The peptide-substrate-binding domain of human collagen prolyl 4-hydroxylases. Backbone assignments,secondary structure,and binding of proline-rich peptides

The collagen prolyl 4-hydroxylases (C-P4Hs) catalyze the formation of 4-hydroxyproline by the hydroxylation of proline residues in -Xaa-Pro-Gly-sequences. The vertebrate enzymes are alpha 2 beta 2 tetramers in which protein-disulfide isomerase serves as the beta subunit. Two isoforms of the catalytic alpha subunit have been identified and shown to form [alpha(I)]2 beta 2 and [alpha(II)]2 beta 2 tetramers, the type I and type II C-P4Hs, respectively. The peptide-substrate-binding domain of type I C-P4H has been shown to be located between residues 138 and 244 in the 517-residue alpha(I) subunit and to be distinct from the catalytic domain that is located in the C-terminal region. We report here that a recombinant human C-P4H alpha(I) polypeptide Phe144-Ser244 forms a folded domain consisting of five alpha helices and one short beta strand. This structure is quite different from those of other proline-rich peptide-binding modules, which consist mainly of beta strands. Binding of the peptide (Pro-Pro-Gly)2 to this domain caused major chemical shifts in many backbone amide resonances, the residues showing the largest shifts being mainly hydrophobic, including three tyrosines. The Kd values determined by surface plasmon resonance and isothermal titration calorimetry for the binding of several synthetic peptides to the alpha(I) and the corresponding alpha(II) domain were very similar to the Km and Ki values for these peptides as substrates and inhibitors of the type I and type II C-P4H tetramers. The Kd values of the alpha(I) and alpha(II) domains for (Gly-Pro-4Hyp)5 were much higher than those for (Pro-Pro-Gly)5, indicating a marked decrease in the affinity of hydroxylated peptides for the domain. Many characteristic features of the binding of peptides to the type I and type II C-P4H tetramers can thus be explained by the properties of binding to this domain rather than the catalytic domain.     

The second type II module from human matrix metalloproteinase 2: structure, function and dynamics.

BACKGROUND: Matrix metalloproteinase 2 (MMP-2, gelatinase A, 72 kDa type IV collagenase) has an important role in extracellular matrix degradation during cell migration and tissue remodeling. It is involved in development, inflammation, wound healing, tumor invasion, metastasis and other physiological and pathological processes. The enzyme cleaves several types of collagen, elastin, fibronectin and laminin. Binding to collagen is mediated by three repeats homologous to fibronectin type II modules, which are inserted in the catalytic domain in proximity to the active site. RESULTS: We have determined the NMR solution structure of the second type II module from human MMP-2 (col-2). The module exhibits a typical type II fold with two short double-stranded antiparallel beta sheets and three large loops packed around a cluster of conserved aromatic residues. Backbone amide dynamics, derived from (15)N relaxation experiments, correlate well with solvent accessibility and intramolecular hydrogen bonding. A synthetic peptide with the collagen consensus sequence, (Pro-Pro-Gly)(6), is shown to interact with the module. CONCLUSIONS: Spectral perturbations induced by (Pro-Pro-Gly)(6) binding reveal the region involved in the interaction of col-2 with collagen. The binding surface comprises exposed aromatic residues Phe21, Tyr38, Trp40, Tyr47, Tyr53 and Phe55, and the neighboring Gly33-Gly37 segment.     

Histone peptide AKRHRK enhances H(2)O(2)-induced DNA damage and alters its site specificity

Histone proteins are involved in compaction of DNA and the protection of cells from oxygen toxicity. However, several studies have demonstrated that the metal-binding histone reacts with H(2)O(2), leading to oxidative damage to a nucleobase. We investigated whether histone can accelerate oxidative DNA damage, using a minimal model for the N-terminal tail of histone H4, CH(3)CO-AKRHRK-CONH(2), which has a metal-binding site. This histone peptide enhanced DNA damage induced by H(2)O(2) and Cu(II), especially at cytosine residues, and induced additional DNA cleavage at the 5'-guanine of GGG sequences. The peptide also enhanced the formation of 8-oxo-7,8-dihydro-2'-deoxyguanosine and ESR spin-trapping signal from H(2)O(2) and Cu(II). Cyclic redox reactions involving histone-bound Cu(II) and H(2)O(2), may give rise to multiple production of radicals leading to multiple hits in DNA. It is noteworthy that the histone H4 peptide with specific sequence AKRHRK can cause DNA damage rather than protection under metal-overloaded condition.     

