PH, (APEH, acylpeptide hydrolase), which catalyzes the hydrolysis of the terminal acetylated amino acid preferentially from small acetylated peptides. The acylpeptide hydrolase is a homotetrameric protein of 300 kDa with each subunit consisting of 732 amino acid residues. A range of molecular masses have been reported for APH by previous investigators, including 80 kD, 75 kD and 69 kD. This antibody can recognize at least 2 isoforms of APH.