Neocarzinostatin: spectral characterization and separation of a non-protein chromophore

Reactivity of Enyne-Allenes Generated via an Alder-Ene Reaction

Tandem transformations of 1,3-diynyl propiolate derivatives are described. The Alder-ene reaction generates an enyne-allene, which undergoes a formal 1,7-H shift or a Diels-Alder reaction, depending on the substituent on the alkyne. A terminal or aryl-substituted alkyne promotes a 1,7-H shift to generate a new enyne-allene, which undergoes a Myers-Saito cycloaromatization followed by a 1,5-H transfer-mediated cyclization to form highly functionalized benzo-fused 6-membered cycles. The reactivity of the preformed enyne-allene shows comparable reactivity profiles.

Evaluating the use of Apo-neocarzinostatin as a cell penetrating protein

Protein-ligand complex neocarzinostatin (NCS) is a small, thermostable protein-ligand complex that is able to deliver its ligand cargo into live mammalian cells where it induces DNA damage. Apo-NCS is able to functionally display complementarity determining regions loops, and has been hypothesised to act as a cell-penetrating protein, which would make it an ideal scaffold for cell targeting, and subsequent intracellular delivery of small-molecule drugs. In order to evaluate apo-NCS as a cell penetrating protein, we have evaluated the efficiency of its internalisation into live HeLa cells using matrix-assisted laser-desorption ionization-time-of-flight mass spectrometry and fluorescence microscopy. Following incubation of cells with apo-NCS, we observed no evidence of internalisation.

Identification of thymidine-5'-aldehyde at DNA strand breaks induced by neocarzinostatin chromophore

Snake venom phosphodiesterase or endonuclease S1 digestion of neocarzinostatin chromophore-treated DNA, labeled in its thymidine residues, liberates an unusual labeled nucleoside from the 5' end of a drug-induced break. This substance, isolated by reverse-phase HPLC, possesses carbons from both the thymine and the deoxyribose moieties of thymidine in the DNA but, unlike thymidine, is readily degraded at pH 12 to thymine and a sugar fragment. The altered nucleoside was shown to contain a carbonyl group by its reduction with NaBH(4) to form a substance that has the chromatographic properties of thymidine and by its reaction with various hydrazines to form the respective hydrazone derivatives; the carbonyl exists as the 5' aldehyde as shown by its mild chemical oxidation to the carboxylic acid with simultaneous loss of the 5' (3)H. Mass spectral analysis showed a fragmentation pattern compatible with the structure thymidine-5'-aldehyde. These data indicate that the nonprotein chromophore of neocarzinostatin, in the presence of a reducing substance (2-mercaptoethanol) and molecular oxygen, selectively oxidizes the 5' carbon of nucleosides in DNA to the aldehyde, resulting in a strand break and a DNA fragment bearing nucleoside-5'-aldehyde at its 5' end.

Protein–Small Molecule Interactions in Neocarzinostatin, the Prototypical Enediyne Chromoprotein Antibiotic

The enediyne chromoproteins are a class of potent antitumour antibiotics comprising a 1:1 complex of a protein and a noncovalently bound chromophore. The protein is required to protect and transport the highly labile chromophore, which acts as the cytotoxic component by reacting with DNA leading to strand cleavage. A derivative of the best-studied member of this class, neocarzinostatin (NCS), is currently in use as a chemotherapeutic in Japan. The application of the chromoproteins as therapeutics along with their unique mode of action has prompted widespread interest in this area. Notable developments include the discovery of non-natural ligands for the apoproteins and the observation that multiple binding modes are available for these ligands in the binding site. Mutation studies on the apoproteins have revealed much about their stability and variability, and the application of an in vitro evolution method has conferred new binding specificity for unrelated ligands. These investigations hold great promise for the application of the apoproteins for drug-delivery, transport and stabilisation systems.

