Advanced oxidation protein products in plasma: stability during storage and correlation with other clinical characteristics

Proteins are susceptible to free radical damage. We measured advanced oxidation protein products (AOPP) in the plasma of 56 hospitalised patients. Concentrations of AOPP were expressed as chloramine-T equivalents by measuring absorbance in acidic conditions at 340 nm in the presence of potassium iodide. We also determined erythrocyte sedimentation rate (ESR), circulating urea, creatinine, glucose, uric acid, electrolytes, lipids, total proteins and fractions and fibrinogen. Twenty-four samples were processed both immediately and after 7, 15, 30, 90, 180 and 438 days of storage at both at -20 degrees C and -80 degrees C (aliquots were frozen and thawed only once) to evaluate AOPP stability. The remaining 32 samples were also processed for thiobarbituric-acid-reactive substances (TBARS). Mean AOPP concentration in all 56 patients was 48.3+/-37.2 microM. Mean basal concentration of AOPP in the 24 plasma samples (55.0+/-47.1 microM) showed no significant change at each intermediate determination, yet significantly increased after 438 days of storage both at -80 degrees C (96.6+/-83.2, p