Digestive Enzymes in Insectivorous Plants

III. Acid Proteases in the Genus Nepenthes and Drosera peltata

Abstract

Further investigations of the acid protease in the genus Nepenthes were made. Zymograms obtained by polyacrylamide-gel thin layer electrophoresis revealed that the crude secretion of Nepenthes had at least 4 protease components, whereas a purified preparation had only one. A purified preparation from an extract of Drosera peltata, which was also investigated comparing with that from Nepenthes secretion, had one protease component, which resembled the purified Nepenthes protease in electrophoretical mobility.
Some properties of the purified protease from the extract of Drosera peltata were also investigated. With casein as a substrate the optimum pH for the enzyme action was about 3. At 40°C the enzyme was stable at pH's from 3.0 to 9.0; at 60°C it was most stable at pH 5. p-Chloromercuribenzoate and diisopropylfluorophosphate
had no effect on the Drosera protease under the present experimental conditions.
The substrate specificity of Drosera proteases was determined with several peptide fragments of known structure. The results showed that the enzyme preferentially splits peptide bonds at both the carboxyl and amino sides of aspartic acid residue, dominantly at the former side, at the carboxyl side of alanine, and probably at the carboxyl side of lysine.
The present studies clearly demonstrate that the proteases in Nepenthes and Drosera peltata closely resemble each other in almost all the properties investigated.