Effects of Ionizable Residues on the Absorption Spectrum and Initial Electron-Transfer Kinetics in the Photosynthetic Reaction Center of Rhodobacter sphaeroides

Effects of ionizable amino acids on spectroscopic properties and electron-transfer kinetics in the photosynthetic reaction center (RC) of Rhodobacter sphaeroides are investigated by site-directed mutations designed to alter the electrostatic environment of the bacteriochlorophyll dimer that serves as the photochemical electron donor (P). Arginine residues at homologous positions in the L and M subunits (L135 and M164) are changed independently: Arg L135 is replaced by Lys, Leu, Glu, and Gln and Arg M164 by Leu and Glu. Asp L155 also is mutated to Asn, Tyr L164 to Phe, and Cys L247 to Lys and Asp. The mutations at L155, L164, and M164 have little effect on the absorption spectrum, whereas those at L135 and L247 shift the long-wavelength absorption band of P to higher energies. Fits to the ground-state absorption and hole-burned spectra indicate that the blue shift and increased width of the absorption band in the L135 mutants are due partly to changes in the distribution of energies for the zero-phonon absorption line and partly to stronger electron-phonon coupling. The initial electron-transfer kinetics are not changed significantly in most of the mutants, but the time constant increases from 3.0 +/- 0.2 in wild-type RCs to 4.7 +/- 0.2 in C(L247)D and 7.0 +/- 0.3 ps in C(L247)K. The effects of the mutations on the solvation free energies of the product of the initial electron-transfer reaction (P(+)) and the charge-transfer states that contribute to the absorption spectrum ( and ) were calculated by using a distance-dependent electrostatic screening factor. The results are qualitatively in accord with the view that electrostatic interactions of the bacteriochlorophylls with ionized residues of the protein are strongly screened and make only minor contributions to the energetics and dynamics of charge separation. However, the slowing of electron transfer in the Cys L247 mutants and the blue shift of the spectrum in some of the Arg L135 and Cys L247 mutants cannot be explained consistently by electrostatic interactions of the mutated residues with P and B(L); we ascribe these effects tentatively to structural changes caused by the mutations.

Spectral characterization of catechol estrogen quinone (CEQ)-derived DNA adducts and their identification in human breast tissue extract

Estrogens, including the natural hormones estrone (E(1)) and estradiol (E(2)), are thought to be involved in tumor induction. Catechol estrogen quinones (CEQ) derived from 4-hydroxyestrone (4-OHE(1)) and 4-hydroxyestradiol (4-OHE(2)) react with DNA and form depurinating N7Gua and N3Ade adducts that might be responsible for tumor initiation (Cavalieri, E. L., et al. (2000) J. Natl. Cancer Inst. Monogr. 27, 75). Current detection limits for the CEQ-derived DNA adducts by high-performance liquid chromatography with multichannel electrochemical detection are in the picomole range. To improve the limit of detection (LOD) for CEQ-derived DNA adducts, spectrophotometric monitoring was investigated. Spectroscopic studies of 4-OHE(1)-1-N3Ade, 4-OHE(1)-1-N7Gua, 4-OHE(2)-1-N3Ade, and 4-OHE(2)-1-N7Gua adduct standards were performed at 77 and 300 K. Upon laser excitation at 257 nm, the 4-OHE(1)- and 4-OHE(2)-derived N7Gua and N3Ade adducts are strongly phosphorescent at T = 77 K. No phosphorescence was observed at 300 K. Both N3Ade and N7Gua adduct types have weak phosphorescence origin bands near 383 and 385 nm, respectively. The corresponding phosphorescence lifetimes are 1.11 +/- 0.05 and 0.37 +/- 0.05 s. The LOD, based on phosphorescence measurements, is in the low femtomole range. The concentration LOD is approximately 10(-9) M, i.e., similar to that recently obtained for CEQ-derived N-acetylcysteine conjugates (Jankowiak, R., et al. (2003) Chem. Res. Toxicol. 16, 304). The LOD in capillary electrophoresis (CE) with field-amplified sample stacking and absorbance detection is about 3 x 10(-8) M. To verify whether CEQ-derived DNA adducts are formed in humans or not, tissue extracts from two breast cancer patients were analyzed by CE interfaced with room temperature absorption and low temperature (laser-excited) phosphorescence spectroscopies. For the first time, formation of CEQ-derived DNA adducts is shown in humans. For example, the level of 4-OHE(1)-1-N3Ade in the breast tissue extract from a patient with breast carcinoma (8.40 +/- 0.05 pmol/g of tissue) is larger by a factor of about 30 than that in the breast tissue sample from a woman without breast cancer (0.25 +/- 0.05 pmol/g of tissue). In contrast, similar amounts of 4-OHE(2)-1-N3Ade were observed in both types of tissue. Although more breast tissue samples from women with and without breast cancer need to be studied, these results suggest that the N3Ade adducts could serve as biomarkers to predict the risk of breast cancer.

