High-affinity Ca2+ binding inhibits autoactivation of rat trypsinogen

The recent discovery that mutation Asn21 --> Ile in the human cationic tryp sinogen (Tg) is associated with hereditary pancreatitis has brought into fo cus the functional role of amino acid 21 in mammalian Tgs. In the present p aper, the effect of mutations Thr21 --> Asn and Thr21 --> Ile on the Ca2+ d ependence of zymogen activation was investigated, using the autolysis-resis tant rat Tg mutant Arg117 --> His. In the absence of Ca2+, rat Tg exhibited low but significant basal autoactivation, which was inhibited by micromola r concentrations of Ca2+ (IC50 2.6 mu M) Interestingly, basal autoactivatio n was diminished in both mutants, and no further inhibition by micromolar C a2+ was detectable. Millimolar Ca2+ concentrations markedly and comparably stimulated autoactivation of wild-type and mutant zymogens (ECS, 1.7-2.4 mM ). The results indicate that rat Tg is subject to dual regulation by Ca2+, allowing zymogen stabilization in a low-Ca2+ environment and efficient acti vation in a high-Ca2+ milieu. (C) 2000 Academic Press.