title: Fluoroquinolones stimulate the DNA cleavage activity of topoisomerase IV by promoting the binding of Mg(2+) to the second metal binding site.
abstract: BACKGROUND
  Fluoroquinolones target bacterial type IIA topoisomerases, DNA gyrase and topoisomerase IV (Topo IV). Fluoroquinolones trap a topoisomerase-DNA covalent complex as a topoisomerase-fluoroquinolone-DNA ternary complex and ternary complex formation is critical for their cytotoxicity. A divalent metal ion is required for type IIA topoisomerase-catalyzed strand breakage and religation reactions. Recent studies have suggested that type IIA topoisomerases use two metal ions, one structural and one catalytic, to carry out the strand breakage reaction.
  
  
  METHODS
  We conducted a series of DNA cleavage assays to examine the effects of fluoroquinolones and quinazolinediones on Mg(2+)-, Mn(2+)-, or Ca(2+)-supported DNA cleavage activity of Escherichia coli Topo IV.
  
  
  RESULTS
  In the absence of any drug, 20-30 mM Mg(2+) was required for the maximum levels of the DNA cleavage activity of Topo IV, whereas approximately 1mM of either Mn(2+) or Ca(2+) was sufficient to support the maximum levels of the DNA cleavage activity of Topo IV. Fluoroquinolones promoted the Topo IV-catalyzed strand breakage reaction at low Mg(2+) concentrations where Topo IV alone could not efficiently cleave DNA.
  
  
  CONCLUSIONS
  At low Mg(2+) concentrations, fluoroquinolones may stimulate the Topo IV-catalyzed strand breakage reaction by promoting Mg(2+) binding to metal binding site B through the structural distortion in DNA. As Mg(2+) concentration increases, fluoroquinolones may inhibit the religation reaction by either stabilizing Mg(2+) at site B or inhibition the binding of Mg(2+) to site A. This study provides a molecular basis of how fluoroquinolones stimulate the Topo IV-catalyzed strand breakage reaction by modulating Mg(2+) binding.
authors.affiliation: Department of Pharmacology, University of Minnesota Medical School, 6-120 Jackson Hall, 321 Church Street SE, Minneapolis, MN 55455, USA. Electronic address: brown235@umn.edu.
authors.forename: Lisa M
authors.initials: LM
authors.lastname: Oppegard
authors.affiliation: Division of Medicinal and Natural Products Chemistry, Department of Pharmaceutical Sciences and Experimental Therapeutics, University of Iowa, 115 S Grand Ave., S321 Pharmacy Building, Iowa City, IA 52242, USA. Electronic address: heidiannschwanz@gmail.com.
authors.forename: Heidi A
authors.initials: HA
authors.lastname: Schwanz
authors.affiliation: Division of Medicinal and Natural Products Chemistry, Department of Pharmaceutical Sciences and Experimental Therapeutics, University of Iowa, 115 S Grand Ave., S321 Pharmacy Building, Iowa City, IA 52242, USA. Electronic address: tyrelltowle@gmail.com.
authors.forename: Tyrell R
authors.initials: TR
authors.lastname: Towle
authors.affiliation: Division of Medicinal and Natural Products Chemistry, Department of Pharmaceutical Sciences and Experimental Therapeutics, University of Iowa, 115 S Grand Ave., S321 Pharmacy Building, Iowa City, IA 52242, USA. Electronic address: robert-kerns@uiowa.edu.
authors.forename: Robert J
authors.initials: RJ
authors.lastname: Kerns
authors.affiliation: Department of Pharmacology, University of Minnesota Medical School, 6-120 Jackson Hall, 321 Church Street SE, Minneapolis, MN 55455, USA. Electronic address: hiasa001@umn.edu.
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Hiasa
mesh_terms: D001665:Binding Sites; D002118:Calcium / Q000494:pharmacology; D002384:Catalysis; D053837:DNA Cleavage / Q000187:drug effects*; D027101:DNA Topoisomerase IV / Q000378:metabolism*; D024841:Fluoroquinolones / Q000494:pharmacology*; D008274:Magnesium / Q000378:metabolism* / Q000494:pharmacology
chemical_list: D024841:Fluoroquinolones; D027101:DNA Topoisomerase IV; D008274:Magnesium; D002118:Calcium
keywords: DNA gyrase; Divalent metal ion; Fluoroquinolone; Quinazolinedione; Topoisomerase IV