Contrasts in the actions of protein antibiotics on deoxyribonucleic acid structure and function

Sequential 1H NMR assignments and secondary structure of aponeocarzinostatin in solution

Sequential assignments and secondary structural analysis have been accomplished for the 113-residue apoprotein of the antitumor drug neocarzinostatin (NCS) from Streptomyces carzinostaticus. A total of 98% of the main-chain and 77% of the side-chain resonances have been sequence specifically assigned by use of information from coherence transfer experiments and by sequential and interstrand NOEs. Because of the complexity of the NCS spectrum, several sequential assignment strategies were employed to complete the analysis. Apo-NCS consists of three antiparallel beta-sheeted domains by NMR analysis. There is an extensive four-strand antiparallel beta-sheet, and two two-stranded domains. One of the two-strand domains is contiguous, S72-N87, with chain reversal occurring through the region L77-R82. The other two-stranded domain has the section G16-A24 antiparallel with respect to the region S62-R70. This secondary structure is consistent with the crystal structure of holo-NCS at 2.8-A resolution.

Crystal structure analysis of auromomycin apoprotein (macromomycin) shows importance of protein side chains to chromophore binding selectivity

The crystal structure of macromomycin, the apoprotein of the antitumor antibiotic auromomycin, has been determined and refined at 1.6-A resolution. The overall structure is composed of a flattened seven-stranded antiparallel beta-barrel and two antiparallel beta-sheet ribbons. The barrel and the ribbons define a deep cleft that is the chromophore binding site. The cleft is very accessible and in this structure is occupied by two 2-methyl-2,4-pentanediol and two water molecules. The overall shape of the binding site is similar to that of the analogue actinoxanthin. Highly specific side chains that are not conserved between different analogues extend into the binding site and may be important to the chromophore binding specificity.

Mode of reversible binding of neocarzinostatin chromophore to DNA: evidence for binding via the minor groove

Two general approaches have been taken to understand the mechanism of the reversible binding of the nonprotein chromophore of neocarzinostatin to DNA: (1) measurement of the relative affinity of the chromophore for various DNAs that have one or both grooves blocked by bulky groups and (2) studies on the influence of adenine-thymine residue-specific, minor groove binding agents such as the antibiotics netropsin and distamycin on the chromophore-DNA interaction. Experiments using synthetic DNAs containing halogen group (Br, I) substituents in the major groove or natural DNAs with glucosyl moieties projecting into the major groove show that obstruction of the major groove does not decrease the binding stoichiometry or the binding constant for the DNA-chromophore interaction. Chemical methylation of bases in both grooves of calf thymus DNA, resulting in 13% methylation of N-7 of guanine in the major groove and 7% methylation of N-3 of adenine in the minor groove, decreases the binding affinity and increases the size of the binding site for neocarzinostatin chromophore. Similar results were obtained whether binding parameters were determined directly by spectroscopic measurements or indirectly by measuring the ability of the DNA to protect the chromophore against degradation. On the other hand, netropsin and distamycin compete with neocarzinostatin chromophore for binding to the minor groove of DNA, as shown by their decrease in the ability of poly(dA-dT) to protect the chromophore against degradation and their reduction in chromophore-induced DNA damage as measured by thymine release.(ABSTRACT TRUNCATED AT 250 WORDS)

Neocarzinostatin abstracts a hydrogen during formation of nucleotide 5'-aldehyde on DNA

The oxidative reaction of polydeoxyadenylic-deoxythymidylic acid [poly(dA-dT)] with neocarzinostatin that produces 5'-thymidine aldehyde esterified to the 5'-end of strand breaks proceeds with hydrogen abstraction. The abstracted hydrogen is covalently bound to the non-protein component of neocarzinostatin; only a small amount (5%) is washed out into solvent. These data rule out a peroxyl radical as the primary DNA damaging species involved in the production of the 5'-aldehyde group. In contrast to earlier reports, it is demonstrated that alpha-tocopherol is not an inhibitor of the reaction.

Structural study of crystalline auromomycin. Preliminary X-ray diffraction study of auromomycin and macromomycin

Auromomycin and macromomycin from the organism Streptomyces macromomyceticus have been crystallized. The X-ray diffraction pattern of crystals of each molecule is consistent with space group P2(1)2(1)2 with cell parameters a = 46.45 A, b = 54.34 A and c = 42.03 A for auromomycin, and a = 46.45 A, b = 54.52 A and c = 41.54 A for macromomycin. Diffraction analysis of auromomycin is in progress.

