Prooxidant and antioxidant activities of bilirubin and its metabolic precursor biliverdin: a structure-activity study

Bilirubin, which is derived from its metabolic precursor biliverdin, is the end product of heme catabolism. It has been proposed as a physiological antioxidant present in human extracellular fluids. We have earlier shown that bilirubin in the presence of the transition metal ion Cu(II) causes strand cleavage in DNA through generation of reactive oxygen species, particularly the hydroxyl radical. Thus bilirubin possesses both antioxidant and prooxidant properties. In order to understand the chemical basis of various biological properties of bilirubin, we have studied the structure-activity relationship between bilirubin and its precursor biliverdin. The latter has also been reported to possess both antioxidant and toxic properties. In the present studies bilirubin was found to be more effective in the DNA cleavage reaction and a more efficient reducer of Cu(II). The rate of formation of hydrogen peroxide and hydroxyl radicals by the compounds also showed a similar pattern. The relative antioxidant activity was also examined by studying the effect of these compounds on DNA cleavage by a hydroxyl radical generating system and their quenching effect on hydroxyl radicals. The results indicate that bilirubin is more active both as an antioxidant as well as an oxidative DNA cleaving agent. A model for binding of copper to bilirubin has been proposed where two copper ions are bound to two molecules of bilirubin through their terminal pyrrole nitrogens. In order to account for the enhanced copper reducing capacity of bilirubin we have further proposed that an additional copper binding site is provided for in the case of bilirubin due to the absence of a double bond between pyrrole rings II and III. Further it would appear that the structural features of the bilirubin molecule which are important for its prooxidant action are also the ones that render it a more effective antioxidant.     

Gene amplification in amphotericin B-resistant Leishmania tarentolae

Two Leishmania tarentolae cells were selected step by step for resistance to the polyene antibiotic amphotericin B, a second-line drug against the parasite Leishmania. One of the mutants was cross-resistant to ketoconazole. DNA amplification was observed in both mutants. The amplicons were extrachromosomal circles and were derived from different chromosomes. In one mutant the circle was unusually stable as it remained within the cell despite numerous passages in the absence of the drug. A circumstantial link between the copy number of amplicons and the resistance levels was established. Gene transfection experiments indicated that the link between the locus amplified and the resistance levels was not straightforward and possibly several mutations act together to lead to amphotericin B resistance.     

Efficient presentation of both cytosolic and endogenous transmembrane protein antigens on MHC class II is dependent on cytoplasmic proteolysis

Peptides from extracellular proteins presented on MHC class II are mostly generated and loaded in endolysosomal compartments, but the major pathways responsible for loading peptides from APC-endogenous sources on MHC class II are as yet unclear. In this study, we show that MHC class II molecules present peptides from proteins such as OVA or conalbumin introduced into the cytoplasm by hyperosmotic pinosome lysis, with efficiencies comparable to their presentation via extracellular fluid-phase endocytosis. This cytosolic presentation pathway is sensitive to proteasomal inhibitors, whereas the presentation of exogenous Ags taken up by endocytosis is not. Inhibitors of nonproteasomal cytosolic proteases can also inhibit MHC class II-restricted presentation of cytosolically delivered protein, without inhibiting MHC class I-restricted presentation from the same protein. Cytosolic processing of a soluble fusion protein containing the peptide epitope I-Ealpha(52-68) yields an epitope that is similar to the one generated during constitutive presentation of I-Ealpha as an endogenous transmembrane protein, but is subtly different from the one generated in the exogenous pathway. Constitutive MHC class II-mediated presentation of the endogenous transmembrane protein I-Ealpha is also specifically inhibited over time by inhibitors of cytosolic proteolysis. Thus, Ag processing in the cytoplasm appears to be essential for the efficient presentation of endogenous proteins, even transmembrane ones, on MHC class II, and the proteolytic pathways involved may differ from those used for MHC class I-mediated presentation.     

