Copper(II).bleomycin, iron(III).bleomycin, and copper(II).phleomycin: comparative study of deoxyribonucleic acid binding

The kinetics and mechanism of binding of Cu-(II).bleomycin, Fe(III).bleomycin, and Cu(II).phleomycin to DNA were studied by using fluorometry, equilibrium dialysis, electric dichroism, and temperature-jump and stopped-flow spectrophotometry. The affinity of Cu(II).bleomycin for DNA was greater than that of metal-free bleomycin but less than that of Fe(III).bleomycin. Cu(II).bleomycin exhibited a two-step binding process, with the slow step indicating a lifetime of 0.1 s for the Cu(II).bleomycin.DNA complex. Fe(III).bleomycin binding kinetics indicated the presence of complexes having lifetimes of up to 22 s. DNA was lengthened by 4.6 A/molecule of bound Cu(II).bleomycin and by 3.2 A/bound Fe(III).bleomycin but not at all by Cu(II).phleomycin, suggesting that both bleomycin complexes intercalate while the phleomycin complex does not. However, phleomycin exhibited nearly the same specificity of DNA base release as bleomycin. These results suggest that the coordinated metal ion plays a major role in the binding of metal-bleomycin complexes to DNA but that intercalation is neither essential for DNA binding and degradation nor primarily responsible for the specificity of DNA base release by these drugs.     

Bleomycin may be activated for DNA cleavage by NADPH-cytochrome P-450 reductase

In the presence of NADPH and O2, NADPH-cytochrome P-450 reductase was found to activate Fe(III)-bleomycin A2 for DNA strand scission. Consistent with observations made previously when cccDNA was incubated in the presence of bleomycin and Fe(II) + O2 or Fe(III) + C6H5IO, degradation of DNA by NADPH-cytochrome P-450 reductase activated Fe(III)-bleomycin A2 produced both single- and double-strand nicks with concomitant formation of malondialdehyde (precursors). Cu(II)-bleomycin A2 also produced nicks in SV40 DNA following activation with NADPH-cytochrome P-450 reductase, but these were not accompanied by the formation of malondialdehyde (precursors). These findings confirm the activity of copper bleomycin in DNA strand scission and indicate that it degrades DNA in a fashion that differs mechanistically from that of iron bleomycin. The present findings also-establish the most facile pathways for enzymatic activation of Fe(III)-bleomycin and Cu(II)-bleomycin, provide data concerning the nature of the activated metallobleomycins, and extend the analogy between the chemistry of cytochrome P-450 and bleomycin.     

Effect of chelating agents and metal ions on the degradation of DNA by bleomycin.

The degradation of DNA by bleomycin was studied in the absence and in the presence of added reducing agents, including 2-mercaptoethanol, dithiothreitol, reduced nicotinamide adenine dinucleotide phosphate, H2O2, and ascorbate, and in the presence of a superoxide anion generating system consisting of xanthine oxidase and hypoxanthine. In all cases, breakage of DNA was inhibited by low concentrations of chelators; where examined in detail, deferoxamine mesylate was considerably more potent than (ethylenedinitrilo)tetraacetic acid. Iron was found to be present in significant quantities in all reaction mixtures. Thus, the pattern of inhibition observed is attributed to the involvement of contaminating iron in the degradation of DNA by bleomycin. Cu(II), Zn(II), and Co(II) inhibit degradation of DNA by bleomycin and Fe(II) in the absence of added reducing agents. A model is proposed in which the degradation of DNA in these systems is dependent on the oxidation of an Fe(II)-bleomycin-DNA complex.     

Oxygen transfer from bleomycin-metal complexes

Both Fe(III) and Cu(II) complexes of bleomycin (BLM), but not N-acetyl BLM . Fe(III), mediated the transfer of oxygen from iodosobenzene to organic substrates. In analogy with results obtained using certain cytochrome P-450 analogs, cis-stilbene was converted cleanly to the respective oxide, while no more than traces of trans-stilbene oxide were formed from trans-stilbene under identical conditions. The possible relevance of these observations to the degradation of DNA by bleomycin was also studied. In both the presence and absence of O2, BLM . Cu(II) . C6H5IO effected DNA degradation, as judged by the release of [3H]thymine from radiolabeled Escherichia coli DNA. These findings provide a valuable new assay system for the study of bleomycin analogs and suggest the possibility that bleomycin may function as an "oxygen transferase" in its degradation of DNA in situ.     