The hydrogen peroxide/copper ion system, but not other metal-catalyzed oxidation systems, produces protein-bound dityrosine

Dityrosine formation leads to the cross-linking of proteins intra- or intermolecularly. The formation of dityrosine in lens proteins oxidized by metal-catalyzed oxidation (MCO) systems was estimated by chemical and immunochemical methods. Among the four MCO systems examined (H(2)O(2)/Cu, H(2)O(2)/Fe-ethylenediaminetetraacetic acid (Fe-EDTA), ascorbate/Cu, ascorbate/Fe-EDTA), the treatment with H(2)O(2)/Cu preferentially caused dityrosine formation in the lens proteins. The success of oxidative protein modification with all the MCO systems was confirmed by carbonyl formation estimated using 2,4-dinitrophenylhydrazine. The loss of tyrosine by the MCO systems was partly due to the formation of protein-bound 3,4-dihydroxyphenylalanine. The formation of dityrosine specific to H(2)O(2)/Cu was confirmed by using poly-(Glu, Ala, Tyr) and N-acetyl-tyrosine as a substrate. The dissolved oxygen concentration in the H(2)O(2)/Cu system hardly affected the amount of dityrosine formation, suggesting that dityrosine generation by the H(2)O(2)/Cu system is independent of oxygen concentration. Moreover, the combination of copper ion with H(2)O(2) is the most effective system for dityrosine formation among various metal ions examined. The addition of reducing agents, glutathione or ascorbic acid, into the H(2)O(2)/Cu system suppressed the generation of the dityrosine moiety, suggesting effective quench of tyrosyl radicals by the reducing agents.     

Structure of proline 3-hydroxylase. Evolution of the family of 2-oxoglutarate dependent oxygenases.

Iron (II)/2-oxoglutarate (2-OG)-dependent oxygenases catalyse oxidative reactions in a range of metabolic processes including the hydroxylation of proline and lysine residues during the post-translational modification of collagen. 2-OG oxygenases commonly require ascorbate for full activity. In the vitamin C deficient disease, scurvy, reduced activity of 2-OG oxygenases results in impaired formation of collagen. Here we report the crystal structure of bacterial proline 3-hydroxylase from Streptomyces sp., an enzyme which hydroxylates proline at position 3, the first of a 2-OG oxygenase catalysing oxidation of a free alpha-amino acid. Structures were obtained for the enzyme in the absence of iron (to 2.3A resolution, R=20.2%, Rfree=25.3%) and that complexed to iron (II) (to 2.4A resolution, R=19.8%, Rfree=22.6%). The structure contains conserved motifs present in other 2-OG oxygenases including a 'jelly roll' beta strand core and residues binding iron and 2-oxoglutarate, consistent with divergent evolution within the extended family. The structure differs significantly from many other 2-OG oxygenases in possessing a discrete C-terminal helical domain. Analysis of the structure suggests a model for proline binding and a mechanism for uncoupling of proline and 2-OG turnover.     

Site-specific and random fragmentation of Cu,Zn-superoxide dismutase by glycation reaction. Implication of reactive oxygen species.

Site-specific and random fragmentation of human Cu,Zn-superoxide dismutase (Cu,Zn-SOD) was observed following the glycation reaction (the early stage of the Maillard reaction). The fragmentation proceeded in two steps. In the first step, Cu,Zn-SOD was cleaved at a peptide bond between Pro62 and His63, as judged by amino acid analysis and sequencing of fragment peptides, yielding a large (15 kDa) and a small (5 kDa) fragment. In the second step, random fragmentation occurred. The ESR spectrum of the glycated Cu,Zn-SOD suggested that reactive oxygen species was implicated in the both steps of fragmentation. The same fragmentations were seen upon exposure of the enzyme to an H2O2 bolus. Catalase completely blocked both steps of the fragmentation process, whereas EDTA blocked only the second step. Incubation with glucose resulted in a time-dependent release of Cu2+ from the Cu,Zn-SOD molecule. The released Cu2+ then likely participated in a Fenton's type of reaction to produce hydroxyl radical, which may cause the nonspecific fragmentation. Evidence that EDTA abolished only the second step of fragmentation induced by an H2O2 bolus supports this mechanism. This is the first report that a site-specific fragmentation of a protein is caused by reactive oxygen species formed by the Maillard reaction.     

