Properties of the induced acid phosphatase and of the constitutive acid phosphatase of Euglena

The induced acid phosphatase (EC 3.1.3.2) of Euglena gracilis has been solubilized and partially purified. The enzyme has a wide range of substrate specificity and, as predicted from earlier studies on whole cells, the ratio of its activity at pH 5 to its activity at pH 7 in presence of 5 mM fluoride is 1.1 for $\text{p-}\text{nitrophenylphosphate}$ as substrate. The enzyme is competitively inhibited by arsenate and phosphate, but exhibits mixed competitive-non-competitive inhibition with molybdate. The enzyme migrates towards the cathode when electrophoresis is performed on cellulose acetate strips at pH 8.2. Euglena also contains several other acid phosphatases. The two major constitutive acid phosphatases, which remain particle-bound after a variety of extraction procedures, differ in their thermal stability from each other and from the induced phosphatase. The ratio of activity at pH 5 to the activity at pH 7 plus 5 mM fluoride for the mixture of these two constitutive enzymes is 15. These observations establish that the increase in acid phosphatase activity occurring in Euglena in response to phosphate deprivation is due to the synthesis of a separate enzyme. The heat of thermal inactivation of the purified induced enzyme is about four times larger than the heat of denaturation computed from the effect of temperature on the rate of reversion of the induced phosphatase in vivo.