Copper-dependent cleavage of DNA by bleomycin

DNA strand scission by bleomycin in the presence of Cu and Fe was further characterized. It was found that DNA degradation occurred readily upon admixture of Cu(I) or Cu(II) + dithiothreitol + bleomycin, but only where the order of addition precluded initial formation of Cu(II)--bleomycin or where sufficient time was permitted for reduction of the formed Cu(II)--bleomycin to Cu(I)--bleomycin. DNA strand scission mediated by Cu + dithiothreitol + bleomycin was inhibited by the copper-selective agent bathocuproine when the experiment was carried out under conditions consistent with Cu chelation by bathocuproine on the time scale of the experiment. Remarkably, it was found that the extent of DNA degradation obtained with bleomycin in the presence of Fe and Cu was greater than that obtained with either metal ion alone. A comparison of the sequence selectivity of bleomycin in the presence of Cu and Fe using 32P-end-labeled DNA duplexes as substrates revealed significant differences in sites of DNA cleavage and in the extent of cleavage at sites shared in common. For deglycoblemycin and decarbamoylbleomycin, whose metal ligation is believed to differ from that of bleomycin itself, it was found that the relative extents of DNA cleavage in the presence of Cu were not in the same order as those obtained in the presence of Fe. The bleomycin-mediated oxygenation products derived from cis-stilbene were found to differ in type and amount in the presence of added Cu vs. added Fe. Interestingly, while product formation from cis-stilbene was decreased when excess Fe was added to a reaction mixture containing 1:1 Fe(III) and bleomycin, the extent of product formation was enhanced almost 4-fold in reactions that contained 5:1, as compared to 1:1, Cu and bleomycin. The results of these experiments are entirely consistent with the work of Sugiura [Sugiura, Y. (1979) Biochem. Biophys. Res. Commun. 90, 375-383], who first demonstrated the generation of reactive oxygen species upon admixture of O2 and Cu(I)--bleomycin.     

Activated bleomycin. A transient complex of drug, iron, and oxygen that degrades DNA

"Activated bleomycin" is an oxygenated iron drug complex which embodies the drug's DNA-cleaving activity. This activity is exercised on DNA, if present, but if DNA is absent, the drug itself is inactivated. Hyperfine interactions in the EPR spectra of activated bleomycin prepared with 57Fe(II) and 17O2 demonstrate the presence of iron as Fe(III) and of bound oxygen originating in dioxygen. Bleomycin can also be activated with Fe(III) and either H2O2 or ethyl hydroperoxide. These latter reactions do not produce a ferrous intermediate nor do they require O2. But O2 is required for the reaction of activated bleomycin with DNA to yield the malondialdehyde-like chromogens used to monitor DNA degradation. The attack on DNA is quantitatively concurrent with the decay of activated bleomycin, however generated.     

Deglyco-bleomycin. Degradation of DNA and formation of a structurally unique Fe(II) . CO complex

In analogy with bleomycin, deglyco-bleomycin B2 has been found to form a stable, diamagnetic complex with Fe(II) and CO. Although the stoichiometry of this complex appeared to be the same as that formed with bleomycin, the geometry of the deglyco-bleomycin complex was fundamentally different, especially as regards orientation of the beta-aminoalanine moiety. In the presence of Fe(II) and O2, deglyco-bleomycin A2 and deglyco-bleomycin B2 were found to release [3H]thymine from radiolabeled PM-2 DNA; when employed at limiting concentrations, deglyco-bleomycin A2 and B2 gave about half as much [3H]thymine release as the respective bleomycins. In view of the spectral evidence (Burger, R. M., Horwitz, S. B., Peisach, J., and Wittenberg, J. B. (1979) J. Biol. Chem. 254, 12299-12302) that Fe(II) . bleomycin . CO has the same geometry as the complex formed by initial association of bleomycin, Fe(II), and O2, the accumulated data suggest strongly that all metal complexes of bleomycin (derivatives) capable of DNA degradation need not have the same geometry.     

