The Binding of Tetracycline Analogs to Conalbumin in the Absence and Presence of Cupric Ions

Equilibrium and kinetic studies on the interaction of tetracyclines with calcium and magnesium

The interaction of Ca2+ and Mg2+ with three Tetracycline antibiotics (tetracycline, chlorotetracycline, and oxytetracycline) has been investigated. Spectrophotometric measurements have been used to determine the apparent association constant for this interaction as a function of pH. It is shown that the results are consistent with a model in which the metal ion can form complexes with both the fully-deprotonated and mono-protonated forms of the Tetracycline. The temperature-jump relaxation method has been used to measure the kinetics of formation of the complexes of Mg2+ with the Tetracyclines. The results are compared with those of previous studies of Mg2+ complex formation reactions and it is shown that the data is consistent with the normal dissociative model. A possible role for metal ion chelation in the mechanism of antibacterial action of the Tetracyclines is discussed.

Oxidation photosensitized by tetracyclines

Irradiation with 365-nm UV light of aerated aqueous solutions of tetracycline gives rise to oxygen uptake when the pH of the solution is above 7.5. The kinetics of the reaction were followed using a polarographic oxygen electrode at a range of pH values for seven currently prescribed tetracyclines. Variation of tetracycline concentration, UV light intensity, and temperature showed the characteristics normally associated with a sensitized photo-oxygenation mechanism rather than a free-radical process. Copper(II) ions inhibited the photo-oxidation of tetracycline, apparently by complex formation. The tetracyclines were tested for photosensitizing capability with oxidizable acceptors. In aqueous solution, no photosensitizing effect could be seen, but methanol solutions of 2,5-dimethylfuran and dl-limonene were oxidized at considerably increased rates when small amounts of tetracyclines were present. This observations has implications for the mechanism of in vivo photosensitivity reactions that occur when tetracyclines are taken internally.