An acidic protease from the grass carp intestine (Ctenopharyngodon idellus)

The acidic Protease was extracted from the intestine of the grass carp (Ctenopharyngodon idellus) by 0.1 M sodium phosphate buffer, pH 7.0 at 4 °C after neat intestine was defatted with acetone, and partially purified by ammonium sulfate precipitation, gel filtration chromatography and ionic exchange chromatography. SDS–PAGE electrophoresis showed that the enzyme was homogeneous with a relative molecular mass of 28,500. Substrate-PAGE at pH7.0 showed that the purified acidic protease has only an active component. Specificity and inhibiting assays showed that it should be a cathepsin D. The optimal pH and optimal temperature of the enzyme were pH2.5 and 37 °C, respectively. It retained only 20% of its initial activity after incubating at 50 °C for 30 min. The enzyme lost 81% of its activity after incubation with pepstatin A at room temperature, but was not inhibited by soybean trypsin inhibitor or phenylmethylsulfonyl fluoride (PMSF). Its Vmax and Km values were determined to be 3.57 mg/mL and 0.75 min− 1, respectively.