lignin amount and composition still needs to be clarified.Based on in vitro data, it has been generally accepted that the methylation reactions in lignin biosynthesis occur exclusively at the cinnamic acid level and that they are catalyzed by a bispecific caffeic acid/5-hydroxyferulic acid O-methyltransferase (COMT) (1). However, the analysis of transgenic tobacco and poplar with suppressed COMT activity has shown that