Association of antitumor antibiotic Mithramycin with Mn2+ and the potential cellular targets of Mithramycin after association with Mn2+.

Mithramycin (MTR), an aureolic acid group of antitumor antibiotic is used for the treatment of several types of tumors. We have reported here the association of MTR with an essential micronutrient, manganese (Mn2+). Spectroscopic methods have been used to characterize and understand the kinetics and mechanism of complex formation between them. MTR forms a single type of complex with Mn2+ in the mole ratio of 2:1 [MTR: Mn2+] via a two step kinetic process. Circular dichroism (CD) spectroscopic study indicates that the complex [(MTR)(2) Mn2+] has a right-handed twist conformation similar in structure with the complexes reported for Mg2+ and Zn2+. This conformation allows binding via minor groove of DNA with (G, C) base preference during the interaction with double-stranded B-DNA. Using absorbance, fluorescence, and CD spectroscopy we have shown that [(MTR)(2) Mn2+] complex binds to double-stranded DNA with an apparent dissociation constant of 32 mu M and binding site size of 0.2 (drug/nucleotide). It binds to chicken liver chromatin with apparent dissociation constant value 298 mu M. Presence of histone proteins in chromatin inhibits the accessibility of the complex for chromosomal DNA. We have also shown that MTR binds to Mn2+ containing metalloenzyme manganese superoxide dismutase from Escherichia coli.