pH dependence of charge transfer between tryptophan and tyrosine in dipeptides

Time-resolved absorption spectroscopy has been employed to study the directionality and rate of charge transfer in W-Y and Ac-W-Y dipeptides as a function of pH. Excitation with 266-nm nanosecond laser pulses produces both W⋅ (or [⋅WH]⁺, depending on pH) and Y⋅. Between pH 6 and 10, W⋅ to was found to oxidize Y with k_X⋅=9.0×10⁴ s⁻¹ and 1.8×10⁴ s⁻¹ for the W-Y and Ac-W-Y dipeptide systems, respectively. The intramolecular charge transfer rate increases as the pH is lowered over the range 6>pH>2. For 10<pH<12, the rate of radical transport for the W-Y dipeptide decreases and becomes convoluted with other radical decay processes, the timescales of which have been identified in studies of control dipeptides Ac-F-Y and W-F. Further increases in pH prompt the reverse reaction to occur, W-Y⋅→W⋅-Y⁻ (Y⁻, tyrosinate anion), with a rate constant of k_X⋅=1.2×10⁵ s⁻¹. The dependence of charge transfer directionality between W and Y on pH is important to the enzymatic function of several model and natural biological systems as discussed here for ribonucleotide reductase.     

ADMA induces monocyte adhesion via activation of chemokine receptors in cultured THP-1 cells

Asymmetric dimethylarginine (ADMA), an endogenous NOS inhibitor, is also an important inflammatory factor contributing to the development of atherosclerosis (AS). The present study was to test the effect of ADMA on angiotensin (Ang) II-induced monocytic adhesion. Human monocytoid cells (THP-1) or isolated peripheral blood monocyte cells (PBMCs) were incubated with Ang II (10⁻⁶ M) or exogenous ADMA (30 μM) for 4 or 24 h in the absence or presence of losartan or antioxidant PDTC. In cultured THP-1 cells, Ang II (10⁻⁶ M) for 24 h elevated the level of ADMA in the medium, upregulated the protein expression of protein arginine methyltransferase (PRMT) and decreased the activity of dimethylarginine dimethylaminohydrolase (DDAH). Both of Ang II and ADMA increased monocytic adhesion to human umbilical vein endothelial cells (HUVECs), elevated the levels of monocyte chemoattractant protein (MCP)-1, interleukin (IL)-8 and tumor necrosis factor (TNF)-α and upregulated CCR₂ and CXCR₂ mRNA expression, concomitantly with increase in reactive oxygen species (ROS) generation and activation of nuclear factor (NF)-κB. Pretreatment with losartan (10 μM) or PDTC (10 μM) abolished the effects mediated by Ang II or ADMA. In isolated PBMCs from healthy individuals, ADMA upregulated the expression of CXCR₂ mRNA, which was attenuated by losartan (10 μM), however, ADMA had no effect on surface protein expression of CCR₂. The present results suggest that ADMA may be involved in monocytic adhesion induced by Ang II via activation of chemokine receptors by ROS/NF-κB pathway.     

Pyrazole inhibitors of coactivator associated arginine methyltransferase 1 (CARM1)

This study reports the identification and Hits to Leads optimization of inhibitors of coactivator associated arginine methyltransferase (CARM1). Compound 7b is a potent, selective inhibitor of CARM1.     

Sequence-structure-function analysis of the bifunctional enzyme MnmC that catalyses the last two steps in the biosynthesis of hypermodified nucleoside mnm5s2U in tRNA

