Prostatic acid phosphatase (PAP), also prostatic specific acid phosphatase (PSAP), is an enzyme produced by the prostate. It may be found in increased amounts in men who have prostate cancer or other diseases.
Certain medications can cause temporary increases or decreases in acid phosphatase levels. Manipulation of the prostate gland through massage, biopsy or rectal exam before a test may increase the level.
Its physiological function may be associated with the liquefaction process of semen.
PSAP was used to monitor and assess progression of prostate cancer until the introduction of prostate specific antigen (PSA), which has now largely displaced it. Recent work, suggesting it has a role in prognosticating intermediate and high-risk prostate cancer, has led to renewed interest in this marker.
PAP may play an important role in the transmission of HIV. Researchers at the University of Ulm in Germany found that PAP forms fibers made of amyloid. They called the fibers Semen-derived Enhancer of Virus Infection (SEVI) and showed that they capture HIV virions promoting their attachment to target cells. The association of PAP with HIV may increase the ability of the virus to infect human cells "by several orders of magnitude." PAP may be a future target of efforts to combat the spread of HIV infection.
A study at the University of North Carolina and University of Helsinki suggested that PAP could have potent antinociceptive, antihyperalgesic, and antiallodynic effects that last longer than morphine. One dose of PAP lasted for up to three days, much longer than the five hours gained with a single dose of morphine. When in distress, nerve cells release a chemical known as adenosine triphosphate (ATP) which in turn invokes a painful sensation. ATP is broken down into AMP which PAP converts into adenosine, a molecule known to suppress pain. 
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1cvi: CRYSTAL STRUCTURE OF HUMAN PROSTATIC ACID PHOSPHATASE
1nd5: Crystal Structures of Human Prostatic Acid Phosphatase in Complex with a Phosphate Ion and alpha-Benzylaminobenzylphosphonic Acid Update the Mechanistic Picture and Offer New Insights into Inhibitor Design
1nd6: Crystal Structures of Human Prostatic Acid Phosphatase in Complex with a Phosphate Ion and alpha-Benzylaminobenzylphosphonic Acid Update the Mechanistic Picture and Offer New Insights into Inhibitor Design
2hpa: STRUCTURAL ORIGINS OF L(+)-TARTRATE INHIBITION OF HUMAN PROSTATIC ACID PHOSPHATASE