Enzymatic Browning and Polyphenol Oxidase of Mung Bean Sprout during Cold Storage

Abstract

Mung bean sprouts turn brown during cold storage. Here, we cloned polyphenol oxidase (PPO) mRNA, and then examined its expression and active form to clarify the mechanism of this browning. A PPO cDNA encoding 592 amino acids showed high homology to PPO genes of Fabaceae plants and had highly conserved motifs, including the active site and transit peptide to the plastid thylakoid. Expression of PPO mRNA was almost constant. The translated PPO protein was considered to migrate into the thylakoid, but the active PPO was mainly present in the cytosol or soluble fraction, and its molecular mass was 31 kDa, smaller than the translated protein. The membrane structure of mung bean sprouts is heavily disrupted during cold storage. These results showed that PPO transported into the thylakoid is solubilized and decomposed to its active form, which oxidizes phenolic compounds in the cytosol to turn sprouts brown during cold storage.