Ionization and divalent cation dissociation constants of nalidixic and oxolinic acids

The ionization constants and some divalent cation dissociation constants of nalidixic and oxolinic acids, both specific inhibitors of bacterial DNA replication, have been determined. The carboxylic pK_a′ values are 6.1 and 6.9 at 25° for nalidixic and oxolinic acids, respectively. These values indicate that intramolecular hydrogen-bonding stabilizes the un-ionized form of these compounds in aqueous solution. Both compounds bind divalent cations; the divalent cation dissociation constants for oxolinic acid are somewhat smaller that those for nalidixic acid. We suggest that both compounds may act by forming a complex in situ with a divalent cation in a metalloprotein involved in DNA replication. The evidence that both drugs inhibit at the same target site is briefly reviewed.