Metabolism of 125I-labelled trypsin in man: evidence of recirculation

125I-labelled human trypsin metabolism has been investigated in man. Three subjects received 125I-trypsin and 131I-albumin intravenously. Against a background secretin infusion (1 U/kg/h), trypsin decayed biexponentially from the serum with half-lives of 17.5, 21, and 24 minutes for the rapid disappearance phase and 520, 540, and 560 minutes for the slow phase. Between 13% and 38% of the 125I injected was recovered from duodenal juice aspirated continously over 300 minutes. In contrast, less than 1% of the 131I-albumin injected was recovered. When bile and pure pancreatic juice were collected at endoscopic retrograde cholangiopancreatography after intravenous 125I-trypsin in a fourth subject 125I radioactivity was found to be secreted via both these routes. After 125I-trypsin infusion into the duodenum 11% of the total dose was found to be present in the circulation after 75 minutes. These results support the concept that recirculation of trypsin exists in man.