A New Model for Ligand Release

Antitumor antibiotic chromoproteins such as neocarzinostatin involve a labile toxin that is tightly bound by a protective protein with very high affinity but must also be freed to exert its function. Contrary to the prevalent concept of ligand release, we established that toxin release from neocarzinostatin requires no major backbone conformational changes. We report, herein, that subtle changes in the side chains of specific amino acid residues are adequate to gate the release of chromophore. A recombinant wild type aponeocarzinostatin and its variants mutated around the opening of the chromophore binding cleft are employed to identify specific side chains likely to affect chromophore release. Preliminary, biophysical characterization of mutant apoproteins by circular dichroism and thermal denaturation indicate that the fundamental structural characteristics of wild type protein are conserved in these mutants. The chromophore reconstitution studies further show that all mutants are able to bind chromophore efficiently with similar complex structures. NMR studies on 15N-labeled mutants also suggest the intactness of binding pocket structure. Kinetic studies of chromophore release monitored by time course fluorescence and quantitative high pressure liquid chromatography analyses show that the ligand release rate is significantly enhanced only in Phe78 mutants. The extent of DNA cleavage in vitro corresponds well to the rate of chromophore release. The results provide the first clear-cut indication of how toxin release can be controlled by a specific side chain of a carrier protein.

Synthetic Ligands for Apo-Neocarzinostatin

Neocarzinostatin (NCS) is a 1:1 complex of an enediyne chromophore (NCSChrom), non-covalently bound to an 11 kDa protein (apoNCS). We are exploring apoNCS as a generic protein system for sequestering small molecules for therapeutic applications. Here, we disclose a new flavone ligand 1 for apoNCS and present a high-resolution NMR structure of this ligand bound to apoNCS. This is the first high-resolution structure of a completely non-cognate ligand bound to the apoNCS protein. This work provides unambiguous evidence that a completely new class of ligand can bind specifically to apoNCS. Furthermore, the mode of binding is different than that of the naphthoate-based ligands, and for such a simple hydrophobic compound, the new ligand surprisingly binds specifically. This work indicates that apo-Neocarzinostatin has multiple selective and distinct binding modes for small-molecule cargo.

Cold instability of aponeocarzinostatin and its stabilization by labile chromophore

The conformational stability of aponeocarzinostatin, an all-beta-sheet protein with 113 amino-acid residues, is investigated by thermal-induced equilibrium unfolding between pH 2.0 and 10.0 with and without urea. At room temperature, the protein is stable in a pH range of 4.0-10.0, whereas the stability of the protein drastically decreases below pH 4.0. The thermal unfolding of aponeocarzinostatin is reversible and follows a two-state mechanism. By two-dimensional unfolding studies, the enthalpy change, heat capacity change, and free energy change for unfolding of the protein are estimated. Circular dichroism profiles suggest that this protein undergoes both heat- and cold-induced unfolding. The ellipticity changes at far- and near-UV circular dichroism suggest that the tertiary structure is disrupted but the secondary structure remains folded at low temperatures. Interestingly, the labile enediyne chromophore, which is highly stabilized by the protein, is able to protect the protein against cold-induced unfolding, but not the heat-induced unfolding.

Design and Synthesis of a Nitrogen Mustard Derivative Stabilized by Apo-neocarzinostatin

Neocarzinostatin (NCS) is an antitumor antibiotic comprising a 1:1 protein-chromophore complex and exhibits cytotoxic action through DNA cleavage via H-abstraction. Cytotoxic activity resides with the chromophore 1 alone, while the protein (apoNCS) protects and transports labile 1. The naphthoate portion (2) of NCS chromophore (1) is important for binding to apoNCS and DNA intercalation. In this paper we describe our attempts to use apoNCS to improve the hydrolytic stability of novel bifunctional DNA alkylating agents. The nitrogen mustards, melphalan and chlorambucil, were both conjugated to 2, and the biological activities of these conjugates were assessed. Chlorambucil did not benefit from conjugation. The melphalan conjugate (6) formed covalent DNA adducts at guanine bases and exhibited greater in vitro cytotoxic activity than unmodified melphalan. Fluorescence and NMR spectroscopy showed that 6 binds to apoNCS. Binding to apoNCS-protected 6 reduced the extent of hydrolysis of the conjugate. This novel approach demonstrates for the first time that an enediyne apo-protein can be used to improve the stability of substances that are of potential interest in cancer chemotherapy.