Effect of zinc ion on the interaction of some amino acid compounds of copper(II) with hydrogen peroxide.

Reaction of elemental copper and zinc powder mixtures with glycine (NH2.CH2COOH; HA) or aspartic acid (NH2CHCOOHCH2COOH; H2B) (in 1:1:2 ratio, respectively) in the presence of excess hydrogen peroxide (H2O2) at 50 degrees C, results in the formation of a new mixed metal peroxy carbonate compound corresponding to formula [Cu(Zn)2(O2(2-) (CO3)2(H2O)4], while the same reaction with elemental copper powder alone yields merely peroxy amino acid compounds having the formula [Cu(O2(2-)) (HA)2(H2O)] and [Cu(O2(2-)) (H2B) (H2O)2] for glycine and aspartic acid, respectively. These compounds have been characterized by elemental analysis, ESR, and electronic and IR spectra. It is interesting to note that both amino acids are converted to carbonate in the presence of zinc alone. A method analogous to that described above, for the reaction of elemental copper, zinc powder mixtures with succinic acid [(CH2COOH)2] or acetic acid (CH3COOH) in excess H2O2, on the other hand, gave a product essentially comprising copper succinate or acetate, respectively. These observations suggest an interesting and perhaps important phenomenon by which only the simple amino acids such as glycine and aspartic acid are converted to carbonates while their corresponding carboxylic acids form only their respective salts.     

Site-specific fragmentation and modification of albumin by sulfite in the presence of metal ions or peroxidase/H2O2: role of sulfate radical.

Exposure of albumin to sulfite in the presence of Co(II) or peroxidase/H2O2 caused site-specific fragmentation, which was not due to cleavage of methionyl nor tryptophanyl peptide bonds. The reaction of GlyPro with sulfite in the presence of Co(II) or peroxidase/H2O2 led to Gly liberation, suggesting the oxidative cleavage of protein at Pro residues. Sulfite plus Co(II) induced bityrosine production, Trp loss and a new Trp-derived fluorescence. ESR-spin trapping method provided evidence for the formation of sulfate radical (SO4.-) during Co(II)-catalyzed autoxidation of sulfite. The order of reactivity with SO4.- seemed to be Trp greater than GlyPro greater than GlyGly approximately Gly approximately Pro. The results suggest that SO4.- plays an important role in fragmentation and modification of albumin.     

Synthesis and degradation rates of collagens in vivo in whole skin of rats, studied with 1802 labelling.

Rats of synthesis and degradation in vivo of collagens in 0.5 M-acetic acid-soluble and -insoluble extracts from skins of three growing rats were determined by using a labelling procedure involving exposure of the animals to an atmosphere of 18O2 for 36 h. For comparison, rats also received injections of [2H]proline. Serial skin biopsies were taken at frequent intervals over 392 days. Enrichment of 18O and 2H in the hydroxyproline of the collagen fractions was determined by gas chromatography-mass spectrometry. Changes in size of the soluble and insoluble collagen pools were considered in the evaluation of isotope kinetic data. The insoluble collagen fraction showed no degradation. The efflux (mean +/- S.D., expressed as mumol of hydroxyproline) from the soluble collagen pool was estimated to be 59.9 +/- 1.9 per day from the 18O data, and 25.5 +/- 7.5 per day from the 2H results. The finding indicates significant reutilization of 2H-radiolabelled proline for hydroxyproline synthesis. From these isotope data and estimates of size of the collagen pools it was determined that 55% of the collagen disappearing from the soluble pool was due to maturation into insoluble collagens and 45% of the disappearance was a result of actual degradation of soluble collagen. These results confirm the utility of 18O2 as a non-reutilizable label for studies of collagen turnover in vivo.     