Gamma-radiolysis study of the reductive activation of neocarzinostatin by the carboxyl radical

The activation of the antitumor protein antibiotic neocarzinostatin (NCS) by the carboxyl radical CO-2, a one-electron donor obtained selectively from gamma-ray irradiation of nitrous oxide-saturated formate buffer, has been investigated in the presence and in the absence of DNA at pH 4.7 and pH 7.0. The reaction of NCS with CO-2 in the absence of DNA is followed by a marked red shift (420----441 nm) and a pronounced increase (X 8.8) of the fluorescence emission corresponding to the naphthalene moiety of the NCS chromophore. The light absorption spectrum shows in parallel a hypochromic change with considerable fine structure throughout the 250-400 nm wavelength range. When DNA is present, the fluorescence intensity at completion of the reaction is slightly reduced (by 5 to 15 per cent) and the maximum emission wavelength shifted to 436-438 nm. However, the bulk rate of reaction is not altered by DNA and is independent of the pH, of the temperature and of the concentration of NCS. The NCS concentration-independence of the reaction rate is consistent with a high intrinsic rate (k greater than 10(8)M-1 . s-1) for the reaction of CO-2 with the NCS chromophore. Complete reduction of the NCS chromophore involves a total of three electron-equivalents. The final product does not react with oxygen, shows no odd electron spin, and is unable to induce DNA strand scission. Its molecular state, however, is fundamentally different when gamma-ray irradiations are performed with DNA. This bears evidence of short-lived one electron or two-electrons reduced intermediates decaying via non-identical routes depending on the presence of the acceptor DNA. Actually, dose-related strand breaks appear in DNA exposed to the action of NCS and CO-2. Some NCS chromophore-DNA covalent adducts are also found. DNA strand breakage by CO-2-activated NCS is correlated with thymine release and is inhibited by a redox-stable intercalating agent. The DNA-nicking process thus bears resemblance to that reported by other authors using mercaptans to initiate reductive activation of the NCS chromophore. However, some spectral differences are observed between the CO-2-reacted and the thiol-treated chromophores. Moreover, thymine release and strand scission in DNA incubated with CO-2 and NCS proceed under anaerobic conditions. It is proposed that the strict oxygen requirement for DNA damage by NCS in the presence of mercaptans is due, at least partly, to competition between oxygen and thiols for reaction with the same primary deoxyribose radical resulting from DNA attack by the reductively activated NCS chromophore.

Antitumor proteins of Streptomyces macromomyceticus: purification and characterization of auromomycin, macromomycin A, and macromomycin D

Macromomycin A and the two related proteins auromomycin and macromomycin D were isolated from the culture filtrates of Streptomyces macromomyceticus by chromatography on columns of DEAE-cellulose, Amberlite XAD-7, and decylagarose. Antibodies prepared against macromomycin A showed antigenic identity by Ouchterlony double diffusion between the three purified proteins. This similarity was further demonstrated by their behavior on disc gel electrophoresis, the amino acid compositions, and comparative peptide mapping of the aminoethylated derivatives. They differed, however, in other chemical and biological properties. Auromomycin and macromomycin A, pI 5.4, have antibiotic activity, which is absent in macromomycin D, pI 5.2. This antibiotic activity was associated with chromophore groups that were extractable by methanol. High-pressure liquid chromatography of the methanol extracts gave difference profiles for each of the purified proteins. The differences in the three proteins extended to their ultraviolet-visible spectra, fluorescence and circular dichroism, and the changes of these properties with heating. The heat denaturation, with auromomycin and macromomycin melting at 70.5 degrees C and macromomycin D at 57.0 degrees C, was reversible. Changes were noted in the spectra both during and following heating at 80 degrees C; the antibacterial activity was lost in auromomycin and only partially reduced in macromomycin A. The properties of the three proteins support the general similarities in their polypeptide structures, modifications in the properties of which are endowed by the differences in the associated nonprotein chromophores.

On the mechanism of reductive activation in the mode of action of some anticancer drugs

Bioactivation of a number of DNA-specific antitumor drugs depends on oxidoreduction. Bleomycin, neocarzinostatin and anthracycline glycosides are the best known among such drugs in terms of reductive activation processes. Their reduction results in short-lived radical or electrophilic intermediates attacking DNA stereospecifically. The physico-chemical properties of these drugs and the nature of DNA damage are reviewed. Models for DNA-intercalation, electron-donor systems involved in drug metabolisation, and the role of oxygen in radical reactions, are discussed in the light of recent reports.

