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.