Comparison of Phytolacain G,a Cysteine Protease from Fruit of Phytolacca americana,with Phytolacain R

The enzymatic properties of phytolacain G, a protease isolated from green fruit of pokeweed, were compared with those of phytolacain R, a protease obtained from ripe fruit. The optimum pH of phytolacain G was 7.5-8.0 at 37°C using casein as the substrate. The enzyme was strongly inhibited by iodoacetic acid and p-chloromercuribenzoic acid, but not by diisopropyl fluorophosphate or EDTA. These results indicated that phytolacain G was a cysteine protease, like phytolacain R. Nine sites of oxidized insulin B-chain were cleaved by phytolacain G during 20 h of hydrolysis. The six sites cleaved by phytolacain G were also cleaved by phytolacain R. The substrate specificity of phytolacain G was broad, but the preference for hydrophobic residues at the P₂ position was similar to the substrate specificity of papain. The amino-terminal sequence of phytolacain G was not identical with that of phytolacain R; however, the amino acid residues conserved in the papain family were also conserved in this enzyme.