Two trypsin isoforms from the intestine of the grass carp (Ctenopharyngodon idellus)

Two trypsin isoforms (GT-A and GT-B) from the grass carp (Ctenopharyngodon idellus) intestine were isolated and purified. SDS-PAGE electrophoresis showed that GT-A and GT-B had relative molecular masses of 30,740 and 26,400, respectively. Enzyme activity was inhibited by three organic trypsin inhibitors but not by EDTA. They had optimal pH of 8.0 and 8.5, and optimal temperatures of 38.5 and 44.0°C, respectively, when hydrolyzing N–benzoyl-l-arginine ethyl ester·HCl (BAEE). They lost 95.8 and 93.7% of their activities, respectively, after heating for 20 min at 65°C. Their thermal denaturation temperatures, respectively, were 66.3 and 67.3°C. GT-A has a Km value of 21.2 μM and a Vmax of 2.0 × 103 min−1, and GT-B has a Km value of 31.7 μM and a Vmax of 3.3 × 103 min−1. Their physiological efficiencies were 94.3 and 105.3 μM−1 min−1, respectively. The Arrhenius activation energies of GT-A and GT-B were 4.16 and 4.38 kcal/mol, respectively. The activities of GT-A and GT-B were not activated by Ca2+, but their thermostability was improved in the presence of Ca2+. Enzyme activity was reduced in presence of Zn2+, Cu2+, Hg2+ and Al3+. Thermal stabilities of GT-A and GT-B were intermediate between Arctic and tropical fish species, and consistent with the wide range of water temperatures to which grass carp are exposed in most provinces of China.