Metabolism of 131I-labelled human pancreatic cationic trypsin after intraduodenal administration.

Human pancreatic cationic trypsin labelled with 131I was administered into the duodenum in 9 healthy individuals. Five had earlier been proctocolectomized and had ileostomies. Radioactivity was measured in plasma, urine, ileostomy content and feces for a period of 72 h. Radioactivity was present in plasma 15 min after administration. The total recovery of radioactivity was 78-98%, the largest amount being observed in urine during the first 24-hour period. About 20% of the administered radioactive dose was recovered in ileostomy content and 10% in feces. The recovered radioactivity in plasma, urine and extracts of ileostomy content and feces was characterized with dialysis and gel filtration. All radioactivity in plasma and urine corresponded to free 131I, whereas in the extracts radioactivity corresponding to intact enzyme and degradation products as well as a small amount of free 131I was observed. It is concluded that pancreatic trypsin is degraded during intestinal passage as with other proteins. Due to a deiodinating mechanism in the intestine, only free 131I is absorbed into the circulation. This deiodination does not take place in duodenal juice. The absence of high molecular weight radioactivity in plasma argues against an enteropancreatic circulation.