Cathepsin B-like cysteine proteinase activity in sputum and immunohistologic identification of cathepsin B in alveolar macrophages.

Cathepsin B-like cysteine proteinase activity was measured, using a fluorimetric assay, in sputum samples from 27 subjects with chronic obstructive airways disease. Thirteen of the subjects had a current acute respiratory infection. Enzyme activity was detected in 11 of the 13 infected and 12 of the 14 noninfected samples. The mean cathepsin B activity observed in the infected samples was significantly greater than that of the noninfected samples. Most of the cathepsin B-like activity in sputum represented enzyme in free solution but a small proportion appeared to be complexed with alpha 2-macroglobulin. Immunohistochemical localization of cathepsin B in human lung tissue sections detected the enzyme only within macrophages.