Evidence for the carriage of silver by sulphadimidine: inhibition of proteolytic enzymes.

1 Trypsin in free solution and trypsin-sepharose were shown to be inhibited by Ag+ in the absence of Cl-. 2 In the presence of Cl- and absence of a suitable carrier, Ag+ has no inhibitory action on trypsin or chymotrypsin. 3 Sulphadimidine bound Ag+ in the presence of Cl-, and carried the Ag+ to both trypsin and chymotrypsin in free solution as well as to trypsin-sepharose leading to the inhibition of all these enzyme systems. 4 The neutral protease of tumour cell surfaces was inhibited by Ag+ transported by sulphadimidine in the presence of Cl-. 5 Kinetic data demonstrated the requirements for both Ag+ and carrier to effect inhibition, the degree of inhibition being directly related to the molarity of each of these reagents. 6 The known inhibition of trypsin by Ag+ binding to histidine in the active site has been defined in mechanistic terms employing the sulphonamide drug, sulphadimidine, to illustrate this exchange mechanism.