Abstract
The protein-damaging potential of photosensitized tetracycline hydrochloride alone and in combination with the metal ion Cu(II) was assessed using serum albumin as a model protein. Exposure of tetracycline to white light in an aqueous solution triggered the generation of significant amounts of reactive oxygen species (ROS) and engendered substantial protein damage. The appearance of distinct low-molecular-mass protein bands on 10% SDS-polyacrylamide gel ascertained the tetracycline concentration-dependent fragmentation of albumin. Photoexcited tetracycline in combination with 100 μM Cu(II) enhanced the protein fragmentation process with concurrent increase in free radical production. The significant release of acid-soluble amino groups and carbonyl groups from treated albumin provided quantitative estimation of protein fragmentation at 0–1.0 mM concentrations of tetracycline. Cu(II) ions per se did not cause any perceptible protein damage. The results with free radical quenchers suggested the role of hydroxyl radicals (
OH) in tetracycline–Cu(II)-induced protein fragmentation, as no superoxide dismutase (SOD)-mediated quenching effect was noted. The generation of free radicals upon tetracycline photoexcitation and consequent protein fragmentation may be considered as important factors in augmentation of tetracycline-induced phototoxic responses.