Discrete reduction of type I collagen thermal stability upon oxidation

The oxidation of acid-soluble calf skin collagen type I caused by metal-dependent free radical generating systems, Fe(II)/H2O2 and Cu(II)/H2O2, was found to bring down in a specific, discrete way the collagen thermal stability, as determined by microcalorimetry and scanning densitometry. Initial oxidation results in splitting of the collagen denaturational transition into two components. Along with the endotherm at 41 degrees C typical for non-oxidized collagen, a second, similarly cooperative endotherm appears at 35 degrees C and increases in enthalpy with the oxidant concentration and exposure time, while the first peak correspondingly decreases. The two transitions at 35 and 41 degrees C were registered by densitometry as stepwise increases of the collagen-specific volume. Further oxidation results in massive collagen destruction manifested as abolishment of both denaturational transitions. The two oxidative systems used produce identical effects on the collagen stability but at higher concentrations of Cu(II) in comparison to Fe(II). The discrete reduction of the protein thermal stability is accompanied by a decrease of the free amino groups, suggestive of an oxidation attack of the side chains of lysine residues. Since the denaturation temperature of collagen shifts from above to below body temperature (41 degrees C-35 degrees C) upon oxidation, it appears important to account for this effect in a context of the possible physiological implications of collagen oxidation.     

Proton magnetic resonance of the triple helix of poly(prolylprolylglycyl)n with defined degree of polymerization

Proton magnetic resonance spectra at 220 MHz were obtained for deuterium oxide and aqueous solutions of the polytripeptides (Pro-Pro-Gly)_n, which were synthesized as collagen models by the modified solid phase method. At higher temperatures, the signals of the proline C_a-protons for the peptides with n ≦ 5 and for those with n = 10 and 15 demonstrate the presence of cis and trans isomers with respect to the Gly-Pro or Pro-Pro C-N bonds. Glycine C_a-protons give typical AB type patterns. At lower temperatures, as the peptides with n = 5, 10 and 15 form triple helices, all of the resonance peaks become broad, but the whole form of the spectrum is quite similar to that of poly(l-proline) form II. The glycine C_a-proton resonances become barely detectable and the upfield peak of the two proline C_a-proton resonances fade away. At the same time, a new glycine NH resonance appears at a field slightly higher than that of a random coil. It seems to suggest that the formation of triple helices accompanies the conversion of cis proline peptide bonds into all trans bonds, and that the glycine residue environment completely changes in the helix.     

Aspirin potently inhibits oxidative DNA strand breaks: implications for cancer chemoprevention

Epidemiological studies have suggested that the use of aspirin is associated with a decreased incidence of human malignancies, particularly colorectal cancer. Since reactive oxygen species (ROS) are critically involved in multistage carcinogenesis, this study was undertaken to examine the ability of aspirin to inhibit ROS-mediated DNA damage. Hydrogen peroxide (H2O2)+Cu(II) and hydroquinone (HQ) + Cu(II) were used to cause oxidative DNA strand breaks in phiX-174 plasmid DNA. We demonstrated that the presence of aspirin at concentrations (0.5-2 mM) compatible with amounts in plasma during chronic anti-inflammatory therapy resulted in a marked inhibition of oxidative DNA damage induced by either H2O2/Cu(II) or HQ/Cu(II). The inhibition of oxidative DNA damage by aspirin was exhibited in a concentration-dependent manner. Moreover, aspirin was found to be much more potent than the hydroxyl radical scavengers, mannitol and dimethyl sulfoxide, in protecting against the H2O2/Cu(II)-mediated DNA strand breaks. Since the reduction of Cu(II) to Cu(I) is crucially involved in both H2O2/Cu(II)- and HQ/Cu(II)-mediated formation of hydroxyl radical or its equivalent, and the subsequent oxidative DNA damage, we examined whether aspirin could inhibit this Cu(II)/Cu(I) redox cycle. It was observed that aspirin at concentrations that showed the inhibitory effect on oxidative DNA damage did not alter the Cu(II)/Cu(I) redox cycle in either H2O2/Cu(II) or HQ/Cu(II) system. In addition, aspirin was not found to significantly scavenge H2O2. This study demonstrates for the first time that aspirin potently inhibits both H2O2/Cu(II)- and HQ/Cu(II)-mediated oxidative DNA strand breaks most likely through scavenging the hydroxyl radical or its equivalent derived from these two systems. The potent inhibition of oxidative DNA damage by aspirin may thus partially contribute to its anticancer activities observed in humans.     

