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Examines the role of postactivated neocarzinostatin chromophore (NCS-chrom) in the formation of DNA bulge structure. Identification on the DNA chain cleavage properties of NCS-chrom; Occurrence of bulge-DNA sequences; Mechanisms involved in the formation of binding and cleaving species.

Presents the results of a study which characterized the base-catalyzed thiol-independent cleavage reaction of neocarzinostatin chromophore with long single-stranded DNA in order to use this reaction as a selective probe for the tertiary structure of naturally occurring ss nucleic acid. Base-catalyzed drug reaction with RNA.

Investigates the specific binding of the biradical analog of the anticancer antibiotic neocarzinostatin chromophore to bulged DNA. Mechanisms for thiol-dependent cleavage; Oligodeoxynucleotides in the bulged structure; Fluorescence quenching.

Reports three-dimensional structure of glutathione post activated neocarzinostatin chromophore-DNA complex by NMR and distance geometry molecular-dynamics simulation techniques. Description of materials and methods; Results and discussion; Conclusions.

Our previous structure elucidation of the complexes of DNA and postactivated neocarzinostatin chromophore (NCS-chrom) compounds revealed two distinctly different binding modes of this antitumor molecule. A thorough understanding of these results will provide the molecular basis for the binding and DNA chain cleavage properties of NCS-chrom. NCSi-gb is one of the postactivated mimics of NCS-chrom which is formed under thiol-free conditions and is able to bind to DNA. This report describes the structure refinement of the NCSi-gb-bulge-DNA complex [Stassinopoulos, A., Jie, J., Gao, X., and Goldberg, I. H. (1996) Science 272, 1943-1946] and the NMR characterization of the free bulge-DNA and free NCSi-gb. These results reveal that the formation of the complex involves conformational changes in both the DNA and the ligand molecule. Of mechanistic importance for the NCS-chrom-DNA interaction, the two ring systems of the drug are brought closer to each other in the complex. This conformation correlates well with the previously observed marked enhancement of the formation of a DNA bulge cleaving species in the presence of bulge-DNA sequences, due to the promotion of the intramolecular radical quenching of the activated NCS-chrom. Interestingly, the binding of NCSi-gb promotes the formation of a bulge binding pocket; this was not found in the unbound DNA. NCS-chrom is unique among the enediyne antibiotics in its ability to undergo two different mechanisms of activation to form two different DNA binding and cleaving species. The two corresponding DNA complexes are compared. One, the bulge-DNA binder NCSi-gb, involves the major groove, and the second, the duplex binder NCSi-glu which is generated by glutathione-induced activation, involves the minor groove. Since the two NCS-chrom-related ligand molecules contain some common chemical structural elements, such as the carbohydrate ring, the striking differences in their DNA recognition and chain cleavage specificity provide insights into the fundamental principles of DNA recognition and ligand design.

We have developed a new method utilizing affinity capillary electrophoresis (ACE) for the determination of binding stoichiometries in biochemical systems. Using the same concentration of a ligand in the sample and the electrophoresis buffer, the appearance of an inverted peak corresponding to the free ligand in the resulting electropherogram provides a criterion of binding of a ligand to its receptor protein. For both low (fast off rates) and high (slow off rates) affinity systems, analysis of the integration of free ligand peak in electropherograms as a function of the total concentration of a ligand in samples at constant concentration of receptor protein yields the binding stoichiometry of the ligand to the protein. Applications of this technique to studies of (i) the inhibition of carbonic anhydrases (CA, EC 4.2.1.1, from human and bovine erythrocytes) by 4-alkylbenzenesulfonamide 1, (ii) the interaction of a monoclonal antibody to human serum albumin (anti-HSA) with its antigen HSA, and (iii) the binding of streptavidin (from Streptomyces avidinii) to biotin derivatives (monobiotinylated oligodeoxyribonucleotide 2, fluorescein biotin, or Lucifer Yellow biotin) yield stoichiometries of 1:1, 1:2, and 1:4, respectively. For multivalent, tight-binding systems, this ACE method can readily separate stable intermediate species. This method is generally applicable to both tight- and weak-binding systems, requires only nanograms of proteins and ligands, involves no radioactive materials, and does not require changes in electrophoretic mobilities of receptor proteins upon binding with ligands. It thereby provides a rapid, sensitive, and convenient method for measuring binding stoichiometries of ligands to proteins.