NF-kappa B RelA (p65) is essential for TNF-alpha-induced fas expression but dispensable for both TCR-induced expression and activation-induced cell death

The Fas death receptor plays a key role in the killing of target cells by NK cells and CTLs and in activation-induced cell death of mature T lymphocytes. These cytotoxic pathways are dependent on induction of Fas expression by cytokines such as TNF-alpha and IFN-gamma or by signals generated after TCR engagement. Although much of our knowledge of the Fas death pathway has been generated from murine studies, little is known about regulatory mechanisms important for murine Fas expression. To this end, we have molecularly cloned a region of the murine Fas promoter that is responsible for mediating TNF-alpha and PMA/PHA-induced expression. We demonstrate here that induction of Fas expression by both stimuli is critically dependent on two sites that associate with RelA-containing NF-kappaB complexes. To determine whether RelA and/or other NF-kappaB subunits are also important for regulating Fas expression in primary T cells, we used CD4 T cells from RelA(-/-), c-Rel(-/-), and p50(-/-) mice. Although proliferative responses were significantly impaired, expression of Fas and activation-induced cell death was unaffected in T cells obtained from these different mice. Importantly, we show that unlike fibroblasts, which consist primarily of RelA-containing NF-kappaB complexes, T cells have high levels of both RelA and c-Rel complexes, suggesting that Fas expression in T cells may be dependent on redundant functions of these NF-kappaB subunits.     

CD134L expression on dendritic cells in the mesenteric lymph nodes drives colitis in T cell-restored SCID mice

Transfer of CD45RB(high) CD4+ T cells to immune-deficient mice in the absence of regulatory T cells leads to a Th1-mediated colitis. In this study, we show that intestinal inflammation is characterized by a 15-fold increase in the number of CD134L+ (OX40L+)-activated DC in the mesenteric lymph nodes (MLNs) compared with BALB/c mice. This was important functionally, as administration of an anti-CD134L mAb inhibited the proliferation of T cells in the MLNs as well as their expression of the gut-homing integrin alpha(4)beta(7). Most importantly, the anti-CD134L mAb completely blocked development of colitis. Surprisingly, CD134L was found to be expressed by a proportion of dendritic cells (DC) in the MLNs of unreconstituted SCID mice, suggesting that CD134L can be induced on DC in the absence of T cell-derived signals. These results indicate that some DC in the MLNs of SCID mice express an activated phenotype and that CD134L expression by these cells is involved in the development of colitis induced by T cell transfer. Accumulation of CD134L+ DC was inhibited by cotransfer of regulatory T cells, suggesting that inhibition of the accumulation of activated DC is one mechanism by which these cells prevent immune pathology.     

Effect of embryonic axis removal and exogenous calcium on carboxypeptidase I of mung bean seedling cotyledons

Removal of the embryonic axis prevents the normal decline of carboxypeptidase (Cpase) I in mung bean seedling cotyledons. Cpase I activity and protein, the latter manifested on western blots, almost completely disappear about 24 h before the cotyledon abscises. Of the 3 proteolytic enzyme patterns, only that of Cpase I can be restored by an exogenous supply of 10 m M CaCl₂ in the agar growth medium. The calcium effect is dependent on [CaCl₂] and is not manifested in the presence of chelators and calcium channel blockers. For detached cotyledons to show the normal low level of Cpase I by the eighth day of growth, calcium had to be supplied during seed imbibition and throughout the entire time from removal of the axis. The difference between detached cotyledons in the absence and presence of calcium was greatest when the cotyledons were detached 4–6 days after seed imbibition. Loss of Cpase I activity and protein can be demonstrated in vitro, with the maximum level of Cpase I-degrading activity measured 4 days after seed imbibition under the same growth conditions used to study the calcium effect. It is sensitive to pepstatin and has a pH optimum of 3, suggesting that this Cpase I-degrading activity is due to an aspartic protease.     