Single-cell nonphotochemical hole burning of ovarian surface epithelial carcinoma and normal cells

Persistent spectral nonphotochemical hole-burning (NPHB) spectroscopy has recently been applied to dye molecules in cells. The sensitivity of NPHB to the nanoenvironment of the probe is well established. It has been shown that NPHB applied to bulk suspensions of cultured human cells can distinguish between normal and cancer cells. Thus, NPHB has potential as a diagnostic cancer tool. For this reason, the methodology is referred to as hole-burning imaging, by analogy with MRI. The optical dephasing time (T(2)) of the dye in hole-burning image replaces the proton T(1) relaxation time in MRI. In addition to the T(2) mode of operation, there are four other modes including measurement of the spectral hole growth kinetics (HGK). Reported here is that the selectivity and sensitivity of NPHB operating in the HGK mode allow for distinction between normal and carcinoma cells at the single-cell level. The ovarian cell lines are ovarian surface epithelial cells with temperature-sensitive large T antigens (analogously normal) and ovarian surface epithelial carcinoma (OV167) cells. The mitochondrial specific dye used was rhodamine 800 (Molecular Probes). This carbocationic dye is highly specific for the outer and inner membranes of mitochondria. In line with the results for bulk suspensions of the two cell lines, the hole-burning efficiency for OV167 cells was found to be significantly higher than that for normal cells. Theoretical analysis of the HGK data leads to the conclusion that the degree of structural heterogeneity for the probe-host configurations in OV167 cells is lower than in the normal cells. Possible reasons for this are given.

Carcinoma and SV40-transfected normal ovarian surface epithelial cell comparison by nonphotochemical hole burning

Results are presented of nonphotochemical-hole-burning experiments on the mitochondrial specific dye rhodamine 800 incubated with two human ovarian surface epithelial cell lines: OSE(tsT)-14 normal cells and OV167 carcinoma cells. This dye is selective for the plasma and inner membranes of the mitochondria, as shown by confocal microscopy images. Dispersive hole-growth kinetics of zero-phonon holes are analyzed with theoretical fits, indicating that subcellular structural heterogeneity of the carcinoma cell line is lower relative to the analogous normal cell line. Broadening of holes in the presence of an applied electric field (Stark effect) was used to determine the permanent dipole moment change for the S(0)-->S(1) transition in the two cell lines. For the carcinoma cell line, the permanent dipole moment change value is a factor of 1.5 higher than for the normal cell line. It is speculated that this difference may be related to differences in mitochondrial membrane potentials in the two cell lines.