Solution Structure of a Novel Chromoprotein Derived from Apo-Neocarzinostatin and a Synthetic Chromophore

The natural complex Neocarzinostatin comprises a labile chromophore noncovalently bound to an 11.2 kDa protein. We present the first high-resolution structure of a novel complex derived from the recombinant apoprotein bound to a non-natural synthetic chromophore. Fluorescence and nuclear magnetic resonance spectroscopy were used to probe the strength and location of binding. Binding occurred in a location similar to that observed for the chromophore in the natural Neocarzinostatin complex, but with a distinct orientation. These results provide structural evidence that the apoprotein can readily accommodate small druglike entities, other than the natural chromophore within its binding cleft. The clinical use of the natural complex described by others, together with the results reported here, suggests potential applications for small molecule binding by apo-Neocarzinostatin.

Development of an enantioselective synthetic route to neocarzinostatin chromophore and its use for multiple radioisotopic incorporation

A convergent, enantioselective synthetic route to the natural product neocarzinostatin chromophore (1) is described. Synthesis of the chromophore aglycon (2) was targeted initially. Chemistry previously developed for the synthesis of a neocarzinostatin core model (4) failed in the requisite 1,3-transposition of an allylic silyl ether when applied toward the preparation of 2 with use of the more highly oxygenated substrates 27 and 54. An alternative synthetic plan was therefore developed, based upon a proposed reduction of the epoxy alcohol 58 to form the aglycon 2, a transformation that was achieved in a novel manner, using a combination of the reagents triphenylphosphine, iodine, and imidazole. The successful route to 1 and 2 began with the convergent coupling of the epoxydiyne 15, obtained in 9 steps (43% overall yield) from D-glyceraldehyde acetonide, and the cyclopentenone (+)-14, prepared in one step (75-85% yield) from the prostaglandin intermediate (+)-16, affording the alcohol 22 in 80% yield and with > or =20:1 diastereoselectivity. The alcohol 22 was then converted into the epoxy alcohol 58 in 17 steps with an average yield of 92% and an overall yield of 22%. Key features of this sequence include the diastereoselective Sharpless asymmetric epoxidation of allylic alcohol 81 (98% yield); intramolecular acetylide addition within the epoxy aldehyde 82, using Masamune's lithium diphenyltetramethyldisilazide base (85% yield); selective esterification of the diol 84 with the naphthoic acid 13 followed by selective cleavage of the chloroacetate protective group in situ to furnish the naphthoic acid ester 85 in 80% yield; and elimination of the tertiary hydroxyl group within intermediate 88 using the Martin sulfurane reagent (79% yield). Reductive transposition of the product epoxy alcohol (58) then formed neocarzinostatin chromophore aglycon (2, 71% yield). Studies directed toward the glycosylation of 2 focused initially on the preparation of the N-methylamino --> hydroxyl replacement analogue 3, an alpha-D-fucose derivative of neocarzinostatin chromophore, formed in 42% yield by a two-step Schmidt glycosylation-deprotection sequence. For the synthesis of 1, an extensive search for a suitable 2'-N-methylfucosamine glycosyl donor led to the discovery that the reaction of 2 with the trichloroacetimidate 108, containing a free N-methylamino group, formed the alpha-glycoside 114 selectively in the presence of boron trifluoride diethyl etherate. Subsequent deprotection of 114 under mildly acidic conditions then furnished the labile chromophore (1). The synthetic route was readily modified for the preparation of singly and doubly (3)H- and (14)C-labeled 1, compounds unavailable by other means, for studies of the mechanism of action of neocarzinostatin in vivo.

