PARTIAL-PURIFICATION AND PROPERTIES OF PLANTAIN POLYPHENOL OXIDASE

Free and bound polyphenol oxidase activities were identified in plantain; the soluble form increases with the maturity of the fruit. No difference was found between the two fractions during the purification process. An eight-fold partially purified preparation was obtained after gel filtration and ion-exchange chromatography. The enzyme has an optimum pH of 6.5. The apparent molecular weights, 30 000 +/- 5000 and 70 000 +/- 500, estimated by gel filtration are indicative of the presence of multiple forms. The ther-mal inactivation pattern agrees with the presence of two isoenzymes with different thermal stabilities, the values obtained for E(A) being 18 kJ/mol for the heat-resistant form. The greatest activity was found when 4-methyl catechol, catechol, pyrogallol, dopamine and L-dopa were used as substrates. The presence of trace quantities of dopamine in plantain pulp (2 mug/g of fresh matter) was established by high-performance liquid chromatography (HPLC). The plantain-polyphenol oxidase is inhibited by copper complexing agents and by reducing agents. One-hundred per cent inhibition was noted in the presence of sodium diethyl dithiocarbamate, L-cysteine, sodium metabisulphite and ascorbic acid (10(-3) m). At this concentration only 40% of inhibition was obtained with potassium cyanide. Determination of browning during flour preparation shows that the enzyme may be efficiently inactivated using sodium metabisulphite. This compound also prevents nonenzymatic browning during the slicing and drying steps of preparation of plantain flour.