Glutathione S-transferases as antioxidant defence agents confer pyrethroid resistance in Nilaparvata lugens

Selection of a laboratory colony of the brown planthopper Nilaparvata lugens with the pyrethroids permethrin and lambda-cyhalothrin increased its resistance to both insecticides. Biochemical analysis and synergistic studies with metabolic inhibitors indicated that elevated glutathione S-transferases (GSTs) with a predominant peroxidase activity conferred resistance to both pyrethroids, whereas esterases conferred part of the resistance to permethrin. Purified esterases hydrolysed permethrin at a slow rate, but incubation of either pyrethroid or their primary metabolites with partially purified GSTs had no effect on the metabolic profile. Although GSTs were sensitive to inhibition by both pyrethroids, they did not serve as binding proteins, as previously hypothesized [Grant and Matsumura (1988) Insect Biochem. 18, 615-622]. We demonstrate that pyrethroids, in addition to their neurotoxic effect, induce oxidative stress and lipid peroxidation in insects. Pyrethroid exposure induced lipid peroxides, protein oxidation and depleted reduced glutathione. Elevated GSTs in the resistant strains attenuated the pyrethroid-induced lipid peroxidation and reduced mortality, whereas their in vivo inhibition eliminated their protective role. We therefore hypothesize that the main role of elevated GSTs in conferring resistance in N. lugens is through protecting tissues from oxidative damage. Our study extends the GSTs' range of efficacy to pyrethroid insecticides and possibly explains the role of elevated GSTs in other pyrethroid-resistant insects.

Quantification of pyrethroid insecticides from treated bednets using a mosquito recombinant glutathione S-transferase

Recombinant glutathione S-transferase (agGST1-6) from the malaria vector mosquito Anopheles gambiae Giles (Diptera: Culicidae) was expressed in Escherichia coli using a pET3a vector system. The expressed enzyme was biochemically active with reduced glutathione (GSH) and 1-chloro-2,4-dinitrobenzene (CDNB). Activity of agGST1-6 with GSH and CDNB was inhibited to different degrees by both alpha-cyano and non-alpha-cyano pyrethroid insecticides. This inhibition was used to develop an assay for quantification of pyrethroids. Standard curves of insecticide concentration against percentage of enzyme inhibition or volume of iodine solution were established by spectrophotometry and iodine volumetric titration, respectively, for permethrin and deltamethrin. These assays allowed estimation of pyrethroid concentrations both spectrophotometrically and visually. For the residue assay of each insecticide, a cut-off point of 50% of the initial pyrethroid impregnation concentration was used, which should differentiate between biologically active and inactive treated bednets. The cross-reactivity of the primary permethrin photodegradants (3-phenoxyalcohol and 3-phenoxybenzoic acid) with the recombinant agGST1-6 was assayed in the same system. No agGST1-6 inhibition by the insecticide metabolites was observed, suggesting that the system is unaffected by primary permethrin metabolites and will accurately measure insecticide parent compound concentrations. The estimated pyrethroid insecticide concentrations, given spectrophotometrically and by iodine titration assay, were comparable to those obtained by direct HPLC quantification of residues extracted from bednets. Hence, it should be relatively easy to adapt this method to produce a test kit for residue quantification in the field.

High-performance liquid chromatography interfaced with fluorescence line-narrowing spectroscopy for on-line analysis

We have demonstrated, for the first time, that high-performance liquid chromatography (HPLC) can be interfaced with fluorescence line-narrowing spectroscopy (FLNS) for on-line identification and characterization of analytes. Interfacing centered primarily on the design and construction of a novel liquid helium cryostat that accommodates variable-sized quartz tubes/capillaries suitable for HPLC as well as capillary electrophoresis/electrochromatography. In addition to the high spectral resolution afforded by FLNS, analyzing the separated components at 4.2 K minimizes photodegradation from the excitation source and provides indefinite detection times for signal averaging. The proof-of-principle for the HPLC-FLNS system is first demonstrated with a mixture of four structurally similar polycyclic aromatic hydrocarbons and then applied to the analysis of DNA adducts from mouse skin exposed to the carcinogen dibenzo[a,l]pyrene. With femtomole detection limits, HPLC-FLNS can be used for real-world analyses of complex mixtures.

