A possible role for neutral proteolysis in the degradation of intracellular proteins

A neutral proteinase has been solubilized from rat intestinal muscle by extraction at low ionic strength. It has an apparent mol. wt. of 33,000 and is stable only around neutral pH. Characterization studies with specific inhibitors and substrates have shown it to be a trypsin-like serine proteinase. The rat proteinase and bovine pancreatic trypsin have equivalent activities as measured with peptide and denatured protein substrates but the rat proteinase is about 300 times more active than an equimolar amount of trypsin towards proteins in their native conformation. It has been shown that the activity of the rat proteinase can be modulated (1) by changing the conformation of the substrate protein(s) and (2) by means of an endogenous inhibitor. The inhibitor has been purified to homogeneity from rat intestinal muscle. It has a mol. wt. of 8,000 and binds only weakly to the rat proteinase (Ki approximately equal to 10(-6) M). It did not inhibit any of the other proteinases tested. The implications for such a proteinase--inhibitor system in the non-lysosomal pathway of intracellular protein degradation are considered.