A Repressible Alkaline Phosphatase in Neurospora crassa

The repressible alkaline phosphatase in Neurospora crassa was investigated with respect to a number of its enzymatic properties. Ethylenediaminetetraacetic acid and its structural analogue, N-hydroxyethylethylenediaminetriacetic acid, stimulate the activity of this enzyme. This stimulation by EDTA is not associated with a measurable change in the pK of a protein functional group that is essential for enzymatic activity, as evidenced by kinetic data obtained over a pH range between 5.6 and 10.6. A number of other chelating agents that were tested did not stimulate the activity of this phosphatase, although several caused inactivation in the absence of substrate. The enzyme is more stable toward denaturation in the presence of certain divalent cations such as Mg2+ or Zn2+. Under a variety of experimental conditions, these metal ions can also facilitate the recovery of phosphatase activity after exposure to various denaturing agents. Although metal-binding sites on the alkaline phosphatase seem to be involved in the maintenance and restoration of its active structure, an absolute metal requirement for phosphatase activity remains in doubt.