Inhibition of a Zn(II)-containing enzyme, alcohol dehydrogenase, by anticancer antibiotics, mithramycin and chromomycin A3.

One of the major attributes for the biological action of the aureolic acid anticancer antibiotics chromomycin A(3) (CHR) and mithramycin (MTR) is their ability to bind bivalent cations such as Mg(II) and Zn(II) ions and form high affinity 2:1 complexes in terms of the antibiotic and the metal ion, respectively. As most of the cellular Zn(II) ion is found to be associated with proteins, we have examined the effect of MTR/CHR on the structure and function of a representative structurally well characterized Zn(II) metalloenzyme, alcohol dehydrogenase (ADH) from yeast. MTR and CHR inhibit enzyme activity of ADH with inhibitory constants of micromolar order. Results from size-exclusion column chromatography, dynamic light scattering, and isothermal titration calorimetry have suggested that the mechanism of inhibition of the metalloenzyme by the antibiotics is due to the antibiotic-induced disruption of the enzyme quaternary structure. The nature of the enzyme inhibition, the binding stoichiometry of two antibiotics per monomer, and comparable dissociation constants for the antibiotic and free (or substrate-bound) ADH imply that the association occurs as a consequence of the binding of the antibiotics to Zn(II) ion present at the structural center. Confocal microscopy shows the colocalization of the antibiotic and the metalloenzyme in HepG2 cells, thereby supporting the proposition of physical association between the antibiotic(s) and the enzyme inside the cell.