Hypoxia inhibits amino acid uptake in human lung fibroblasts.

Hypoxia and amino acid deprivation downregulate expression of extracellular matrix genes in lung fibroblasts. We examined the effect of hypoxia on amino acid uptake and protein formation in human lung fibroblasts. Low O(2) tension (0% O(2)) suppressed incorporation of [(3)H]proline into type I collagen without affecting [(35)S]methionine labeling of other proteins. Initial decreases in intracellular [(3)H]proline incorporation occurred after 2 h of exposure to 0% O(2), with maximal suppression of intracellular [(3)H]proline levels at 6 h of treatment. Hypoxia significantly inhibited the uptake of radiolabeled proline, 2-aminoisobutyric acid (AIB), and 2-(methylamino)isobutyric acid (methyl-AIB) while inducing minor decreases in leucine transport. Neither cycloheximide nor indomethacin abrogated hypoxia-related suppression of methyl-AIB uptake. Efflux studies demonstrated that hypoxia inhibited methyl-AIB transport in a bidirectional fashion. The downregulation of amino acid transport was not due to a toxic effect; function recovered on return to standard O(2) conditions. Kinetic analysis of AIB transport revealed a 10-fold increase in K(m) accompanied by a small increase in maximal transport velocity among cells exposed to 0% O(2). These data indicate that low O(2) tension regulates the system A transporter by decreasing transporter substrate affinity.     

High yield of endoreduplication induced by ICRF-193: a topoisomerase II catalytic inhibitor

Previous studies have demonstrated that phenolic compounds, including genistein (4',5,7-trihydroxyisoflavone) and resveratrol (3,4',5-trihydroxystilbene), are able to protect against carcinogenesis in animal models. This study was undertaken to examine the ability of genistein and resveratrol to inhibit reactive oxygen species (ROS)-mediated strand breaks in phi X-174 plasmid DNA. H(2)O(2)/Cu(II) and hydroquinone/Cu(II) were used to cause oxidative DNA strand breaks in the plasmid DNA. We demonstrated that the presence of genistein at micromolar concentrations resulted in a marked inhibition of DNA strand breaks induced by either H(2)O(2)/Cu(II) or hydroquinone/Cu(II). Genistein neither affected the Cu(II)/Cu(I) redox cycle nor reacted with H(2)O(2) suggest that genistein may directly scavenge the ROS that participate in the induction of DNA strand breaks. In contrast to the inhibitory effects of genistein, the presence of resveratrol at similar concentrations led to increased DNA strand breaks induced by H(2)O(2)/Cu(II). Further studies showed that in the presence of Cu(II), resveratrol, but not genistein was able to cause DNA strand breaks. Moreover, both Cu(II)/Cu(I) redox cycle and H(2)O(2) were shown to be critically involved in resveratrol/copper-mediated DNA strand breaks. The above results indicate that despite their similar in vivo anticarcinogenic effects, genistein and resveratrol appear to exert different effects on oxidative DNA damage in vitro.     

The applicability of subtilisin Carlsberg in peptide synthesis.

The synthesis of peptide bonds catalysed by subtilisin Carlsberg was studied in different hydrophilic organic solvents with variable H2O concentration. Z-Val-Trp-OMe and Z-Ala-Phe-OMe were used as acyl donors, and a series of amino acid derivatives, di- and tripeptides of the general structure Xaa-Gly, Gly-Xaa, Gly-Gly-Xaa (Xaa represents all natural L-amino acids except cysteine) and other peptides were used as nucleophiles. A comparative study of the enzymatic synthesis in aqueous DMF (50%, v/v) and acetonitrile containing 10% (v/v) of H2O demonstrated that the yields of peptide products were higher in most cases when acetonitrile with low H2O concentration was used. The acylation of weak nucleophiles was improved in organic solvents with very low H2O concentration (2%). The reactions in anhydrous Bu(t)-OH proceeded with substantially lower velocity. Generally, the restricted nucleophile specificity of the enzyme for glycine and hydrophilic amino acid residues in P1' position, as well as numerous side reactions, limit the utilization of subtilisin in peptide synthesis, especially in the case of the segment condensations. Contrary to the published data, we have proved that proline derivatives were not acylated in any media with the help of subtilisin Carlsberg. Effective ester hydrolysis of a protected nonapeptide corresponding to the N-terminal sequence of dicarba-eel-calcitonin catalysed by subtilisin was achieved.     

