Direct binding of bilirubin to GST-VWF73-H, VWF, and rADAMTS13, which is eliminated by bilirubin oxidase treatment. A, B, and C show the binding of unconjugated bilirubin at various concentrations (0-62.5 μM) to immobilized GST-rVWF73-H, plasma VWF, and rADAMTS13, respectively. D, E, and F show the binding of unconjugated bilirubin that was pretreated for 15 min with bilirubin oxidase (2 U/ml) to GST-VWF73-H, plasma VWF, and rADAMTS13, respectively. All data shown were the net response units after subtracting the response units in the control cell that was activated and blocked with 1 M ethanolamine (pH 8.5) without being immobilized with a protein of interest.