Substrate Specificity and Inhibition of Polyphenoloxidase (PPO) From a Dwarf Variety of Banana (Muss Cavendishii, L.)

Banana polyphenoloxidases oxidize specifically o-diphenols. The Km values were low for dopamine, epinephrine, and norepinephrine. It was lower for L-dopa as compared with D-dopa. The most effective inhibitor was ascorbic acid followed by cysteine and then sodium metabisulfite. Diethyldithiocarbamate, at pH 7.0, was not as effective as the above inhibitors. Ammonium ion and the oxidized β-nicotine adenine dinucleotide acted as activators whereas Fe⁺², Fe+³, Al⁺³, Ca⁺² and Zn⁺² showed various degrees of inhibition. On the other hand, C⁺¹, Cu⁺², Mg⁺² and Mn⁺² did not affect the enzyme activity. Mercaptoethanol (17 mM) completely inactivated the enzymes. On dialysis 30% of the activity was restored with regeneration of two isozymes.