Iron-bleomycin interactions with oxygen and oxygen analogues. Effects on spectra and drug activity.

Despite extensive structural dissimilarities, iron . bleomycin complexes and heme-containing oxygenases display remarkable similarities in binding oxygen antagonists and in spectral properties deriving from bound iron. Fe(II)-bleomycin reversibly forms a complex with either CO or isocyanide (lambda max = 384 and 497 nm, respectively), either of which interfere with its oxygen-dependent cleavage of DNA. A similar but paramagnetic complex forms with NO (lambda max = 470 nm; AN = 24 G). In contrast, cyanide enhances bleomycin activity against DNA. Complexes of bleomycin and FE(III), formed either by direct association or by autoxidation of the Fe(II) . bleomycin complex, exhibit indistinguishable EPR and visible spectra, which change characteristically with pH. At neutral pH, Fe(III) . bleomycin is a low spin complex (g = 2.45, 2.18, 1.89; lambda max = 365, 384 nm) and, at low pH, it is a high spin rhombic complex (geff = 9.4, 4.3; lambda max = 430 nm). These complexes are interconvertible (pK 4.3). Fe(II) . bleomycin oxidation, although reversible by spectral criteria, is accompanied by drug inactivation unless DNA is present.     

M?ssbauer, EPR and NMR studies of the acid-induced reduction and changes in spin state of ferric bleomycin

Iron-57 M?ssbauer, electron paramagnetic resonance (EPR) and H-1 nuclear magnetic resonance (NMR) studies of iron-bleomycin complexes in the pH range from 1.0 to 6.0 are reported. Sequential protonation of the ligands produces a variety of high-spin and low-spin complexes of the metal. Of particular interest is the reversible equilibrium between Fe(III)- and oxygen-stable Fe(II)-bleomycin. Below pH 3.5 Fe(II) complexes form, with maximal reduction occurring at approximately pH 2. At still lower pH, Fe(III) complexes unassociated with bleomycin become dominant. The observed reduction in the absence of exogenous reducing agents suggests the possible involvement of intramolecular autoreduction in bleomycin-mediated DNA degradation.     

Stimulation of iron(II) bleomycin activity by phosphate-containing compounds

Orthophosphate and phosphate derivatives including pyrophosphate, hexametaphosphate, ATP, ADP, and inositol hexaphosphate enhance the extent of DNA degradation by iron(II) bleomycin. These phosphate-containing compounds increase both the release of free nucleic base and that of base propenals which are DNA cleavage products, probably by enhancing the efficiency with which Fe(II) is recruited into the drug. Phosphate action occurs during drug activation prior to the attack on DNA. In addition, phosphates affect the stability of the activated drug complex, overcome the inhibition observed with high concentrations of DNA, and reduce the size of the DNA fragment necessary for reacting with the drug. Phosphate derivatives bind to iron(II) bleomycin and alter its optical spectrum. An analysis of titration data for pyrophosphate and inositol hexaphosphate indicates that each phosphate compound binds to more than one iron(II) bleomycin molecule. With ATP, ADP, and 2,3-diphosphoglycerate, only a single phosphate-containing compound binds to the ferrous drug complex. The affinity for ATP is sufficiently high as to suggest that the ternary complex formed in vitro may exist physiologically.     

Synthetic analogues and biosynthetic intermediates of bleomycin. Metal-binding, dioxygen interaction, and implication for the role of functional groups in bleomycin action mechanism

In order to clarify the role of bleomycin functional groups in action mechanism, the metal-binding, dioxygen activation, and DNA cleavage of several synthetic analogues and biosynthetic intermediates of bleomycin have been investigated. The present results support that 1) the beta-aminoalaninepyrimidine-beta-hydroxyhistidine portion of the bleomycin molecule substantially participates in the Fe(II) and dioxygen interactions, 2) the transposition of the pyrimidine (or pyridine) and imidazole groups in the Fe(II)-coordination is essential for the effective binding and activation of molecular oxygen by the bleomycin ligands, and 3) the gulose-mannose moiety plays an important role as an environmental factor for the efficient dioxygen reduction and DNA cleavage, although the sugar portion does not contribute significantly to the nucleotide specificity in the DNA strand scission. Certain oligopeptides are able to mimic the metal-binding and dioxygen activation by bleomycin, but not induce the effective DNA cleavage. Probably, the bithiazole DNA interaction site of bleomycin delivers the iron/dioxygen chemistry to particularly the DNA (formula, see text) nucleotide sequences.     

