Acid phosphatase-1 (orthophosphoric monoester phosphohydrolase, acid optimum, EC 3.1.3.2), the major phosphatase in adult Drosophila melanogaster, has been purified to apparent homogeneity. The final product is a glycoprotein homodimer with a subunit molecular weight of about 50,000, as measured by its electrophoretic mobility in denaturing conditions on polyacrylamide gels containing sodium dodecyl sulfate. It has a turnover number of 1720 1-naphthyl phosphate molecules hydrolyzed/s by each acid phosphatase-1 molecule at 37 degrees C, pH 5.0. An average fly contains about 5 ng of enzyme. Pure acid phosphatase-1 displays heterogeneity in isoelectric focusing, with a major band at pH 5.3. The enzyme hydrolyzes a wide variety of phosphate monoesters, including AMP, glucose 6-phosphate, ATP, choline phosphate, or phosphoproteins. The maximum reaction rates are different for all substrates, and some substrates appear to inhibit the reaction at high substrate concentrations. The Michaelis constants for 1-naphthyl phosphate and p-nitrophenyl phosphate are 79 microM and 68 microM, respectively, at pH 5.0 and 37 degrees C. The optimum pH level for 1-naphthyl phosphate is 4.5. Acid phosphatase-1 is inhibited by L(+)-tartrate (but not D(-)-tartrate), phosphate, and fluoride. The reaction rate increases 2.1-fold for every 10 degrees C rise in temperature. Above 48 degrees C, the rate of thermal denaturation is greater than the rate of the enzyme reaction.