Inhibition kinetics of a trypsin-like neutral proteinase on the surface of Ehrlich ascites-tumour cells.

The data indicate that trypsin-Sepharose has markedly different inhibition kinetics from trypsin in free solution. The Sepharose granules (i) offer protection from inhibition in the first place (e.g. for active-site titrants; Fig. Id) and/or (ii) facilitate the displacement of inhibitor from trypsin by excess substrate (Figs. la, Ib and Ic). It would seem that when an inhibitor such as ovomucoid is preincubated with trypsin-Sepharose, all the enzyme molecules are initially complexed. On adding substrate, a certain proportion of trypsin-inhibitor complexes dissociate with regain of trypsin activity, this proportion being controlled by the relative concentrations of ovomucoid and substrate competing for enzyme active sites. As the relative concentration of inhibitor increases, greater inhibition results. We conclude that surface-bound trypsin is far more effective enzymically than trypsin in free solution in the presence of inhibitors. Similar observations have been made on a trypsin-like neutral proteinase on tumour-cell surfaces (Steven et al., 1980) that may have significance for tumour invasion.