Interactions between fluoroquinolones, Mg2+, DNA and DNA gyrase, studied by phase partitioning in an aqueous two-phase system and by affinity chromatography.

The primary target of fluoroquinolones has been identified as the enzyme DNA gyrase, but the mechanism of action of these antibacterial agents is still a matter of controversy. Using partitioning in aqueous polyethylene glycol (PEG)-dextran systems, the affinities of several fluoroquinolones for DNA were determined with accuracy and at equilibrium. It was proved that the binding was strongly dependent on the ability of the drugs to bind Mg2+, with KA values of about 40 000 l mol-1, but was poorly related to the antibacterial activity [minimal inhibitory concentration (MIC) and gyrase inhibition]. Using affinity chromatography on immobilized fluoroquinolone, it was shown that DNA gyrase was unable to bind fluoroquinolones in the absence of DNA, but that a DNA-quinolone-gyrase complex was formed in the presence of Mg2+.