Studies on the pancreatic secretory trypsin inhibitor in plasma and its complex with trypsin in vivo and in vitro

Complexes between human or canine trypsin and the pancreatic secretory trypsin inhibitor (PSTI) from the same species were studied in vitro and in vivo. The following results were obtained. (1) Human or dog PSTI-trypsin complex without serum did not show any signs of dissociation after 3 h incubation at room temperature. (2) Immediate separation of reaction mixtures of human or canine serum and the corresponding PSTI-trypsin complexes by gel filtration showed that 60--70% of the trypsin was found in complex with alpha2-macroglobulin and the remainder in equal amounts in complex with alpha1-antitrypsin and PSTI, respectively. (3) The results of in vivo studies in dog indicated a similar rapid dissociation of the complexes in the circulation. (4) The elimination for intravenously administered 125I-labelled PSTI was rapid to about 20% of the initial value with a half-life of about 8 min for the initial part of the curve. No organ accumulation of the labelled inhibitor was found. (5) Most of the radioactivity injected was recovered in the urine bound to degradation products but part of it was bound to biologically active inhibitor.