Repetitive interactions observed in the crystal structure of a collagen-model peptide, [(Pro-Pro-Gly)9]3

The crystal structure of a collagen-model peptide [(Pro-Pro-Gly)(9)](3) has been determined at 1.33 A resolution. Diffraction data were collected at 100 K using synchrotron radiation, which led to the first structural study of [(Pro-Pro-Gly)(n)](3) under cryogenic conditions. The crystals belong to the P2(1) space group with cell parameters of a = 25.95, b = 26.56, c = 80.14 Angstroms and beta = 90.0 degrees. The overall molecular conformation was consistent with the left-handed 7/2-helical model with an axial repeat of 20 A for native collagen. A total of 332 water molecules were found in an asymmetric unit. Proline residues in adjacent triple-helices exhibited three types of hydrophobic interactions. Furthermore, three types of hydrogen-bonding networks mediated by water molecules were observed between adjacent triple-helices. These hydrophobic interactions and hydrogen-bonding networks occurred at intervals of 20 Angstroms along the c-axis based on the previous sub-cell structures [(Pro-Pro-Gly)(n)](3) (n = 9, 10), which were also seen in the full-cell structure of [(Pro-Pro-Gly)(10)](3). Five proline residues at the Y position in the X-Y-Gly triplet were found in a down-puckering conformation, this being inconsistent with the recently proposed propensity-based hypothesis. These proline residues were forced to adopt opposing puckering because of the prevailing hydrophobic interaction between triple-helices compared with the Pro:Pro stacking interaction within a triple-helix.     

Copper redox-dependent activation of 2-tert-butyl(1,4)hydroquinone: formation of reactive oxygen species and induction of oxidative DNA damage in isolated DNA and cultured rat hepatocytes

The biotransformation of butylated hydroxyanisole (BHA), a possible carcinogenic food antioxidant, includes o-demethylation to 2-tert-butyl(1,4)hydroquinone (TBHQ) which can subsequently be oxidized to 2-tert-butyl(1,4)paraquinone (TBQ). In this study, we have examined the capacity of Cu, a nuclei- and DNA-associated transition metal, to mediate the oxidation of TBHQ. In phosphate buffered saline (PBS), autooxidation of TBHQ to TBQ was not detectable, while Cu(II) at micromolar concentrations strongly catalyzed the oxidation of TBHQ to TBQ. Oxidation of TBHQ by Cu(II) was accompanied by the utilization of O(2) and the concomitant generation of H(2)O(2). Using electron spin resonance spectroscopy, it was observed that Cu(II) mediated the one electron oxidation of TBHQ to a semiquinone anion radical. The formation of a semiquinone anion radical, the utilization of O(2) and the generation of H(2)O(2) and TBQ could be completely blocked by bathocuproinedisulfonic acid (BCS) and reduced glutathione (GSH), two Cu(I)-chelators. 4-Pyridyl-1-oxide-N-tert-butylnitrone (POBN)-spin trapping experiments showed that the reaction of TBHQ with Cu(II) resulted in the generation of POBN-CH(3) and POBN-CH(OH)CH(3) adducts in the presence of dimethyl sulfoxide (DMSO) and ethanol, respectively, suggesting the formation of hydroxyl radical or a similar reactive intermediate. The formation of POBN-CH(3) adduct from the TBHQ/Cu(II)+DMSO could be completely inhibited by catalase, GSH or BCS, indicating that the hydroxyl radical or its equivalent is generated from the interaction of H(2)O(2) with Cu(I). Incubation of supercoiled phiX-174 plasmid DNA with the TBHQ/Cu(II) resulted in extensive DNA strand breaks, which could be prevented by catalase or BCS. Incubation of rat hepatocytes with TBHQ in PBS led to increased formation of 8-hydroxy-2'-deoxyguanosine (8-OHdG) in nuclear DNA. The TBHQ-induced formation of 8-OHdG was markedly reduced in the presence of cell permeable Cu(I)-specific chelator, bathocuproine or neocuproine, suggesting that a Cu(II)/Cu(I) redox mechanism may also be involved in the induction of oxidative DNA damage by TBHQ in hepatocytes. Taken together, the above results conclusively demonstrate that the activation of TBHQ by Cu(II) results in the formation of TBQ, semiquinone anion radical and reactive oxygen species (ROS), and that the ROS formed may participate in oxidative DNA damage in both isolated DNA and intact cells. These reactions may contribute to the carcinogenicity as well as other biochemical activities observed with BHA in animals. To our knowledge this study provides the first evidence that endogenous cellular Cu may be capable of bioactivating TBHQ, leading to oxidative DNA damage in cultured cells.     

