Inhibition of neutrophil elastase by mucus glycoprotein

A number of structurally diverse polyanions have been found to inhibit human leukocyte elastase (HLE) activity. The purpose of this study was to examine the effects of mucus glycoprotein (mucin), one of the most plentiful high molecular weight polyanions in the respiratory tracts, on HLE activity. Human airway mucin and bovine submaxillary mucin at concentrations of 0.4 to 2.8 mg/ml both markedly inhibited the elastolytic activity of 50 nM HLE, with maximum inhibition approaching 90%. The degree of inhibition was the same regardless of whether the mucin, elastase, and elastin were simultaneously combined or whether the mucin was added to elastase 20 min prior to adding elastin, indicating that mucin is a rapid-acting inhibitor. The shape of the inhibition curve resembled that of curves obtained using heparin and HLE. Mucin had little inhibitory effect on pancreatic elastase, which is structurally related to but less cationic than HLE. The inhibition of HLE by mucin was blocked by 1 M NaCl. Removal of sulfate esters by acid-catalyzed solvolysis markedly reduced the inhibitory effect of bovine submaxillary mucin. These results indicate that inhibition of HLE by mucin involves binding of the positively charged HLE molecules to the negatively charged sulfated carbohydrates in the mucin. Mucin was also found to substantially reduce the antiprotease activity of secretory leukocyte protease inhibitor, a low molecular weight cationic protein known to bind to mucin.