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

Commercially obtained E. coli ss -galactosidase was stored at 25 degreesC i n buffer containing 1 mM MgCl2 and in buffer containing no added MgCl2. Sam ples were removed at set times and the activity of individual enzyme molecu les assayed. When stored in the presence of 1 mM magnesium, the number of a ctive molecules did not change over a 2.5-h period. When stored in the abse nce of added MgCl2, over half the enzyme molecules became inactive within t he first hour. However, those molecules which retained activity remained ac tive for the duration of the experiment. This indicates that there may exis t two populations of E. coli ss -galactosidase, one which requires storage in the presence of the higher concentration of Mg2+ in order to remain acti ve. There was no observed correlation between this requirement for magnesiu m and reaction rate. Additionally, the presence of the 1 mM MgCl2 was found to decrease the average activity of the ss -galactosidase molecules under the conditions employed.