Purification and biochemical characterization of ionically unbound polyphenol oxidase from Musa paradisiaca leaf

An ionically unbound and thermostable polyphenol oxidase (PPO) was extracted from the leaf of Musa paradisiaca. The enzyme was purified 2.54-fold with a total yield of 9.5% by ammonium sulfate precipitation followed by Sephadex G-100 gel filtration chromatography. The purified enzyme exhibited a clear single band on native polyacrylamide gel electrophoresis (PAGE) and sodium dodecyl sulfate (SDS) PAGE. It was found to be monomeric protein with molecular mass of about 40 kD. The zymographic study using crude extract as enzyme source showed a very clear band around 40 kD and a faint band at around 15 kD, which might be isozymes. The enzyme was optimally active at pH 7.0 and 50°C temperature. The enzyme was active in wide range of pH (4.0-9.0) and temperature (30-90°C). From the thermal inactivation studies in the range 60-75°C, the half-life (t(1/2)) values of the enzyme ranged from 17 to 77?min. The inactivation energy (Ea) value of PPO was estimated to be 91.3?kJ mol(-1). It showed higher specificity with catechol (K(m)?=?8?mM) as compared to 4-methylcatechol (K(m)?=?10?mM). Among metal ions and reagents tested, Cu(2+), Fe(2+), Hg(2+), Mn(2+), Ni(2+), protocatechuic acid, and ferrulic acid enhanced the enzyme activity, while K(+), Na(+), Co(2+), kojic acid, ascorbic acid, ethylenediamine tetraacetic acid (EDTA), sodium azide, β-mercaptoethanol, and L-cysteine inhibited the activity of the enzyme.     

PGC-1α, a key modulator of p53, promotes cell survival upon metabolic stress

Metabolic stress results in p53 activation, which can trigger cell-cycle arrest, ROS clearance, or apoptosis. However, what determines the p53-mediated cell fate decision upon metabolic stress is not very well understood. We show here that PGC-1α binds to p53 and modulates its transactivation function, resulting in preferential transactivation of proarrest and metabolic target genes. Thus glucose starvation results in p53-dependent cell-cycle arrest and ROS clearance, but abrogation of PGC-1α expression results in extensive apoptosis. Additionally, prolonged starvation results in PGC-1α degradation concomitant with induction of apoptosis. We have also identified RNF2, a Polycomb group (PcG) protein, as the cognate E3 ubiquitin ligase. Starvation of mice where PGC-1α expression is abrogated results in loss of p53-mediated ROS clearance, enhanced p53-dependent apoptosis, and consequent severe liver atrophy. These findings provide key insights into the role of PGC-1α in regulating p53-mediated cell fate decisions in response to metabolic stress.     

Association by guilt: identification of DLX5 as a target for MeCP2 provides a molecular link between genomic imprinting and Rett syndrome

Rett syndrome (RTT) is an X-linked dominant neurodevelopmental disorder affecting almost exclusively girls. Although mutations in methyl-CpG-binding protein (MeCP2) are known to be associated with RTT, gene expression patterns are not significantly altered in MeCP2-deficient cells. A recent study1 identified MeCP2-mediated histone modification and formation of a higher-order chromatin loop structure specifically associated with silent chromatin at the Dlx5-Dlx6 locus in normal cells, and its absence thereof in RTT patients. This altered expression of Dlx5 through loss of silent chromatin loop formation provides a molecular mechanism underlying RTT and proposes a novel role for MeCP2 in chromatin organization and imprinting.     

Abiotic metal stress enhances diosgenin yield in Dioscorea bulbifera L. cultures

Lower concentrations of CuSO₄ (25–75 M) in the MS medium supplemented with 0.1 mg l^–1 IAA+5.0 mg l^–1 Kn+500 mg l^–1 CH+10 mg l^–1 Cyst hyd enhanced the growth of regenerants of Dioscorea bulbifera L. CuSO₄ (75 M) induced an appreciable diosgenin yield in the regenerants compared to those obtained on media without Cu. The presence of Cu thus seems to stimulate diosgenin production. The regenerants also differentiated bulbils on lower concentrations of Cu. At CuSO₄ (100 M), however, cultures showed poor growth as well as a low diosgenin yield. Increased proline and protein contents were recorded in cultures grown on Cu-enriched media.     

Dynamics of the native and the ligand-bound structures of eosinophil cationic protein: network of hydrogen bonds at the catalytic site

Eosinophil Cationic Protein (ECP) is sequentially and structurally similar to ribonuclease A (RNase A). It belongs to the RNase A family of proteins and the RNA catalysis is essential to its biological function. In the present study, we have generated the dinucleotide-bound structures of ECP by docking the dinucleotides to a number of molecular dynamics (MD) generated ECP structures. The stability of the docked enzyme-ligand complexes was ascertained by extensive MD simulations. The modes of ligand binding are explored by essential dynamics studies. The role of water molecules in the stability of the complex and in the catalysis was investigated. The active site residues form a complex network of connections with the ligand and with a water molecule. The catalytic mechanism of the RNA cleavage is examined on the basis of the active site geometry obtained by the simulations.     

DNA-binding behavior of ruthenium(II) complexes containing both group 15 donors and 2,2':6',2''-terpyridine

Tetrafluoroborate salts of cationic ruthenium complexes [Ru(kappa(3)-tpy)(EPh(3))(2)Cl](+) (tpy=2,2':6',2'-terpyridine; E=P, 1 or As, 2) containing both the group 15 donor ligands and tpy and their representative substitution products are reported. Weak interaction {C-H...X (X=Cl, F and pi) and pi-pi interaction} studies revealed the presence of a double helical motif in complex 1, while the complex 2 assumes a single helical motif. Intercalative mode of interaction of the complexes 1 and 2 with calf thymus DNA (ctDNA) has been supported by absorption titration studies.     

Regulation of HAX-1 anti-apoptotic protein by Omi/HtrA2 protease during cell death

Omi/HtrA2 is a nuclear-encoded mitochondrial serine protease that has a pro-apoptotic function in mammalian cells. Upon induction of apoptosis, Omi translocates to the cytoplasm and participates in caspase-dependent apoptosis by binding and degrading inhibitor of apoptosis proteins. Omi can also initiate caspase-independent apoptosis in a process that relies entirely on its ability to function as an active protease. To investigate the mechanism of Omi-induced apoptosis, we set out to isolate novel substrates that are cleaved by this protease. We identified HS1-associated protein X-1 (HAX-1), a mitochondrial anti-apoptotic protein, as a specific Omi interactor that is cleaved by Omi both in vitro and in vivo. HAX-1 degradation follows Omi activation in cells treated with various apoptotic stimuli. Using a specific inhibitor of Omi, HAX-1 degradation is prevented and cell death is reduced. Cleavage of HAX-1 was not observed in a cell line derived from motor neuron degeneration 2 mice that carry a mutated form of Omi that affects its proteolytic activity. Degradation of HAX-1 is an early event in the apoptotic process and occurs while Omi is still confined in the mitochondria. Our results suggest that Omi has a unique pro-apoptotic function in mitochondria that involves removal of the HAX-1 anti-apoptotic protein. This function is distinct from its ability to activate caspase-dependent apoptosis in the cytoplasm by degrading inhibitor of apoptosis proteins.