A study of the mechanism of ferritin formation. The effect of pH, ionic strength and temperature, inhibition by imidazole and kinetic analysis

The rate of ferritin formation in the buffers 4-morpholinepropanesulphonic acid (Mops), 4-morpholineethanesulphonic acid (Mes) and imidazole at pH values from 5.0 to 6.5 is quite similar. However, the rate of iron deposition is much greater in Mops and Mes at pH values above 6.5 than in imidazole. Increasing the concentration of imidazole inhibits ferritin formation and also leads to a transformation in the shape of the kinetic curves observed. This inhibiton is also observed at constant ionic strength but is not found for non-complexing buffers such as Mops. An inhibition of ferritin formation in imidazole and in Mops buffers is also observed with increasing ionic strength. We conclude that the unprotonated form of imidazole inhibits iron deposition, possibly by binding to the active site of the apoferritin molecule. The temperature dependence of iron deposition was examined. An optimum temperature of 50 degrees C was found but the Arrhenius plots were non-linear. On the basis of these and previous results, a kinetic model is developed which accounts well for ferritin formation at pH values below 6.5 and above 7.0 in non-complexing buffers. The model does not account for the kinetics observed at pH values close to neutrality.