Reaction of DNA-bound Co(II)bleomycin with dioxygen.

The aerobic oxidation of Co(II)bleomycin bound to calf thymus DNA has been investigated in relation to the mechanism of reaction in solution in the absence of DNA. Kinetics of dioxygenation of the Co(II) complex were followed by spectrophotometric and electron spin resonance spectroscopy as well as dioxygen analysis. The reaction is slower than when carried out in solution; its rate is inversely related to the ratio of DNA base pairs to Co(II)bleomycin. The subsequent oxidation reaction, observed spectrophotometrically and by dioxygen analysis, is second order in cobalt complex. The calculated second order rate constant is also inversely related to the base pair to metal complex ratio. Once this ratio exceeds three, the reaction rate slows significantly with each additional increment of DNA added to the starting reaction mixture. Taking advantage of the high stability of O(2)-Co(II)bleomycin bound to greater than a 3-fold excess of DNA base pairs, it could be demonstrated that the rate constant for oxidation of two O(2)-Co(II)bleomycin molecules is much slower than that for O(2)-Co(II)bleomycin plus Co(II)bleomycin. With the same technique it was observed that the metal centers of O(2)-Co(II)bleomycin and Fe(II)bleomycin also undergo oxidation. The binding to DNA of both solution products of the oxidation of Co(II)bleomycin by O2 was examined by 1H NMR spectroscopy. Peroxy-Co(III)bleomycin, Form I, binds with higher affinity than Co(III)bleomycin, Form II. At lower ionic strength, the size of the DNA binding site for each form is about 2 base pairs/molecule of drug.     

Copper(I)-bleomycin: structurally unique complex that mediates oxidative DNA strand scission

Copper(I)-bleomycin [Cu(I) X BLM] was characterized in detail by 13C and 1H NMR. Unequivocal chemical shift assignments for Cu(I) X BLM and Cu(I) X BLM X CO were made by two-dimensional 1H-13C correlated spectroscopy and by utilizing the observation that Cu(I) X BLM was in rapid equilibrium with Cu(I) and metal-free bleomycin, such that individual resonances in the spectra of BLM and Cu(I) X BLM could be correlated. The binding of Cu(I) by bleomycin involves the beta-aminoalaninamide and pyrimidinyl moieties, and possibly the imidazole, but not N alpha of beta-hydroxyhistidine. Although no DNA strand scission by Cu(II) X BLM could be demonstrated in the absence of dithiothreitol, in the presence of this reducing agent substantial degradation of [3H]DNA was observed, as was strand scission of cccDNA. DNA degradation by Cu(I) X BLM was shown not to depend on contaminating Fe(II) and not to result in the formation of thymine propenal; the probable reason(s) for the lack of observed DNA degradation in earlier studies employing Cu(II) X BLM and dithiothreitol was (were) also identified. DNA strand scission was also noted under anaerobic conditions when Cu(II) X BLM and iodosobenzene were employed. If it is assumed that the mechanism of DNA degradation in this case is the same as that under aerobic conditions (i.e., with Cu(I) X BLM + O2 in the presence of dithiothreitol), then Cu X BLM must be capable of functioning as a monooxygenase in its degradation of DNA.     

