CXXC domain of human DNMT1 is essential for enzymatic activity

DNA cytosine methylation is one of the major epigenetic gene silencing marks in the human genome facilitated by DNA methyltransferases. DNA cytosine-5 methyltransferase 1 (DNMT1) performs maintenance methylation in somatic cells. In cancer cells, DNMT1 is responsible for the aberrant hypermethylation of CpG islands and the silencing of tumor suppressor genes. Here we show that the catalytically active recombinant DNMT1, lacking 580 amino acids from the amino terminus, binds to unmethylated DNA with higher affinity than hemimethylated or methylated DNA. To further understand the binding domain of enzyme, we have used gel shift assay. We have demonstrated that the CXXC region (C is cysteine; X is any amino acid) of DNMT1 bound specifically to unmethylated CpG dinucleotides. Furthermore, mutation of the conserved cysteines abolished CXXC mediated DNA binding. In transfected COS-7 cells, CXXC deleted DNMT1 (DNMT1 (DeltaCXXC)) localized on replication foci. Both point mutant and DNMT1 (DeltaCXXC) enzyme displayed significant reduction in catalytic activity, confirming that this domain is crucial for enzymatic activity. A permanent cell line with DNMT1 (DeltaCXXC) displayed partial loss of genomic methylation on rDNA loci, despite the presence of endogenous wild-type enzyme. Thus, the CXXC domain encompassing the amino terminus region of DNMT1 cooperates with the catalytic domain for DNA methyltransferase activity.     

Purple membrane lipid control of bacteriorhodopsin conformational flexibility and photocycle activity.

Specific lipids of the purple membrane of Halobacteria are required for normal bacteriorhodopsin structure, function, and photocycle kinetics [Hendler, R.W. & Dracheva, S. (2001) Biochemistry (Moscow)66, 1623-1627]. The decay of the M-fast intermediate through a path including the O intermediate requires the presence of a hydrophobic environment near four charged aspartic acid residues within the cytoplasmic loop region of the protein (R. W. Hendler & S. Bose, unpublished results). On the basis of the unique ability of squalene, the most hydrophobic purple membrane lipid, to induce recovery of M-fast activity in Triton-treated purple membrane, we proposed that this uncharged lipid modulates an electrostatic repulsion between the membrane surface of the inner trimer space and the nearby charged aspartic acids of the cytoplasmic loop region to promote transmembrane alpha-helical mobility with a concomitant increase in the speed of the photocycle. We examined Triton-treated purple membranes in various stages of reconstitution with native lipid suspensions using infrared spectroscopic techniques. We demonstrate a correlation between the vibrational half-width parameter of the protein alpha-helical amide I mode at 1660 cm-1, reflecting the motional characteristics of the transmembrane helices, and the lipid-induced recovery of native bacteriorhodopsin properties in terms of the visible absorbance maxima of ground state bacteriorhodopsin and the mean decay times of the photocycle M-state intermediates.     

Copper removal by immobilized Microcystis aeruginosa in continuous flow columns at different bed heights: study of the adsorption/desorption cycle

Microcystis aeruginosa immobilized in a natural polymer was tested for its potential to remove Cu²⁺ ions from aqueous solution in a continuous, downflow packed columnar reactor. Various parameters like flow rate, bed height and contact time required for maximum removal of test metals by the immobilized Microcystis aeruginosa were optimized. An increase in bed height from 2 to 10 cm resulted in an apparent decrease in biosorption capacity from 8.94 to 5.34 mg g^–1, but more Cu²⁺ solution was purified at the higher bed height. Efficiency of metal recovery from Cu²⁺-loaded biomass and its subsequent regeneration was also determined. Immobilized M. aeruginosa was found to be effective in Cu²⁺ removal from solution for up to 10 cycles of adsorption–desorption and 1 M HCl is very efficient desorbent for regeneration of Microcystis biomass for reuse.     

Photochemical labeling of membrane hydrophobic core of human erythrocytes using a new photoactivable reagent 2-[3H]diazofluorene

Photoactivable reagents have been useful for studying the structural aspects of membrane hydrophobic core. We have reported earlier (Anjaneyulu, P.S.R., and Lala, A. K. (1982) FEBS Lett. 146, 165-167) the use of diazofluorene as a probe for fluorescent photochemical labeling of hydrophobic core in artificial membranes. To quantitate and enhance the monitoring ability of this probe, we have synthesized 2-[3H]diazofluorene of high specific activity. This reagent rapidly partitions into phosphatidylcholine vesicles and selectively labels the fatty acyl chains of phosphatidylcholine. The insertion yield (13%) is not affected by the presence of scavengers like reduced glutathione. 2-[3H]Diazofluorene also readily partitions into erythrocyte membranes and on photolysis labels the membrane. The overall insertion was 48% with 9.7% in protein fraction and the rest in lipids. The distribution of radioactivity in labeled protein fraction was restricted to integral membrane proteins with Band 3 being the major protein labeled. There is little or no labeling associated with extrinsic proteins like spectrin. Further analysis of labeled Band 3 by treatment with chymotrypsin indicated that the labeling was restricted to the membrane spanning CH-17 and CH-35 fragments. No labeling of the cytoplasmic fragment of Band 3 could be observed. 2-[3H]Diazofluorene should prove useful for studying integral membrane proteins and their membrane-spanning regions.     

Solid-State Stability Issues of Drugs in Transdermal Patch Formulations

The transdermal patch formulation has many advantages, including noninvasiveness, an ability to bypass the first-pass metabolism, low dosage requirements, and prolonged drug delivery. However, the instability of solid-state drugs is one of the most critical problems observed in transdermal patch products. Therefore, a well-characterized approach for counteracting stability problems in solid-state drugs is crucial for improving the performance of transdermal patch products. This review provides insight into the solid-state stability of drugs associated with transdermal patch products and offers a comprehensive update on the various approaches being used for improving the stability of the active pharmaceutical ingredients currently being used.     

Elucidation of the anticancer potential and tubulin isotype-specific interactions of β-sitosterol

Beta-sitosterol (β-SITO), a phytosterol present in many edible vegetables, has been reported to possess antineoplastic properties and cancer treatment potential. We have shown previously that it binds at a unique site (the ‘SITO-site’) compared to the colchicine binding site at the interface of α- and β-tubulin. In this study, we investigated the anticancer efficacy of β-SITO against invasive breast carcinoma using MCF-7 cells. Since ‘isotypes’ of β-tubulin show tissue-specific expression and many are associated with cancer drug resistance, using computer-assisted docking and atomistic molecular dynamic simulations, we also examined its binding interactions to all known isotypes of β-tubulin in αβ-tubulin dimer. β-SITO inhibited MCF-7 cell viability by up to 50%, compared to vehicle-treated control cells. Indicating its antimetastatic potential, the phytosterol strongly inhibited cell migration. Immunofluorescence imaging of β-SITO-treated MCF-7 cells exhibited disruption of the microtubules and chromosome organization. Far-UV circular dichroism spectra indicated loss of helical stability in tubulin when bound to β-SITO. Docking and MD simulation studies, combined with MM-PBSA and MM-GBSA calculations revealed that β-SITO preferentially binds with specific β-tubulin isotypes (β_II and β_III) in the αβ-tubulin dimer. Both these β-tubulin isotypes have been implicated in drug resistance against tubulin-targeted chemotherapeutics. Our data show the tubulin-targeted anticancer potential of β-SITO, and its potential clinical utility against β_II and β_III isotype-overexpressing neoplasms.