Effect of ascorbic acid, sodium bisulfite, and thiol compounds on mushroom polyphenol oxidase

Purification and characterization of a bacterial nitrophenol oxygenase which converts ortho-nitrophenol to catechol and nitrite

“…The buffers were prepared by dissolving the corresponding salt in water and adjusting the pH with NaOH or H2SO4. Buffers: A, 50 mM succinate (pH 5.5 to 6.0) or 50 mM acetate (pH 5.5 to 6.0); x, 20 mM phosphate (pH 6.0 to 7.5); *, 50 mM Tris (pH 7.5 to 9.0); V, 50 mM ammonium (pH 9 8 ,uM; para-nitrophenol, 6 ,uM), both showed a substrate inhibition at concentrations above 20 ,uM nitrophenol, both required NADPH as a cofactor, and for both the optimal activity was found between pH 7.5 and 8.0. Stoichiometric determinations showed that the conversions of 1 mol of ONP to catechol and of 1 mol of para-nitrophenol to hydroquinone, respectively, were coupled with the consumption of 1 mol of oxygen and 2 mol of NADPH and with the liberation of 1 mol of nitrite.…”

“…It is noteworthy, though, that the enzymatic liberation of nitrite from ONP was a rather slow reaction and required the presence of the reducing agent NADPH in the assay mixture. Since it is known that the reducing agent ascorbic acid converts orthobenzoquinone rapidly to catechol in a spontaneous chemical reaction (3,6), we also determined the action of NADPH on ortho-benzoquinone. We found that 0.1 mM ortho-benzoquinone in 20 mM phosphate buffer (pH 7.5) was completely reduced to catechol in a nonenzymatic reaction within seconds after the addition of 0.4 mM NADPH (data not shown).…”

Diphenol Oxidases, Enzyme-catalysed Browning and Plant Disease Resistance

“…They have also been reported to inhibit catecholase directly (Embs and Markakis, 1965) with modification of the protein structure but still retaining catecholase molecular unity (Sayavedra-Soto and Montgomery, 1986). Golan-Goldhirsh andWhitaker (1984,1985) have suggested that this may be due to a inactivation mechanism (where the enzyme is inactivated during turnover of substrate to product via free radical action) in addition to actual inhibition by sulphites. Finally, and probably most importantly, sulphites are known to complex with quinones forming colourless thioethers (Walker, 1975).…”

Inhibitory effect of pineapple shell extract and its ultrafiltered fractions on polyphenol oxidase activity and browning in fresh-cut banana slices Efecto inhibidor de extracto de cáscara de piña y sus fracciones ultrafiltradas en la actividad polifenoloxidasa y pardeamiento de rodajas de plátano recién cortadas

“…In addition, sodium metabisulfite could decrease the oxygen consumption and would form a complex with the enzyme to give inactive enzyme form (Ricquebourg, Robert-Da Silva, Rouch, & Cadet, 1996). Ascorbate acts as an antioxidant in PPO inhibition because it reduces the initial quinone formed by the enzyme to the original diphenol before it undergoes secondary reactions which lead to browning (Whitaker, 1972) and ascorbic acid at high concentrations may cause irreversible PPO inhibition (Golan-Goldhirsh & Whitaker, 1984). Ü mit Ü nal (2007) also found that 0.8 mM ascorbic acid and 0.01 mM sodium metabisulfite could completely inhibit PPO from Anamur banana (Musa cavendishii).…”