Gene transcription analysis of Saccharomyces cerevisiae exposed to neocarzinostatin protein-chromophore complex reveals evidence of DNA damage, a potential mechanism of resistance, and consequences of prolonged exposure

The natural product neocarzinostatin (NCS), a protein-small molecule complex, exhibits potent antiproliferative activity in mammalian cells but has little apparent effect on the growth of the unicellular eukaryotic organism, Saccharomyces cerevisiae. Here, we show by whole-genome transcription profiling experiments that incubation of S. cerevisiae with NCS leads to dramatic and wide-ranging modifications in the expression profile of yeast genes. Approximately 18% of yeast transcripts are altered by 2-fold or more within 4 h of treatment with NCS. Analysis of the observed transcription profile provides evidence that yeast rapidly and continuously overexpress multiple DNA-damage repair genes during NCS exposure. Perhaps to meet the energetic requirements of continuous DNA-damage repair, yeast cells enter respiration upon prolonged exposure to NCS, although grown in nutrient-rich medium. The NCS protein component is readily transported into S. cerevisiae, as demonstrated by fluorescence microscopy of yeast treated with fluorescently labeled NCS. Transcription profiling experiments with neocarzinostatin protein alone implicate a specific resistance mechanism in yeast that targets the NCS protein component, one involving the nonclassical export pathway. These experiments provide a detailed picture of the effects of exposure to NCS upon yeast and the mechanisms they engage as a response to this protein-small molecule DNA-damaging agent.

Heat-induced unfolding of neocarzinostatin, a small all-β protein investigated by small-angle X-ray scattering 1 1Edited by M. F. Moody

Neocarzinostatin is an all-beta protein, 113 amino acid residues long, with an immunoglobulin-like fold. Its thermal unfolding has been studied by small-angle X-ray scattering. Preliminary differential scanning calorimetry and fluorescence measurements suggest that the transition is not a simple, two-state transition. The apparent radius of gyration is determined using three different approaches, the validity of which is critically assessed using our experimental data as well as a simple, two-state model. Similarly, each step of data analysis is evaluated and the underlying assumptions plainly stated. The existence of at least one intermediate state is formally demonstrated by a singular value decomposition of the set of scattering patterns. We assume that the pattern of the solution before the onset of the transition is that of the native protein, and that of the solution at the highest temperature is that of the completely unfolded protein. Given these, actually not very restrictive, boundary constraints, a least-squares procedure yields a scattering pattern of the intermediate state. However, this solution is not unique: a whole class of possible solutions is derived by adding to the previous linear combination of the native and completely unfolded states. Varying the initial conditions of the least-squares calculation leads to very similar solutions. Whatever member of the class is considered, the conformation of this intermediate state appears to be weakly structured, probably less than the transition state should be according to some proposals. Finally, we tried and used the classical model of three thermodynamically well-defined states to account for our data. The failure of the simple thermodynamic model suggests that there is more than the single intermediate structure required by singular value decomposition analysis. Formally, there could be several discrete intermediate species at equilibrium, or an ensemble of conformations differently populated according to the temperature. In the latter case, a third state would be a weighted average of all non native and not completely unfolded states of the protein but, since the weights change with temperature, no meaningful curve is likely to be derived by a global analysis using the simple model of three thermodynamically well-defined states.

Novel designed enediynes: molecular design, chemical synthesis, mode of cycloaromatization and guanine-specific DNA cleavage

The molecular design and chemical synthesis of novel enediyne molecules related to the neocarzinostatin chromophore (1), and their chemical and DNA cleaving properties are described. The 10-membered enediyne triols 16-18 were effectively synthesized from xylitol (10) in a short step, and found to be quite stable when handled at room temperature. The representative and acylated enediyne 16 was cycloaromatized by 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) in cyclohexa-1,4-diene-benzene to give the benzenoid product 21 through a radical pathway. On the other hand, the enediyne 16 was cycloaromatized by diethylamine in dimethyl sulfoxide-Tris-HCl, pH 8.5 buffer to afford another benzenoid product 22 as a diethylamine adduct through a polar pathway. Furthermore, the enediynes 16-18 were found to exhibit guanine-specific DNA cleavage under weakly basic conditions with no additive.

