Prostatic Acid Phosphatase (ACPP) catalyzes the hydrolysis of a variety of phosphate monoesters, including phosphorylated proteins. The activity optimum of ACPP is in the pH range of 4-6, and the activity is inhibited by L(+)-tartrate. ACPP expression levels are highest in the prostate, with much lower expression in most other tissues. ACPP is a type I integral membrane protein of the plasma membrane and lysosomes, and a secreted form also exists. The concentration of ACPP is elevated in the circulation of prostate cancer patients, making the enzyme a marker for the progression of prostate cancer. Cellular ACPP has been shown to be a protein tyrosine phosphatase. Protein substrates include the epidermal growth factor receptor and HER-2. Cellular ACPP is considered to function as a tumor suppressor.