Titanium has been successfully used for decades in dental and orthopedic implants, but the mechanism for this metal's biocompatible properties have not been determined. Our hypothesis is that this biocompatibility involves interaction between the surface layer of titanium dioxide on the metal implant and reactive oxygen mediators of the inflammatory response. The affect of different titanium surface oxide layers on the reactive oxygen mediators produced during the inflammatory response has never been examined. Peroxynitrite is a highly reactive and unstable compound produced in vivo by the reaction of nitric oxide with superoxide. We investigated if titanium oxides affect the stability of peroxynitrite by promoting its breakdown. Peroxynitrite levels can be measured by its absorbance at 302 nm. At pH= 13.2, we found a 100% increase in the rate of degradation of peroxynitrite in the presence of titanium particles. Peroxynitrite is capable of nitrating 4-hydroxyphenolacetic acid (4-HPA). The nitrated form of 4-HPA can be measured by its absorbance at 432nm. 3-morpholinosydnonimine (SIN-1), a nitric oxide donor, has been shown to produce superoxide during its breakdown resulting in the formation of peroxynitrite. At physiological pH (7.4), a solution of 0.5mM 4-HPA was exposed to 5mM SIN- 1 on passivated titanium surfaces. There was a decrease of 58% nitrated 4-HPA in the solution exposed to passivated titanium compared to controls. Unpassivated titanium surfaces resulted in only a 10% decrease of nitrated 4-HPA while titanium treated with hydrogen peroxide resulted in a 70% decrease in nitrated 4-HPA concentrations compared to controls. Zirconium and palladium were also tested. These experiments suggest that titanium is capable of enhancing the breakdown of the inflammatory compound peroxynitrite which may account for the metal's biocompatible properties.
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