SLPI

Antileukoproteinase also known as secretory leukocyte protease inhibitor (SLPI) is an is_associated_with::enzyme that in humans is encoded by the SLPI is_associated_with::gene. SLPI is a highly is_associated_with::cationic single-chain is_associated_with::protein with eight intramolecular is_associated_with::disulfide bonds. It is found in large quantities in bronchial, cervical, and nasal is_associated_with::mucosa, saliva, and seminal fluids. SLPI inhibits human is_associated_with::leukocyte elastase, human is_associated_with::cathepsin G, human is_associated_with::trypsin, neutrophil elastase, and mast cell is_associated_with::chymase. is_associated_with::X-ray crystallography has shown that SLPI has two homologous domains of 53 and 54 amino acids, one of which exhibits anti-protease activity (C-terminal domain). The other domain (N-terminal domain) is not known to have any function.

Function
This gene encodes a secreted inhibitor which protects epithelial tissues from serine proteases. It is found in various secretions including seminal plasma, cervical mucus, and bronchial secretions, and has affinity for trypsin, leukocyte elastase, and cathepsin G. Its inhibitory effect contributes to the immune response by protecting epithelial surfaces from attack by endogenous proteolytic enzymes; the protein is also thought to have broad-spectrum anti-biotic activity.

Clinical significance
The gene for SLPI is expressed by cells at many mucosal surfaces located in the tissues of the lungs, cervix, is_associated_with::seminal vesicles, and is_associated_with::parotid ducts. SLPI is also one of the dominantly present proteins in nasal epithelial lining fluid and other nasal secretions. Many diseases, such as is_associated_with::emphysema, is_associated_with::cystic fibrosis, and is_associated_with::idiopathic pulmonary fibrosis, are characterized by increased levels of neutrophil elastase. SLPI is one of the major defenses against the destruction of pulmonary tissues and epithelial tissues by neutrophil elastase. SLPI is considered to be the predominant elastase inhibitor in secretions, while α1-antitrypsin is the predominant elastase inhibitor in tissues. Several diseases, including those listed, are actually the result of SLPI and α1-antitrypsin defenses being overwhelmed by neutrophil elastase. It has been suggested that recombinant human SLPI be administered to treat symptoms of cystic fibrosis, genetic emphysema, and is_associated_with::asthma. In addition, SLPI has occasionally been monitored in an effort to coordinate its levels with different pathological conditions. Increased levels of SLPI in nasal secretions and bronchoalveolar fluids may be denotive of inflammatory lung conditions or allergic reactions, and increased levels of SLPI in plasma may be indicative of is_associated_with::pneumonia.

Increased levels of SLPI in saliva and plasma may also be an indicator of is_associated_with::HIV infection. This is evident due to the virtual nonexistence of HIV transmission through oral-to-oral contact. This antiviral activity is due to the interference of SLPI in events that are mediated by protease, such as entry into the host cell and replication of viral genetic material. Studies have shown that decreasing levels of SLPI in saliva also decreases its anti-HIV activity. What makes SLPI such a topic of interest is that it exhibits anti-HIV properties in physiological conditions, rather than artificial ones.

Interactions
SLPI has been shown to interact with is_associated_with::PLSCR1.