Assessment of GAPDH enzymatic activity in total RBCs and RBC supernatants at storage days 2, 21, and 42. GAPDH activity was measured utilizing a commercial kit with detector molecule leading to increased absorbance at 450 nm in a manner directly proportional to GAPDH activity. The assay was carried out in the absence (blue lines) and presence (red lines) of DTT (2.5 mM). Bar plots at right represent the relative initial rates (0-10 minutes) of GAPDH catalysis ± DTT.