MnmC catalyses the last two steps in the biosynthesis of 5-methylaminomethyl-2-thiouridine (mnm(5)s(2)U) in tRNA. Previously, we reported that this bifunctional enzyme is encoded by the yfcK open reading frame in the Escherichia coli K12 genome. However, the mechanism of its activity, in particular the potential structural and functional dependence of the domains responsible for catalyzing the two modification reactions, remains unknown. With the aid of the protein fold-recognition method, we constructed a structural model of MnmC in complex with the ligands and target nucleosides and studied the role of individual amino acids and entire domains by site-directed and deletion mutagenesis, respectively. We found out that the N-terminal domain contains residues responsible for binding of the S-adenosylmethionine cofactor and catalyzing the methylation of nm(5)s(2)U to form mnm(5)s(2)U, while the C-terminal domain contains residues responsible for binding of the FAD cofactor. Further, point mutants with compromised activity of either domain can complement each other to restore a fully functional enzyme. Thus, in the conserved fusion protein MnmC, the individual domains retain independence as enzymes. Interestingly, the N-terminal domain is capable of independent folding, while the isolated C-terminal domain is incapable of folding on its own, a situation similar to the one reported recently for the rRNA modification enzyme RsmC.     

Organoselenides from Nicotiana tabacum genetically modified to accumulate selenium

Nicotiana tabacum L. (tobacco) plants were transformed to overexpress a selenocysteine methyltransferase gene from the selenium hyperaccumulator Astragalus bisulcatus (Hook.) A. Gray (two-grooved milkvetch), and an ATP-sulfurylase gene from Brassica oleracea L. var. italica (broccoli). Solvent extraction of leaves harvested from plants treated with selenate revealed five selenium-containing compounds, of which four were identified by chemical synthesis as 2-(methylseleno)acetaldehyde, 2,2-bis(methylseleno)acetaldehyde, 4-(methylseleno)-(2E)-nonenal, and 4-(methylseleno)-(2E,6Z)-nonadienal. These four compounds have not previously been reported in nature.     

Aromatase inhibitory, radical scavenging, and antioxidant activities of depsidones and diaryl ethers from the endophytic fungus Corynespora cassiicola L36

Isolation of a broth extract of the endophytic fungus Corynespora cassiicola L36 afforded three compounds, corynesidones A (1) and B (3), and corynether A (5), together with a known diaryl ether 7. Compounds 1, 3, 5, and 7 were relatively non-toxic against cancer cells, and inactive toward normal cell line, MRC-5. Corynesidone B (3) exhibited potent radical scavenging activity in the DPPH assay, whose activity was comparable to ascorbic acid. Based on the ORAC assay, compounds 1, 3, 5, and 7 showed potent antioxidant activity. However, the isolated natural substances and their methylated derivatives (1−8) neither inhibited superoxide anion radical formation in the XXO assay nor suppressed TPA-induced superoxide anion generation in HL-60 cell line. Corynesidone A (1) inhibited aromatase activity with an IC₅₀ value of 5.30 μM.     

Plant meristems: <Emphasis Type="Italic">CLAVATA3/ESR</Emphasis>-related signaling in the shoot apical meristem and the root apical meristem

The plant meristems, shoot apical meristem (SAM) and root apical meristem (RAM), are unique structures made up of a self-renewing population of undifferentiated pluripotent stem cells. The SAM produces all aerial parts of postembryonic organs, and the RAM promotes the continuous growth of roots. Even though the structures of the SAM and RAM differ, the signaling components required for stem cell maintenance seem to be relatively conserved. Both meristems utilize cell-to-cell communication to maintain proper meristematic activities and meristem organization and to coordinate new organ formation. In SAM, an essential regulatory mechanism for meristem organization is a regulatory loop between WUSCHEL (WUS) and CLAVATA (CLV), which functions in a non-cell-autonomous manner. This intercellular signaling network coordinates the development of the organization center, organ boundaries and distant organs. The CLAVATA3/ESR (CLE)-related genes produce signal peptides, which act non-cell-autonomously in the meristem regulation in SAM. In RAM, it has been suggested that a similar mechanism can regulate meristem maintenance, but these functions are largely unknown. Here, we overview the WUS–CLV signaling network for stem cell maintenance in SAM and a related mechanism in RAM maintenance. We also discuss conservation of the regulatory system for stem cells in various plant species. S. Sawa is the recipient of the BSJ Award for Young Scientist, 2007.