Degradation of structurally modified DNAs by bleomycin group antibiotics

Bleomycin-mediated DNA strand scission has been shown to be diminished at certain sequences in proximity to 5-methylcytidines. We have investigated the molecular basis of this observed diminution using selective bleomycin (BLM) modifications at the C-terminus. Of the four different bleomycin congeners investigated, only bleomycin A2 and bleomycin BAPP were substantially affected by cytidine methylation. We have also examined the effect of other DNA modifications on bleomycin-mediated strand scission. Methylation at the N6 position of adenosine resulted in diminution of DNA cleavage by all four bleomycin congeners. The presence of bulky 5-(glucosyloxy)methyl groups in the major groove of T4 DNA had little effect on the efficiency of DNA strand scission mediated by bleomycin A2 or B2, suggesting the absence of important steric interactions between Fe(II).BLM and DNA in the major groove. In contrast, DNA cleavage mediated by bleomycin congeners was very sensitive to a major DNA conformational change, the B----Z transition. Salt and MgCl2 titrations of the DNA copolymers poly(dG-dC).poly(dG-dC) and poly(dG-MedC).poly(dG-MedC) demonstrated that bleomycin A2 and B2 did not cleave Z-DNA efficiently. In addition, circular dichroism titrations of these copolymers revealed that both bleomycin congeners increased the cation concentration necessary to induce the B----Z transition, implying that bleomycin preferentially binds to and stabilizes B-form DNA. These results are consistent with a model in which cytidine methylation at appropriate sequences of DNA is sufficient to induce subtle conformational changes that render the helix unreceptive to cleavage by some bleomycin congeners.     

Deglyco-peplomycin metal complexes on DNA fibers: a role of the sugar moiety for the stability and the orientation of the complexes

Binding structures of metal complexes of deglyco-peplomycin (dPEP) on DNA were investigated by comparing dPEP complexes with those of bleomycin (BLM) using DNA fiber EPR spectroscopy. A low spin species of Fe(III)dPEP observed in the DNA pellet changed irreversibly to several high spin species after the fabrication of the DNA fibers. The g values of the high spin species were different from those of Fe(III)BLM. The high spin species could not be nitrosylated reductively to ON-Fe(II)dPEP, suggesting that some nitrogen atoms coordinated to the Fe(III) were displaced on the DNA fibers. On the other hand, O(2)-Co(II)dPEP remained intact on the fibers similarly to O(2)-Co(II)BLM but with an increased randomness in the orientation on the DNA. In contrast to Cu(II)BLM, a considerable amount of Cu(II)dPEP bound almost randomly on B-form DNA fibers. These results indicated that the sugar moiety in peplomycin or bleomycin is playing an important role in enhancing the stability of the metal-binding domain and in the stereospecificity of the binding on DNA.     

Oxygenated iron bleomycin. A short-lived intermediate in the reaction of ferrous bleomycin with O2.

The reaction of Fe(II) . bleomycin with O2 to yield Fe(III) . bleomycin has been resolved into two kinetic events by stopped-flow spectrophotometry. The first event is first order with respect to both bleomycin and O2 and may be regarded as a second order reaction (k = 6.1 x 10(3) M-1s-1 at 2 degrees C). The first product has no EPR spectrum. The optical spectrum resembles those of Fe(II) . bleomycin complexes with CO, NO, and ethyl isocyanide. We propose that the first product is an Fe(II) . bleomycin . O2 complex. The second kinetic event is first order with respect to the first accumulated product (k = 0.11 s-1 at 2 degrees C) and independent of oxygen concentration. The product of this reaction is indistinguishable from Fe(III) . bleomycin by optical and EPR spectroscopy.     

Bleomycin-iron can degrade DNA in the presence of excess ethylenediaminetetraacetic acid in vitro

The antineoplastic drug bleomycin, when complexed to Fe(II), causes both single- and double-stranded lesions in DNA in vitro. EDTA is commonly used to inhibit the reaction of bleomycin-Fe with DNA, presumably by removing the metal cofactor. In this study, we utilized a simple assay involving the conversion of supercoiled plasmid DNA to the nicked or linear forms to further investigate the ability of bleomycin-Fe to degrade DNA in the presence of EDTA. We found that a 1:1 complex of bleomycin and Fe can degrade plasmid DNA even in the presence of a 10(6) molar excess of EDTA over bleomycin. Furthermore, we found that the half-life for inactivation of bleomycin-Fe by excess EDTA is about 1.5 h. Finally, we demonstrate that excess bleomycin associated with the outer plasma membranes of cells can damage DNA after the cells are lysed in buffers containing EDTA and SDS. These results suggest that EDTA may not be an efficient inhibitor of the reaction of bleomycin-Fe with DNA.     

