Characterization and purification of polyphenol oxidase from artichoke (Cynara scolymus L.)
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In this study, the polyphenol oxidase (PPO) of artichoke (Cynara scolymus L.) was first purified by a combination of (NH4)(2)SO4 precipitation, dialysis, and a Sepharose 4B-L-tyrosine-p-aminobenzoic acid affinity column. At the end of purification, 43-fold purification was achieved. The purified enzyme migrated as a single band on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Polyacrylamide gel electrophoresis indicated that PPO had a 57 kDa molecular mass. Second, the contents of total phenolic and protein of artichoke head extracts were determined. The total phenolic content of artichoke head was determined spectrophotometrically according to the Folin-Ciocalteu procedure and was found to be 425 mg 100 g(-1) on a fresh weight basis. Protein content was determined according to Bradford method. Third, the effects of substrate specificity, pH, temperature, and heat inactivation were investigated on the activity of PPO purified from artichoke. The enzyme showed activity to 4-methylcatechol, pyrogallol, catechol, and L-dopa. No activity was detected toward L-tyrosine, resorsinol, and p-cresol. According to V-max/K-m values, 4-methylcatechol (1393 EU min(-1) mM(-1)) was the best substrate, followed by pyrogallol (1220 EU min(-1) mM(-1)), catechol (697 EU min(-1) mM(-1)), and L-dopa (102 EU min(-1) mM(-1)). The optimum pH values for PPO were 5.0, 8.0, and 7.0 using 4-methylcatechol, pyrogallol, and catechol as substrate, respectively. It was found that optimum temperatures were dependent on the substrates studied. The enzyme activity decreased due to heat denaturation of the enzyme with increasing temperature and inactivation time for 4-methylcatechol and pyrogallol substrates. However, all inactivation experiments for catechol showed that the activity of artichoke PPO increased with mild heating, reached a maximum, and then decreased with time. Finally, inhibition of artichoke PPO was investigated with inhibitors such as L-cysteine, EDTA, ascorbic acid, gallic acid, D,L-dithiothreitol, tropolone, glutathione, sodium azide, benzoic acid, salicylic acid, and 4-aminobenzoic acid using 4-methylcatechol, pyrogallol, and catechol as substrate. The presence of EDTA, 4-aminobenzoic acid, salicylic acid, gallic acid, and benzoic acid did not cause the inhibition of artichoke PPO. A competitive-type inhibition was obtained with sodium azide, L-cysteine, and D,L-dithiothreitol inhibitors using 4-methylcatechol as substrate; with L-cysteine, tropolone, D,L-dithiothreitol, ascorbic acid, and sodium azide inhibitors using pyrogallol as substrate; and with L-cysteine, tropolone, D,L-dithiotreitol, and ascorbic acid inhibitors using catechol as a substrate. A mixed-type inhibition was obtained with glutathione inhibitor using 4-methylcatechol as a substrate. A noncompetitive inhibition was obtained with tropolone and ascorbic acid inhibitors using 4-methylcatechol as substrate, with glutathione inhibitor using pyrogallol as substrate, and with glutathione and sodium azide inhibitors using catechol as substrate. From these results, it can be said that the most effective inhibitor for artichoke PPO is tropolone. Furthermore, it was found that the type of inhibition depended on the origin of the PPO studied and also on the substrate used.