Effect of bilirubin on ATPase activity of human erythrocyte membranes

The effect of bilirubin on ATPase activity of human erythrocyte membranes was studied to assess the possible mechanism of bilirubin toxicity on membrane function. Bilirubin inhibited (Na+ + K+)-ATPase activity of erythrocyte membranes and the degree of inhibition varied according to the reaction temperature, showing a stronger inhibition at lower temperature. A discontinuous point in Arrhenius plots of ATPase activity of erythrocyte membranes was shifted to a higher temperature and the activation energies were then increased in the presence of bilirubin. The experiments are consistent in suggesting that bilirubin, presumably by direct binding, induces a change of the physical state of the phospholipids associated with the ATPase activity of erythrocyte membranes.