Purification and properties of a protease from the sarcocarp of bead tree fruit

A protease was purified from bead tree fruit (Melia azedarach L. var. japonica Makino) in four steps, including HPLC gel-filtration. The M(r) of the enzyme, named melain, was estimated to be 25,000 on SDS-PAGE and on HPLC gel filtration. Melain contained a carbohydrate moiety. Using casein as a substrate, the optimum pH of the enzyme was 7.5-8.5 at 37 degrees. The enzyme was inhibited by iodoacetic acid, but was not inhibited by phenylmethanesulphonyl fluoride or EDTA. The enzyme had a wide specificity for peptide substrates such as oxidized insulin B-chain. All split sites (P1 and/or P'1) were hydrophobic or charged amino acid residues. The enzymatic properties of this protease were similar to those of phytolacain, an enzyme from the fruit of pokeweed, Phytolacca americana.