Binding and cleavage characteristics of the complexes formed between the neocarzinostatin chromophore and single site containing oligonucleotides

It is shown by fluorescence spectroscopy that the post-activated form of neocarzinostatin chromophore (NCSi-glu) can form stable complexes with single-site oligonucleotides (SSOs) featuring sequences known to be involved in double stranded (AGC.GCT, AGT.ACT, AGA.TCT, ACA.TGT) or single stranded (AGG.CCT) cleavage (attacked residues in bold). Furthermore, the same SSOs form cleavage productive complexes with native neocarzinostatin chromophore (NCS chrom) over a similar concentration range. The productive complexes yield damage similar to that observed if the same sequence is part of a longer DNA piece. Previously identified double stranded site sequences ATT.AAT and TAT.ATA are shown to contain overlapping attack sites. Binding order preference derived from fluorescence quenching experiments for NCSi-glu is consistent with constants derived by quantitative cleavage affinity binding experiments with NCS chrom. This confirms the similarity in interactions between the NCSi-glu and NCS chrom and justifies the use of NCSi-glu as a stable analog of NCS chrom.

NMR studies of the post-activated neocarzinostatin chromophore-DNA complex. Conformational changes induced in drug and DNA

The glutathione post-activated neocarzinostatin chromophore (NCSi-glu)-DNA complex was studied in detail by 2-D NMR spectroscopy. The complex is a model for understanding the sequence specific cleavage of DNA by the native neocarzinostatin chromophore (NCS chrom), a highly potent enediyne antitumor agent. NMR spectral analysis is presented for the free NCSi-glu, the free DNA duplex and the NCSi-glu-DNA complex. In addition to the previously reported structural details of the complex (Gao, X.; Stassinopoulos, A.; Rice, J. S.; Goldberg, I. H. Biochemistry 1995, 34, 40), we demonstrate that the binding of NCSi-glu in minor groove results in a patch of negatively charged surface covering the otherwise relatively neutral minor groove. The formation of the complex is largely driven by hydrophobic forces and the solvation of the polar surface of the complex. Comparison of the conformations of NCSi-glu and DNA duplex in their free and bound form reveals an induced mutual fit of DNA and NCSi-glu upon complex formation. The reduced NCS chrom represents a DNA binding motif for sequence specific recognition of DNA via intercalation and minor groove interactions.

In vivo efficacy of novel synthetic enediynes 1

We have investigated the biodistribution, toxicity, and antitumor activity of a new type of synthetic compound containing an enediyne functional group capable of benzenoid diradical generation. The design of this cytotoxic molecule was based on the structures of naturally occurring enediyne antibiotics. Compared to the natural compounds, the synthetic enediyne displayed cytotoxicities approaching the natural analogs. Using a tritiated analog, biodistribution studies revealed relatively high uptake levels in kidney, lung, heart, and spleen with moderate levels in all other organs. Antitumor activity was apparent, with significant tumor regression observed in athymic nude mice with established M21 melanomas. Significant tumor antiproliferative effects were observed against L-1210 mouse leukemia, A549 lung carcinomas and PC3 prostate carcinomas in athymic nude mice, and against EMT-6 mouse mammary adenocarcinomas in Balb/cByJ mice. These results suggest that synthetic enediynes may be useful therapeutic compounds since their design reduces systemic toxicity compared to the natural products, without compromising antitumor activity. The relatively low sensitivity of many established cell lines to synthetic enediynes suggests a discrepancy between cell culture and in vivo tumor cytotoxicities. Adaptation of some cell lines for in vivo proliferation may affect their sensitivity to synthetic enediynes.

Antitumor effect of neocarzinostatin conjugated to human/mouse chimeric Fab fragments of the monoclonal antibody A7 on human pancreatic carcinoma

The anticancer agent neocarzinostatin (NCS) was bound covalently to human/mouse chimeric Fab fragments of the monoclonal antibody A7 to form the conjugate chA7Fab-NCS. The antitumor effect of chA7Fab-NCS was tested by measuring the inhibition of 3H-thymidine incorporation into human pancreatic carcinoma cells. The chA7Fab-NCS was approximately 2.3 times as effective as free NCS against human pancreatic carcinoma cells which reacted with the monoclonal antibody A7. The antitumor activity of chA7Fab-NCS was inhibited by excess chA7Fab. ChA7Fab-NCS had an antitumor effect equivalent to free NCS on human pancreatic carcinoma cells which did not react with the monoclonal antibody A7. ChA7Fab-NCS appears to be a potentially useful conjugate for immunotargeting chemotherapy against pancreatic carcinoma.

