INDUCTION OF BASE DAMAGE IN DNA SOLUTIONS BY ULTRASONIC CAVITATION

Ultrasound can damage macromolecules by the mechanical (shearing) and sonochemical (free radical generating) action of ultrasonic cavitation . Attributing macromolecular damage to either direct mechanical stress or to indirect mechanisms involving free radicals or other sonochemic als is a challenging problem. DNA damage induced by ultrasound was eva luated by measuring the formation of purine and pyrimidine products us ing combined gas chromatography-mass spectrometry with selected ion mo nitoring. Samples of DNA were prepared in 10 mmol dm(-3) phosphate buf fered saline (pH 7.4) and saturated with a mixture of argon:oxygen (3: 1). Continuous 2.17 MHz ultrasound exposures at 0.82 mPa spatial peak negative pressure amplitude were performed in a 60 rpm rotating tube e xposure system. Hydrogen peroxide yields were measured after each expo sure to quantify the cavitation activity and ranged up to 350 mu mol d m(-3) for 1-h exposures. Purine and pyrimidine products identified wer e those typically observed following exposure of DNA to hydroxyl radic al-generating systems, such as ionizing radiation, hypoxanthine/xanthi ne oxidase, or hydrogen peroxide in the presence of transition metal i ons. The yields of these products were directly correlated with cavita tion activity as measured by residual hydrogen peroxide concentrations . The yields of DNA products increased in the following order: thymine glycol similar to cytosine glycol>8-oxoAde>FAPyAde similar to 5-HMU s imilar to 5,6-diOHCyt>FAPyGua. Unexpectedly, 8-oxoguanine did not exhi bit a dose-dependent increase above background levels, and this observ ation is inconsistent with processes involving metal ion-dependent for mation of hydroxyl radicals from hydrogen peroxide. In addition, the p roduct yields were far too large to result from the residual hydrogen peroxide. Thus, ultrasonic cavitation appears to have a mode of action distinct from either ionizing radiation or formation of hydroxyl radi cals via Fenton-like reaction with transition metals.