Biotransformation of lantadene A (22β-angeloyloxy-3-oxoolean-12-en-28-oic acid), the pentacyclic triterpenoid, by Alcaligenes faecalis

A bacterial strain capable of biotransformation oflantadene A(22-angeloyloxy-3-oxo-olean-12-en-28-oic acid),the pentacyclic hepatotoxin of lantana (Lantanacamara var. aculeata) has been isolated fromsoil using lantadene A as the sole carbon source. Theorganism is Gram negative, rod shaped, motile,catalase positive and has been identified as Alcaligenes faecalis. The isolate has been found tobe specific for lantadene A and did not utilizelantadene B. In studies using sucrose as an additionalcarbon source, A. faecalis elicitedbiotransformation of lantadene A to its trans isomer22-tigloyloxy-3-oxoolean-12-en-28-oic acid,designated as lantadene X and two other minormetabolites which could not be isolated in pure state.     

The solution structure of a locked nucleic acid (LNA) hybridized to DNA

LNA (Locked Nucleic Acids) is a novel oligonucleotide analogue containing a conformationally restricted nucleotide with a 2'-O, 4'-C-methylene bridge that induces unprecedented thermal affinities when mixed with complementary single stranded DNA and RNA. We have used two-dimensional 1H NMR spectroscopy obtained at 750 and 500 MHz to determine a high resolution solution structure of an LNA oligonucleotide hybridized to the complementary DNA strand. The determination of the structure was based on a complete relaxation matrix analysis of the NOESY cross peaks followed by restrained molecular dynamics calculations. Forty final structures were generated for the duplex from A-type and B-type dsDNA starting structures. The root-mean-square deviation (RMSD) of the coordinates for the forty structures of the complex was 0.32A. The structures were analysed by use of calculated helix parameters. This showed that the values for rise and buckle in the LNA duplex is markedly different from canonical B-DNA at the modification site. A value of twist similar to A-DNA is also observed at the modification site. The overall length of the helix which is 27.3 A. The average twist over the sequence are 35.9 degrees +/- 0.3 degrees. Consequently, the modification does not cause the helix to unwind. The bis-intercalation of the thiazole orange dye TOTO to the LNA duplex was also investigated by 1H NMR spectroscopy to sense the structural change from the unmodified oligonucleotide. We observed that the bis-intercalation of TOTO is much less favourable in the 5'-CT(L)AG-3' site than in the unmodified 5'-CTAG-3' site. This was related to the change in the base stacking of the LNA duplex compared to the unmodified duplex.     

Reverse micelles: a novel tool for H2 production

Reverse micelles serve as a novel tool to entrap enzymes and microbial whole cells within aqueous pockets and can be of great use in enhancing the efficiency and sustainability of the biological system. Photosynthetic bacterium Rhodopseudomonas sphaeroides entrapped inside the aqueous pool of reverse micelles prepared from benzene-sodium lauryl sulphate exhibited 25-fold enhancement of H₂ photoproduction rate (1.67 ml H₂ [mg protein]^–1 h^–1) compared to cells suspended in normal aqueous medium. Hydrogen photoproduction by the bacterium was catalysed by the nitrogenase enzyme system which was supported at a low light intensity of 12 Em^–2 sec^–1 photon flux energy at a wavelength of 520 nm. The optimum temperature for the process was 40 °C.     

Nature of the chromophore binding site of bacteriorhodopsin: the potential role of Arg82 as a principal counterion

The nature of the chromophore binding site of light-adapted bacteriorhodopsin is analyzed by using modified neglect of differential overlap with partial single and double configuration interaction (MNDO-PSDCI) molecular orbital theory to interpret previously reported linear and nonlinear optical spectroscopic measurements. We conclude that in the absence of divalent metal cations in close interaction with Asp85 and Asp212, a positively charged amino acid must be present in the same vicinity. We find that models in which Arg82 is pointed upward into the chromophore binding site and directly stabilizes Asp85 and Asp212 are successful in rationalizing the observed one-photon and two-photon properties. We conclude further that a water molecule is strongly hydrogen bonded to the chromophore imine proton. The chromophore "1Bu*+" and "1Ag*-" states, despite extensive mixing, exhibit significantly different configurational character. The lowest-lying "1Bu*+" state is dominated by single excitations, whereas the second-excited "1Ag*-" state is dominated by double excitations. We can rule out the possibility of a negatively charged binding site, because such a site would produce a lowest-lying "1Ag*-" state, which is contrary to experimental observation. The possibility that Arg82 migrates toward the extracellular surface during the photocycle is examined.