Inactivation of Trypanosoma cruzi and Crithidia fasciculata topoisomerase I by Fenton systems

Fenton systems (H(2)O(2)/Fe(II) or H(2)O(2)/Cu(II)) inhibited Trypanosoma cruzi and Crithidia fasciculata topoisomerase I activity. About 61-71% inactivation was produced by 25 microM Fe(II) or Cu(II) with 3.0 mM H(2)O(2). Thiol compounds and free radical scavengers prevented Fenton system effects, depending on the topoisomerase assayed. With the T. cruzi enzyme, reduced glutathione (GSH), dithiothreitol (DTT), cysteine and N-acetyl-L-cysteine (NAC) entirely prevented the effect of the H(2)O(2)/Fe(II) system; mannitol protected 37%, whereas histidine and ethanol were ineffective. With C. fasciculata topoisomerase, GSH, DTT and NAC protected 100%, cysteine, histidine and mannitol protected 28%, 34% and 48%, respectively, whereas ethanol was ineffective. With the H(2)O(2)/Cu(II) system and T. cruzi topoisomerase, DTT and histidine protected 100% and 60%, respectively, but the other assayed protectors were less effective. Similar results were obtained with the C. fasciculata enzyme. Topoisomerase inactivation by the H(2)O(2)/Fe(II) or H(2)O(2)/Cu(II) systems proved to be irreversible since it was not reversed by the more effective enzyme protectors. It is suggested that topoisomerases could act either as targets of 'reactive oxygen species' (ROS) generated by Fenton systems or bind the corresponding metal ions, whose redox cycling would generate reactive oxygen species in situ.     

Products of Cu(II)-catalyzed oxidation in the presence of hydrogen peroxide of the 1-10, 1-16 fragments of human and mouse beta-amyloid peptide

The interactions of proteins with reactive oxygen species (ROS) may result in covalent modifications of amino acid residues in proteins, formation of protein-protein cross-linkages, and oxidation of the protein backbone resulting in protein fragmentation. In an attempt to elucidate the products of the metal-catalyzed oxidation of the human (H) and mouse (M) (1-10H), (1-10M), (1-16H) and (1-16M) fragments of beta-amyloid peptide, the high performance liquid chromatography (HPLC) and matrix-assisted laser desorption/ionization mass spectrometry (MALDI-TOF MS) methods and Cu(II)/H(2)O(2) as a model oxidizing system were employed. Peptide solution (0.50 mM) was incubated at 37 degrees C for 24 h with metal:peptide:H(2)O(2) molar ratio 1:1:1 for the (1-16H), (1-16M) fragments, and 1:1:2 for the (1-10H), (1-10M) peptides in phosphate buffer, pH 7.4. Oxidation targets for all peptide studied are the histidine residues coordinated to the metal ions. For the (1-16H) peptide are likely His(13) and/or His(14), and for the (1-16M) fragment His(6) and/or His(14), which are converted to 2-oxo-His. Metal-binding residue, the aspartic acid (D(1)) undergoes the oxidative decarboxylation and deamination to pyruvate. The cleavages of the peptide bonds by either the diamide or alpha-amidation pathways were also observed.