Regulation of apoptosis in leukemic cells by analogs of dynemicin A

The naturally occurring enediyne antibiotics, which include calicheamicin, esperamicin, neocarzinostatin, kedarcidin and dynemicin, are a unique class of reactive compounds which can undergo aromatization to produce cytotoxic biradicals and can result in phosphodiester bond breakage of DNA. Synthetic enediynes designed with low molecular complexity are also highly cytotoxic, specifically to human leukemic cells, by a mechanism involving the induction of apoptosis. We have used a variety of biological assays to evaluate the cytotoxic properties of synthetic dynemicin analogs which contain a spectrum of structural modifications and reactivities. It was found that the induction of apoptosis and nuclear degradation by the synthetic compounds did not require an ability to bind or cleave DNA. Prevention of apoptosis was observed in analogs which were electronically stabilized to inhibit aromatic rearrangement and generation of diradicals. The preventive capability of the stabilized analogs was observed against a wide variety of toxic agents including topoisomerase I and II inhibitors, anti-mitotic and DNA anti-metabolite drugs, as well as alkylating agents. The structural determinants involved in inhibiting the induction of apoptosis are described. The significance of these results with respect to relevant mechanism of tumor regression are discussed.

Crystal structure of neocarzinostatin, an antitumor protein-chromophore complex

Structures of the protein-chromophore complex and the apoprotein form of neocarzinostatin were determined at 1.8 angstrom resolution. Neocarzinostatin is composed of a labile chromophore with DNA-cleaving activity and a stabilizing protein. The chromophore displays marked nonlinearity of the triple bonds and is bound noncovalently in a pocket formed by the two protein domains. The chromophore pi-face interacts with the phenyl ring edges of Phe52 and Phe78. The amino sugar and carbonate groups of the chromophore are solvent exposed, whereas the epoxide, acetylene groups, and carbon C-12, the site of nucleophilic thiol addition during chromophore activation, are unexposed. The position of the amino group of the chromophore carbohydrate relative to C-12 supports the idea that the amino group plays a role in thiol activation.

Production, binding and cytotoxicity of human/mouse chimeric monoclonal antibody-neocarzinostatin conjugate

A human/mouse chimeric Fab monoclonal antibody A7 (chFabA7) was covalently coupled to neocarzinostatin (NCS) by the SPDP method at various chFabA7:NCS substitution ratios. The antigen-binding activity of the conjugate, examined by ELISA using fixed antigen-positive colon cancer cells, was identical to that of the parent chFabA7 when one mole of NCS was conjugated, but was reduced with 2 or 3 moles of conjugated NCS. By means of a colony-forming assay, the cytocidal effect of the conjugate on antigen-positive cancer cells was found to be stronger than that of free NCS, whereas in antigen-negative cancer cells it was similar to that of free NCS. This effect was attenuated by adding an excess amount of monoclonal antibody A7. These findings indicate that the conjugate has an antigen-specific cytocidal action, and thus chFabA7-NCS is a promising tool for targeting cancer chemotherapy.

