Substrate specificity and reaction model of the active complex of human plasminogen and streptokinase

Abstract

In this communication, we introduced a soybean trypsin inhibitor in the reaction system to block the activities of trypsin and plasmin like proteolytic enzymes. Since STI was found to be a selective inhibitor for the other serine proteases except SK-PLG complex, in the presence of STI, we were able to quantitize the activity of the SK-PLG complex using artficial chromogenic substrate such as N$^a$-CBZ-L-lysine-p-nitrophenyl exter (CLM). Also, with the kinetic study of SK-PLG complex for synthetic esters and peptides, its substrate specificity has been investigated. This results show that the SK-PLG complex is primarily specific for peptides and esters of N,C-blocking, N-positively charged amino acids which have hydrophobic groups at its a-carboxyl position. Like other serine proteases, the SK-PLG complex was irreversibly inhibited by diisopropylfluorophosphate(DIFP) and diethylpyrocarbonate(DEPC) as well. So, it was concluded that the hydroxyl group of serine and the imidazole group of histine have been constituted a part of the active site of SK-PLG complex. This result was supported by pH-profile of SK-PLG complex using N$^a$-benzoyl-L-arginine-p-nitroanilide as a substrate. And it was observed that the hydroylsis of CLN catalyzed by SK-PLG complex proceeds with an initial burst of p-nitrophenol release, followed by a slower steady-state release of this product. This experimental observation may provides proof that a particular covalent SK-PLG complex-substrate compound is formed as an intermediate in the reaction of SK-PLG complex. This, based on these results investigated, reaction mechanism of SK-PLG complex can be proposed. Also some human plasma proteins, carbonate ion, and chloride ion, etc. affected the activity of SK-PLG complex. It was considered that these components in human blood may be partially concerned in the regulation of blood fibrinolysis related to the SK-PLG complex.