zinostatin and sodium-borohydride

The structures of mercaptan and sodium borohydride reaction products of neocarzinostatin chromophore A (NCS Chrom A) are compared. Implications on the mechanism of activation of NCS are discussed.

Topics: Antibiotics, Antineoplastic; Borohydrides; Chemical Phenomena; Chemistry; Enediynes; Magnetic Resonance Spectroscopy; Molecular Structure; Thioglycolates; Zinostatin

Neocarzinostatin chromophore produces alkali-labile, abasic sites at cytidylate residues in AGC sequences in oligonucleotides in their duplex form. Glutathione is the preferred thiol activator of the drug in the formation of these lesions. The phosphodiester linkages on each side of the abasic site are intact, but when treated with alkali, breaks are formed with phosphate moieties at each end. Similar properties are exhibited by the abasic lesions produced at the purine residue to which the C in AGC is base-paired on the complementary strand. The abasic sites at C residues differ from those produced by acid-induced depurination in the much greater lability of the phosphodiester linkages on both sides of the deoxyribose, in the inability of NaBH4 to prevent alkali-induced cleavage, and in the relative resistance to apurinic/apyrimidinic endonucleases. The importance of DNA microstructure in determining attack site specificity in abasic site formation at C residues is shown not only by the requirement for the sequence AGC but also by the findings that substitution of G by I 5' to the C decreases the attack at C, whereas placement of an I opposite the C markedly enhances the reaction. Quantitation of the abstraction of 3H into the drug from C residues in AGC specifically labeled in the deoxyribose at C-5' or C-1',2' suggests that, in contrast to the attack at C-5' in the induction of direct strand breaks at T residues, abasic site formation at C residues may involve attack at C-1'. Each type of lesion may exist on the complementary strands of the same DNA molecule, forming a double-stranded lesion.

Topics: Antibiotics, Antineoplastic; Base Sequence; Borohydrides; Cytidine Monophosphate; Cytosine Nucleotides; Deoxyribonuclease IV (Phage T4-Induced); DNA-(Apurinic or Apyrimidinic Site) Lyase; Endodeoxyribonucleases; Oligodeoxyribonucleotides; Oxidation-Reduction; Zinostatin

Spectroscopic evidence suggests the presence of a highly strained ether ring (Fig. 1) (possibly an epoxide) in the C12-subunit of the previously determined partial structure 2a (Fig. 2) of the major neocarzinostatin chromophore (NCS-Chrom A) which completes assignment of all the oxygens in the molecule. The main product from mercaptan treatment suggests opening of the ether ring involving the addition of one molecule of mercaptan as well as reduction of the C12-substructure, whereas a parallel two-step reduction occurs on NaBH4 treatment. Both reactions occur with rearrangement of the C12-substructure and the implication for the mechanism of action of NCS-Chrom A in DNA strand scission activity is discussed. The evidence suggests a downward revision of the molecular formula for NCS-Chrom A as well as minor components B and C by two protons.

Topics: Antibiotics, Antineoplastic; Borohydrides; Chemical Phenomena; Chemistry; Enediynes; Ethers, Cyclic; Magnetic Resonance Spectroscopy; Structure-Activity Relationship; Sulfhydryl Compounds; Zinostatin