Background: In order to investigate the therapeutic interaction of an extra-cellular photosensitization reaction, we evaluated the oxidation characteristics of human and bovine serum albumin by this reaction with talaporfin sodium under complete binding with albumin by spectroscopic analysis in a cell-free solution. Methods: The solution was composed of 20. μg/ml talaporfin sodium and 2.1. mg/ml human or bovine serum albumin. A 662. nm laser light was used to irradiate the solution. Visible absorbance spectra of solutions were measured to obtain the oxidized and non-oxidized relative densities of albumin and talaporfin sodium before and after the photosensitization reaction. The defined oxidation path ratio of talaporfin sodium to albumin reflected the oxidation of the solution. Absorbance wavelengths at approximately 240 and 660. nm were used to calculate normalized molecular densities of oxidized albumin and talaporfin sodium, respectively. Results and conclusions: The oxidation path ratio of talaporfin sodium to albumin when binding human serum albumin was approximately 1.8 times larger than that of bovine serum albumin during the photosensitization reaction from 1 to 50J/cm2. We hypothesized that the oxidation path ratio results might have been caused by talaporfin sodium binding affinity or binding location difference between the two albumins, because the fluorescence lifetimes of talaporfin sodium bound to human and bovine serum albumin were 7.0 and 4.9ns, respectively. Therefore, the photodynamic therapeutic interaction might be stronger with human serum albumin than with bovine serum albumin in the case of extracellular photosensitization reaction.
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