May play some biological role in the host defense system on the mucous membrane independently of or in cooperation with other substances in airway mucous or bronchial secretions. Preferentially cleaves the C-terminal side of arginine residues at the P1 position of certain peptides (By similarity). Plays a role in the proteolytic processing of ACE2. Isoform 2 may play a key role in regulating adrenal proliferation by specifically cleaving N-POMC. Monomer. Activated by cleavage and secreted. Highly expressed in the esophagus, tongue, and trachea, low expression was seen in heart, lung, and adrenal gland. Isoform 2 is also highly expressed in the adrenal gland. Belongs to the peptidase S1 family.