Studies on the chemical reactivity of copper bleomycin

The copper(II) complex of the clinically used antitumor agent bleomycin (Blm) has cytotoxic as well as antitumor properties. To understand the relationship of the bleomycin ligand, copper bleomycin, and other possible metal complexes of this agent, kinetic studies of the formation of Cu(II)Blm, ligand substitution reactions of CuBlm with ethylenediaminetetraaletic acid, and the redox reaction of CuBlm with thiols have been completed and interpreted along with previous studies of the thermodynamic stability of Cu2+ with bleomycin. Cu(II)Bm is found to be kinetically and thermodynamically stable in ligand substitution processes and is only slowly reduced and dissociated by sulfhydryl reagents. The rate constant of reduction of the complex by 2-mercaptoethanol (2-ME) at pH 7.4 and 25 degrees C is 9.5 X 10(-3) M-1 sec-1, explaining the inhibition of Fe2+-dependent strand scission of DNA by Cu2+ in the presence of 2-ME. CuBlm forms in preference to Fe(II)Blm and cannot be reduced and dissociated rapidly enough by thiols to liberate Blm and form the reactive iron complex. In agreement with the observed chemical stability of CuBlm, it is also shown that the complex is stable in human plasma and in the presence of Ehrlich cells suspended in ascites fluid. Interestingly, little CuBlm enters these cells to carry out cytotoxic reactions. Finally, it is shown that both Cu2+ and Zn2+, at equivalent concentrations to Fe2+, effectively inhibit the strand scission of DNA by Fe(II)Blm plus oxygen. However, at substoichiometric amounts of Cu2+, the ferroxidase activity of Blm enables the drug to remain effective in the strand-scission reaction, despite the lowered Cu-free Blm/Fe2+ ratio. These results are discussed in light of the proposed mechanism of action of bleomycin.     

Synergistic effects of flavonoids and ascorbate on enhancement in DNA degradation induced by a bleomycin-Fe complex

Flavonoids were examined for synergistic effects with ascorbate on enhancement of DNA degradation induced by a bleomycin(BLM)-Fe complex. The synergistic effects of flavonoids and ascorbate on DNA degradation induced by the BLM-Fe complex were observed to be greater with flavonoids such as isorhamnetin, kaempferol and morin, which accelerated oxidation more markedly in the presence, than in the absence of BLM. Conversely, myricetin and fisetin, which showed oxidation barely accelerated by the addition of BLM, inhibited DNA degradation promoted by ascorbate. Consequently, there was a good correlation between oxidation of flavonoids accelerated by BLM and the extent of DNA degradation promoted synergistically with ascorbate. Our previous studies indicated that oxidation of flavonoids accelerated by BLM and DNA degradation promoted by flavonoids were not correlated with Fe(III)-reducing activity of flavonoids. Those results suggest that Fe(III)-reducing activity of flavonoids is not the only factor determining DNA degradation-promoting activity induced by the BLM-Fe complex. On the other hand, in a Fenton reaction, degradation of 2-deoxy-d-ribose promoted by flavonoids was correlated to the Fe(III)-reducing activity of flavonoids. However, there was not a synergistic interaction between flavonoids and ascorbate in the degradation of 2-deoxy-d-ribose. Therefore, it is suggested that the synergistic DNA degradation caused by flavonoids and ascorbate in the BLM-Fe redox cycle arose from the difference in the reductive processes in which flavonoids and ascorbate mainly act.     

Effects of O2 on the reactions of activated bleomycin

The antitumor drug, bleomycin, interacts with either Fe(II) and O 2 or Fe(III) and H2O2 to form an activated complex which attacks DNA. Under aerobic conditions, both reactions yield similar quantities of free bases and products consisting of base plus deoxyribose carbon atoms 1 to 3. Under anaerobic conditions, activated bleomycin releases only free base. The yield of free base is the same under aerobic or anaerobic conditions, provided DNA is furnished in excess. When the DNA concentration is limiting, more base is released under anaerobic than under aerobic conditions. Drug self-destruction proceeds as quickly and completely in the presence or absence of O2.