The role of monoclonal antibody A7 as a drug modifier in cancer therapy

An anticancer antibiotic, neocarzinostatin (NCS), was covalently conjugated to the murine monoclonal antibody A7 (mAb A7), which recognizes the glycoprotein on the cell surface of human colon cancer. The biological and pharmacological properties of the conjugate (A7-NCS) were examined and compared with those of unconjugated NCS. A7-NCS exhibited a strong binding and cytotoxicity to the cell and an antigen-specific tumor accumulation. Significant tumoricidal effects in vivo were observed in the antigen-positive tumor-bearing mice treated with A7-NCS, whereas NCS mixed with mAb A7 and NCS alone were relatively ineffective. In the antigen-negative tumor, the tumoricidal effect of A7-NCS was lower than in the antigen-positive tumor. The NCS concentration in blood and tumor were significantly elevated by conjugation to mAb A7. The NCS localization in tumor was higher in the antigen-positive tumor than in the antigen-negative tumor. Death due to acute toxicity was observed at a dose of 20 units (U) NCS in mice treated with unconjugated NCS, whereas toxicity was seen with a much higher dose of NCS (100 U) if the drug was conjugated to the mAb. These findings show that mAb A7 confers more favorable pharmacological properties on an anticancer drug, making it potentially more useful for cancer chemotherapy.

Three-dimensional solution structure of apo-neocarzinostatin

Two and three-dimensional solution nuclear magnetic resonance studies of the 11K apoprotein from natural antitumor agent neocarzinostatin (NCS) were extended to elucidation of the high-resolution structure by the use of restrained molecular dynamics computations. The refined structures attained convergency upon three steps of iterative calculations, in which more distance restraints were extracted from experimental data, and the existing distance bounds were optimized on the basis of computed structures. The solution structures of apo-NCS contain seven antiparallel beta-strands, which form two closely located beta-sheets and a short beta-segment. This protein lacks any alpha-helical component. The alignment of the seven beta-strands gives rise to a beta-barrel with an elongated diameter in one direction. The global structure of apo-NCS resembles that of the Ig-fold domain found in immunoglobulins and other structurally related beta-proteins. Residues responsible for side-chain packing and the possible salt-bridge formation important for protein folding were identified. Neocarzinostatin and the analogous proteins are known to exert their biological activity through the interaction of DNA with a chromophoric molecule, which is non-covalently bound to the apo-proteins. This molecular chromophore-binding site in apo-NCS is made of a cavity consisting of residues from the four-beta-stranded sheet and the short beta-segment. Although the solution structures of apo-NCS are similar to that of the analogous apoauromomycin in the crystalline state, difference in the shape of the binding cavities between the two was found. This study provides a structural basis for characterization of the specific recognition and molecular mechanism of the antitumor NCS chromophore binding to its host protein.

Targeted enhancement of the biological activity of the antineoplastic agent, neocarzinostatin. Studies in murine neuroblastoma cells

The development of chemotherapeutic approaches to cancer has been hampered by the toxicity of proposed agents for normal rapidly dividing cells. By using neocarzinostatin, a "pro-drug" which is activated by reduction by thiol compounds, adjunctively with 6-mercaptodopamine, a thiol-containing dopamine analogue, we have been able to enhance neocarzinostatin toxicity for cells of the neural crest tumor neuroblastoma. Thiol compounds that are not neurotransmitter analogues do not act synergistically with neocarzinostatin in this system. Since most normal rapidly dividing cells do not have surface dopamine receptors, we propose this approach for the selective targeting of toxicity for neuroblastoma cells. We further introduce cell-selective activation of prodrugs as a new chemotherapeutic strategy which demands further development.

Three-dimensional solution structure of apo-neocarzinostatin from Streptomyces carzinostaticus determined by NMR spectroscopy

The three-dimensional solution structure of apo-neocarzinostatin has been resolved from nuclear magnetic resonance spectroscopy data. Up to 1034 constraints were used to generate an initial set of 45 structures using a distance geometry algorithm (DSPACE). From this set, ten structures were subjected to refinement by restrained energy minimization and molecular dynamics. The average atomic root mean square deviations between the final ten structures and the mean structure obtained by averaging their coordinates run from 0.085 nm for the best defined beta-sheet regions of the protein to 0.227 nm for the side chains of the most flexible loops. The solution structure of apo-neocarzinostatin is closely similar to that of the related proteins, macromomycin and actinoxanthin. It contains a seven-stranded antiparallel beta-barrel which forms, together with two external loops, a deep cavity that is the chromophore binding site. It is noteworthy that aromatic side chains extend into the binding cleft. They may be responsible for the stabilization of the holo-protein complex and for the chromophore specificity within the antitumoral family.