Active-site titration of serine proteinases acting selectively on cationic substrates by N-alpha-carbobenzoxy-L-arginine p-nitrophenyl ester and N-alpha-carbobenzoxy-L-lysine p-nitrophenyl ester; determination of active enzyme concentration.

A titration method for determination of trypsin-like serine proteinase concentration has been developed by using ZArgONp and ZLysONp, two specific chromogenic amino-acid derivatives which show the characteristics of optimal active-site titrants. Active proteinase concentration has been estimated from the effect of titrant concentration on the amplitude, at time zero, of the time-course for the instantaneous release of p-nitrophenol, preceding the steady-state reaction (burst phase).