Covalent adducts of DNA and the nonprotein chromophore of neocarzinostatin contain a modified deoxyribose

When the nonprotein chromophore of neocarzinostatin was allowed to react with either calf thymus DNA or poly(dA-dT) . poly(dA-dT) in the presence of 2-mercaptoethanol and the DNA was precipitated with ethanol, 5% of the fluorescence attributable to the naphthalene rings of the chromophore coprecipitated with the DNA. Most of this fluorescence remained attached to DNA through successive reprecipitations, suggesting formation of covalent adducts between chromophore and DNA. Enzymatically digested poly(dA-dT) . poly(dA-dT)-chromophore adduct contained, in addition to deoxyadenosine and thymidine, several highly fluorescent hydrophobic products, separable by reverse-phase chromatography, all of which contained both adenine and thymine radiolabel, as well as chromophore radiolabel. One such product consistently had twice as much thymine as adenine, suggesting a structure chromophore-d(TpApT), in which the attached chromophore rendered both phosphodiester bonds refractory to endonuclease S1. This adduct fragment was completely hydrolyzed at pH 12, releasing adenine, 3'-dTMP, and 5'-dTMP. At pH 7, the adduct fragment slowly released chromophore and 3'-dTMP with parallel kinetics, leaving a modified d(ApT), which was cleaved by snake venom phosphodiesterase to yield 5'-dTMP and a modified deoxyadenosine. These hydrolysis patterns are unlike those of any previously characterized base or phosphotriester DNA adduct but rather indicate an altered deoxyadenosine sugar. The formation of adducts containing a modified deoxyribose suggests that deoxyribose may be the site of covalent chromophore attachment. Alteration of this same site, possibly the 5'-carbon of the sugar moiety, may account for the extreme lability of the phosphodiester bond.

DNA breakage activity of the methanol extract of auromomycin

The constituents of the antitumor agent auromomycin have been analyzed to determine their DNA-breakage activities. Spectral analysis showed that the methanol extract contained 70% of the non-peptide chromophore, whereas the residue contained 20%. Amino acid analysis of the methanol extract showed that it contained 21%-26% of the original auromomycin polypeptides. The DNA-degradation activity of the extract was 121% +/- 28% of that of the untreated auromomycin, whereas that of the residue was only 22% +/- 3.8%. Mixing of the residue and the methanol extract resulted in the loss of three-fourths of the total activity. Agarose gel electrophoretic analysis showed that the single-strand DNA breakage activity of the methanol extract was 6.5-fold greater than that of the double-strand DNA-breakage activity. The difference in the total DNA-cleavage activity of the untreated, methanol-treated, and remixed auromomycin preparations may suggest the occurrence of certain non-peptide chromophore-polypeptide interactions in both the untreated and the remixed preparations. This is consistent with the fluorescent changes observed upon mixing of the extract and residue. Fractionation of the methanol extract by Sephadex chromatography revealed that several column fractions which were enriched with non-peptide chromophore relative to the polypeptides contained in them still had significant DNA-degradation activity. These studies suggest that the non-peptide chromophore in the auromomycin preparation may contribute to most of the observed DNA breakage activity.

Roles of chromophore and apo-protein in neocarzinostatin action

The methanol-extractable, nonprotein chromophore of the antitumor, protein antibiotic neocarzinostatin (NCS) has at least the full activity of the parent compound in inhibiting DNA synthesis and growth of HeLa cells and in causing DNA strand breaks in vivo and in vitro. In vitro DNA strand scission by the chromophore is markedly stimulated by 2-mercaptoethanol and is inhibited by guanidine hydrochloride and alpha-tocopherol. By high-pressure liquid chromatography, this activity has been localized to fractions eluting at greater than 90% methanol and having fluorescence emission at 420 nm (excitation at 340 nm). The apo-protein of NCS is inactive by itself but complexes with the chromophore so as to regulate its availability during the in vitro reaction. In DNA strand scission the chromophore acts rapidly at both 0 and 37 degrees C, whereas native and reconstituted NCS are inactive at 0 degrees C and slowly active at 37 degrees C. Complex formation with apo-NCS stabilizes the chromophore. Reconstitution of NCS (pI 3.3) from chromophore and apo-protein (pI 3.2) was shown by both activity studies and isoelectric focusing on polyacrylamide gels. "Pre-NCS," the biosynthetic precursor of NCS, is identical to apo-NCS in amino acid composition, spectral properties, isoelectric focusing on polyacryl-amide gels, and ability to complex with isolated chromophore to form material with all the properties of native NCS.