Effect of bilirubin on the Arrhenius plots for Na,K-ATPase activities of young and adult rat cerebra.

The effect of bilirubin on the temperature dependence of Na,K-ATPase activity was investigated with NaI-treated microsomes prepared from young (13- to 23-day-old) and adult (about 1-year-old) rat cerebra. The Arrhenius plots for the enzymic activities in the young and adult rats showed break temperatures at 26 and 18 degrees C, respectively. In the young rat enzyme, bilirubin caused a shift of the break temperature to 23 and 22 degrees C at concentrations of 60 and 120 microM, respectively, with a significant increase of the activation energy below the break temperature. However, no significant changes in the break temperature and the activation energy were observed in the adult rat enzyme exposed to the same concentrations of the pigment. The results suggest that the lipid environment surrounding the enzyme and modulating its activity in the plasma membranes may differ between the young and adult rat cerebra, and that bilirubin may change the physical state of the lipids related to the activity of the young rat Na,K-ATPase.