CTSS and cystic fibrosis: Other ASL intrinsic factors may also be at play, notably pH-dependent proteolytic cleavage of ENaC by channel activating proteases (CAPs), including serine proteases prostasin, matriptase, furin, and cysteinyl proteases such as cathepsins, The activities of CAPs, which are present at excessively high levels at the apical surface of CF airway epithelial cells, are a significant component in causing Na+ hyperabsorption in CF airways.