Escherichia coli beta-galactosidase is heterogeneous with respect to a requirement for magnesium.

Commercially obtained E. coli beta-galactosidase was stored at 25 degrees C in buffer containing 1 mM MgCl2 and in buffer containing no added MgCl2. Samples were removed at set times and the activity of individual enzyme molecules assayed. When stored in the presence of 1 mM magnesium, the number of active molecules did not change over a 2.5-h period. When stored in the absence of added MgCl2, over half the enzyme molecules became inactive within the first hour. However, those molecules which retained activity remained active for the duration of the experiment. This indicates that there may exist two populations of E. coli beta-galactosidase, one which requires storage in the presence of the higher concentration of Mg2+ in order to remain active. There was no observed correlation between this requirement for magnesium and reaction rate. Additionally, the presence of the 1 mM MgCl2 was found to decrease the average activity of the beta-galactosidase molecules under the conditions employed.