ROLE OF MAGNESIUM-ION IN MITHRAMYCIN DNA INTERACTION - BINDING OF MITHRAMYCIN MG2+ COMPLEXES WITH DNA

Mithramycin is an anticancer drug that blocks macromolecular synthesis via reversible interaction with the DNA template in the presence of bivalent metal ions such as Mg2+. The role of Mg2+ in this antibiotic-DNA interaction is not clear. We approached the problem in two steps via studies on the interactions between (i) mithramycin and Mg2+ and (ii) mithramycin-Mg2+ complex(es) and DNA. Spectroscopic techniques such as absorption, fluorescence, and CD were employed for the purpose. From equilibrium and kinetic studies, we earlier reported that MTR forms two different types of complexes with Mg2+ [Aich, P., and Dasgupta, D. (1990) Biochem. Biophys. Res. Commun. 173, 689]. The two complexes are referred to as complex I (with 1:1 stoichiometry in terms of mithramycin:Mg2+) and complex II (with 2:1 stoichiometry in terms of mithramycin:Mg2+). In this report, we have further characterized these complexes by fluorescence spectroscopy. Interactions of these complexes with calf thymus DNA were examined to elucidate their binding. Evaluation of binding parameters (intrinsic binding constant and stoichiometry) from spectrophotometric and fluorimetric titrations suggests that the complexes bind differently to the same DNA. Measurement of van't Hoff enthalpies for the interaction of the two ligands and DNA shows that the complex I-DNA interaction is exothermic, in contrast to the endothermic nature of the complex II-DNA interaction. This could originate from a difference in the molecular nature of the interactions between the complexes and calf thymus DNA. Our studies to detect the nature of the groove via which these complexes bind to DNA suggest that both complexes approach via the minor groove of the DNA. These studies help us to understand the role of Mg2+ in the DNA-binding properties of mithramycin. A comparative study is also made among the Mg2+- and DNA-binding properties of mithramycin and another aureolic acid class of antibiotic, chromomycin A(3).