Can hydrolyze phosphorylated Ser-, Thr- or Tyr-substrates in vitro. The natural substrate is unknown. H2O + O-phospho-L-seryl-[protein] = L-seryl-[protein] + phosphate H2O + O-phospho-L-threonyl-[protein] = L-threonyl-[protein] + phosphate Inhibited by cadmium, copper, zinc when added activity but with less efficiency. Optimum pH is 7.5-8.5. Optimum temperature is 30-37 degrees Celsius. Neither magnesium nor calcium stimulates activity. Belongs to the PPP phosphatase family. PP-1 subfamily.