Iron-containing acid phosphatases: Characterization of the metal-ion binding site of the enzyme from pig allantoic fluid

All of the iron can be removed from the violet acid phosphatase of pig allantoic fluid by treatment with sodium dithionite at pH 4.9. Of the two moles of iron present per mole of enzyme (40,000 g), half is lost rapidly, and the remainder much more slowly. Removal of half of the iron causes complete loss of acid phosphatase activity. Conditions have been defined for the isolation and complete reconstitution [by Fe(II) and β-mercaptoethanol] of two apoenzymes, designated “iron-free” and “one-iron” apoenzymes. Zn 2+ ions restore most of the acid phosphatase activity to the one-iron apoenzyme but not to the iron-free enzyme. No metal ions other than Fe(II) and Fe(III) restore significant activity to the iron-free apoenzyme, but Zn(II) and Ni(II) bind tightly to it.