Proteolytic enzymes of lung.

The lung usually is considered to be a passive tissue whose main function is to permit gaseous diffusion to and from the blood; however, the lung is also an organ of defense. Its phagocytes remove inhaled dust particles and bacteria. A considerable part of the lung parenchyma consists of mononuclear phagocytes and potential mononuclear phagocytes. Since these cells are not readily available in quantity, a survey of the proteolytic enzymes of lung was made with the hope that such information could be correlated eventually with the enzymes of the mononuclear phagocytes as well as be helpful in understanding the liquefaction’ process of tuberculosis. Fruton (1) demonstrated the presence of leucine aminopeptidase, tripeptidase, and glycylglycine dipeptidase in extracts of rabbit lung. He could detect no hydrolysis of the synthetic substrates for several proteinases, including benzoyl-L-argininamide. Weiss and Halliday (2), however, have reported the presence of an enzyme in rabbit lung which hydrolyzes benzoyl-L-argininamide. Nye (3) and Weiss (4) have also identified a proteinase in lung that is optimally active at pH 3 to 4. The present study confirms the work of Fruton (1) on the peptidases of lung and reports the presence of two additional peptidases. We have also found that lung extracts show two regions of proteinase activity, one at acid pH (Proteinase I) and another at alkaline pH (Proteinase II). A method of purification and some of the properties of Proteinase I are described. Further studies of its specificity are given elsewhere (5). A synthetic substrate, N-acetyl-L-tyrosine ethyl ester, has been found for a component of Proteinase II, but purification of this fraction has not been attempted as yet.