Activation of chymotrypsinogen-A by thrombin preparations

The hydrolysis of chemically modified proteins by Factor Xa and thrombin

Bovine thrombin and Factor Xa were shown to hydrolyse slowly several chemically modified proteins. Both enzymes hydrolyse the proteins at trypsin-susceptible bonds, with arginine, lysine or the synthetically generated S-(beta-aminoethyl)cysteine at the P1 position. Both enzymes, however, cleave at far fewer sites than trypsin. The presence of highly polar groups in the P2 position appears to hinder hydrolysis by Factor Xa or thrombin. The presence of hydrophobic or neutral amino acids around this site may make the site more susceptible to hydrolysis. Differences in the hydrolysis patterns between thrombin and Factor Xa are observed.

Determination of human pancreatic cationic trypsinogen in serum by radioimmunoassay

A specific radioimmunoassay has been developed for human pancreatic cationic trypsin. The assay has been employed for the determination of immunoreactive forms of pancreatic cationic trypsin in blood. The trypsin employed as radioiodinated tracer in the assay was inactivated with tosyl-L-lysine chloromethyl ketone (TLCK) to prevent binding of the tracer to the serum inhibitors while maintaining its immunoreactivity. The average normal serum level determined was 26 ng/ml, with a range of 12--41 ng/ml. Eight of nine patients with acute pancreatic inflammation had at least a 15-fold elevation of total serum immunoreactive cationic trypsin. Cationic trypsinogen and cationic trypsin bound to alpha1-antitrypsin cross-react strongly in the radioimmunoassay. Thus it is possible to measure these potential molecular forms of cationic trypsin in serum. When normal human serum was fractionated on Sephadex G-200, all of the immunoreactive material eluted as a single peak of approximately 23,000 mol wt. No cationic trypsin could be detected in association with alpha1-antitrypsin or alpha2-macroglobulin. The 23,000-mol-wt peak was definitively shown to contain trypsinogen by affinity chromatography and by activation with human enteropeptidase. The identification of cationic trypsinogen in blood implies that the zymogen is secreted into the circulation by the pancreas rather than entering the bloodstream via absorption from the intestine.

Cleavage of a specific bond in troponin C by thrombin

Limited proteolysis of rabbit skeletal troponin C with bovine thrombin yielded two fragments, TH1 (Mr = 11000) containing Ca2+ binding regions I--III and TH2 (Mr = 6000) containing region IV. Determination of the partial sequences of the fragments established the site of cleavage at Arg120-Ala121. Secondary cleavage by thrombin at other arginyl or lysyl residues in troponin C was ruled out by the sequence data and by the amino acid compositions of the two fragments.