Detection and quantification of depurinated benzo[a]pyrene-adducted DNA bases in the urine of cigarette smokers and women exposed to household coal smoke

Polycyclic aromatic hydrocarbons (PAH) are metabolized to electrophiles that can bind to DNA bases and destabilize the N-glycosyl bond, causing rapid depurination of the adducted bases. Recent studies support depurination of DNA as a mechanism central to the genesis of H-ras mutations in PAH-treated mouse skin. Depurinating adducts account for 71% of all DNA adducts formed in mouse skin treated with benzo[a]pyrene (BP). This study analyzed urine of cigarette smokers, coal smoke-exposed women, and nonexposed controls for the presence and quantities of the depurinated BP-adducted DNA bases, 7-(benzo[a]pyren-6-yl)guanine (BP-6-N7Gua) and 7-(benzo[a]pyren-6-yl)adenine (BP-6-N7Ade). Since these adducted bases originate from reaction of the BP radical cation with double-stranded DNA and not with RNA or denatured DNA, their presence in urine is indicative of DNA damage. Urine samples were fractionated by a combination of SepPak extraction and reverse-phase HPLC, and then analyzed by tandem mass spectrometry and capillary electrophoresis with laser-induced fluorescence. BP-adducted bases were detected in the urine from three of seven cigarette smokers and three of seven women exposed to coal smoke, but were not detected in urine from the 13 control subjects. Concentrations were estimated to be 60-340 and 0.1-0.6 fmol/mg of creatinine equivalent of urine for coal smoke-exposed women (maximum possible BP intake of ca. 23 000 ng/day) and cigarette smokers (BP intake of ca. 800 ng/day), respectively, exhibiting a sensitive response to BP exposures. BP-6-N7Gua was present at ca. 20-300 times the concentration of BP-6-N7Ade in the urine of coal smoke-exposed women, but was not detected in the urine of cigarette smokers. This difference may be due to the remarkably different BP exposures experienced by the two groups of PAH-exposed individuals. These results justify more extensive studies of depurinated BP-adducted DNA bases as potential biomarkers of PAH-associated cancer risk.

Molecular characterization of the amplified carboxylesterase gene associated with organophosphorus insecticide resistance in the brown planthopper, Nilaparvata lugens

Widespread resistance to organophosphorus insecticides (OPs) in Nilaparvata lugens is associated with elevation of carboxylesterase activity. A cDNA encoding a carboxylesterase, Nl-EST1, has been isolated from an OP-resistant Sri Lankan strain of N. lugens. The full-length cDNA codes for a 547-amino acid protein with high homology to other esterases/lipases. Nl-EST1 has an N-terminal hydrophobic signal peptide sequence of 24 amino acids which suggests that the mature protein is secreted from cells expressing it. The nucleotide sequence of the homologue of Nl-EST1 in an OP-susceptible, low esterase Sri Lankan strain of N. lugens is identical to Nl-EST1. Southern analysis of genomic DNA from the Sri Lankan OP-resistant and susceptible strains suggests that Nl-EST1 is amplified in the resistant strain. Therefore, resistance to OPs in the Sri Lankan strain is through amplification of a gene identical to that found in the susceptible strain.

Comparison of esterase gene amplification, gene expression and esterase activity in insecticide susceptible and resistant strains of the brown planthopper, Nilaparvata lugens (Stål)

Organophosphorus and carbamate insecticide resistance in Nilaparvata lugens is based on amplification of a carboxylesterase gene, Nl-EST1. An identical gene occurs in susceptible insects. Quantitative real-time PCR was used to demonstrate that Nl-EST1 is amplified 3-7-fold in the genome of resistant compared to susceptible planthoppers. Expression levels were similar to amplification levels, with 1-15-fold more Nl-EST1 mRNA in individual insects and 5-11-fold more Nl-EST1 mRNA in mass whole body homogenates of resistant females compared to susceptibles. These values corresponded to an 8-10-fold increase in esterase activity in the head and thorax of individual resistant insects. Although amplification, expression and activity levels of Nl-EST1 in resistant N. lugens were similar, the correlation between esterase activity and Nl-EST1 mRNA levels in resistant individuals was not linear.