Hysteresis on heating and cooling of E. coli alkaline phosphatase

Measurements of [theta](222) of E. coli phosphatase on heating from 20 degrees to 90 degrees and subsequent cooling to 20 degrees shows a gradual increase in [theta](222) on heating, while cooling shows a symmetric transition centered at 45 degrees . Reheating and cooling shows the same phenomenon. Enzyme heated and cooled once is fully active. The activity of the enzyme depends on its storage conditions (buffer and pH for example), but such changes are least to some extent reversible, especially by heating in different solvents. We conclude the enzyme exists in several forms which are in slow equilibrium with each other, so that the enzyme responds slowly when heated and hence is not at equilibrium during heating/cooling experiments.