The enzyme capable to cleave trypsin active site titrant as substrate is a carboxylesterase with electrophoretic mobility similar to albumin.

An enzyme isolated from Ehrlich ascites plasma and capable of cleaving trypsin active site titrant 4-nitrophenyl-p-guanidinobenzoate (Steven, F.S. and Al-Achmad, R.K. (1983) has been further investigated. The substrate hydrolysis follows Michaelis-Menten kinetics. The molecular mass of the enzyme is 50-70 kDa by gel filtration and SDS polyacrylamide gel electrophoresis. It has the mobility of albumin and coelutes with a carboxylesterase activity on a cation exchange column. (Cbz-Arg-NH)2-Rhodamine, the specific noncompetitive inhibitor of guanidinobenzoatase, also inhibits the carboxylesterase activity. Therefore, the guanidinobenzoatase activity of Ehrlich ascites plasma is a carboxylesterase (EC 3.1.1.1.) which likely originates from blood.