Inhibition of free and bound trypsin-like enzymes.

1. The inhibition of beta-naphthylamidase activity of free trypsin, trypsin-Sepharose and a trypsin-like neutral protease on the surface of tumour cells have been studied in independent systems and with mixtures of free trypsin plus surface-bound trypsin. 2. Kinetic data have demonstrated that high-molecular-weight (protein) inhibitors of free trypsin are less effective inhibitors of trypsin-Sepharose and fail to inhibit the cell surface neutral protease. 3. Inhibition of mixtures of free trypsin plus trypsin-Sepharose follows independent kinetic plots for each component. The free trypsin is reacted before any Sepharose-bound trypsin reacts with high-molecular-weight inhibitors. 4. Low-molecular-weight inhibitors of trypsin also inhibit bound trypsin equally well. 5. Papain-derived peptides from high-molecular-weight inhibitors of trypsin inhibit free trypsin, trypsin-Sepharose and the cell-surface neutral protease almost equally well. 6. Fluorescence microscopy has shown that a high-molecular-weight inhibitor of trypsin does not bind to the tumour cell-surface neutral protease, but it does bind to trypsin-Sepharose. 7. The cell-surface neutral protease has been shown to be capable of activation of latent beta-naphthylamidase activity in the presence of excess extracellular inhibitors of free trypsin. 8. The mechanism by which trypsin-Sepharose remains partially active in the presence of excess inhibitor necessary to inhibit an equivalent quantity of free trypsin has been discussed. 9. These studies indicate that a search for inhibitors which are selectively active against the cell-surface neutral protease and have no action on trypsin-like enzymes in free solution must take into account the modifying effects of the cell surface on neutral protease activity.