Measurement of the Bovine Pancreatic Trypsin Inhibitors by Radioimmunoassay

Abstract

Bovine pancreas contains two polypeptide trypsin inhibitors which are not homologous and differ in their inhibitory activity towards chymotrypsin, kallikrein, elastase and other serine proteinases [1]. The Kunitz inhibitor [2, 3] and the Kazal inhibitor [4, 5] are present in approximately equimolar concentrations in bovine pancreatic tissue [6], yet only the Kazal inhibitor is detectable in the pancreatic juice [6, 7]. The Kazal inhibitor has been named the pancreatic secretory trypsin inhibitor, PSTI [8] because its concentration in the pancreatic juice parallels that of the exocrine secretory proteins [7, 9, 10]. The Kunitz inhibitor is considered the “intracellular” inhibitor [7]. However, no direct information is available concerning the intracellular localization of these inhibitors in the pancreas. A sensitive and specific analytical method for the measurement of Kazal and Kunitz inhibitors at the picogram-nanogram level is required for intracellular distribution studies. The preparation of trace labelled 131I-Kunitz inhibitor by the chloramine T method and its use in a radioimmunoassay have been described by Arndts et al. [11]. In this communication we report the preparation of 125I derivatives of Kazal and Kunitz inhibitors by the lactoperoxidase method and present a radioimmunoassay for each inhibitor.