Not superoxide but an impurity in xanthine oxidase increases endothelin binding to pig coronary artery smooth muscle

To determine the effect of superoxide on endothelin (ET) binding to its receptors in microsomes from pig coronary artery smooth muscle, we used xanthine oxidase (XO) purified from bovine milk + 0.3 mM xanthine (X) to generate superoxide Pretreating the microsomes with XO + X increased the ET-1 binding to ETA and ETB sites However, XO alone also caused the binding to increase. A bacterial XO preparation did not alter the ET-1 binding with or without X. Thus, superoxide does not affect ET-1 binding but bovine milk XO preparation gives an artefact unrelated to superoxide generation.