Purification and characterization of a polyphenol oxidase from the kew cultivar of Indian pineapple fruit

Three isoforms of polyphenol oxidase (PPO) were purified to apparent homogeneity from the Kew cultivar of Indian pineapple fruit in a four-step procedure. The major isoenzyme, with a yield of 45%, was found to be a tetramer of identical subunits of molecular mass approximate to 25 kDa. An ionic strength dependent association-dissociation equilibrium was observed with pineapple PPO. Amino acid analysis of the major isoenzyme indicated the presence of a high content of glutamic acid, glycine, and serine and a low content of the sulfur-containing amino acids. The enzyme was optimally active between pH 6 and 7. The PPO did not show any cresolase activity, and the preferred substrates were diphenols. Ascorbic acid, L-cysteine, and potassium metabisulfite were found to be potent inhibitors of PPO.