The inactivation of native enzymes by a neutral proteinase from rat intestinal muscle

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Abstract
The solubilization and partial purification of a proteinase from the intestinal smooth muscle of rats fed on protein-free diets are described. It has a MW of about 33,000 and it is stable over a narrow pH range. From its susceptibility to known modifiers of proteolytic enzymes, it appears to be a serine proteinase of a trypsin-like nature. Active-site titration with soybean trypsin inhibitor shows that the concentration of proteinase was about 3 .mu.g/g wet wt of intestinal smooth muscle. However, the muscle proteinase demonstrates a marked ability for inactivating enzymes in their native conformation at neutral pH. It is about 100 times more efficient than pancreatic trypsin when the inactivating activities are compared on an approximately equimolar basis. Inactivation of the substrate enzymes is accompanied by limited proteolysis, as demonstrated by sodium dodecyl sulfate/polyacrylamide-gel electrophoresis. An endogenous inhibitor was separated from the proteinase by fractionation with (NH4)2SO4. Contamination of the muscle tissue by lumen, mucosal or blood proteinases and inhibitors is unlikely. A role for the neutral trypsin-like proteinase in initiating the degradation of intracellular enzymes is considered.