Albumin, the main protein in plasma, is prone to different mechanisms of oxidative modification since extracellular fluids contain only small amounts of antioxidant enzymes. The redox state of cysteine-34 of human albumin defines three fractions which allow to monitor albumin oxidation: mercaptalbumin with a free thiol group, nonmercaptalbumin1 containing a disulfide, and nonmercaptalbumin2 with a sulfinic or sulfonic acid group. These fractions can be separated by HPLC and detected with UV or fluorescence detection. The method is very rugged and only simple sample preparation is needed. It has been used to demonstrate albumin oxidation during exercise, aging, and pathologies like diabetes, liver disease, or renal disease. Problems may arise when high endogenous concentrations of bilirubin or certain drugs are present. The redox state of albumin shows high variability and is hence a valuable tool for the investigation of reversible and irreversible modification of plasma protein.
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