Oxidation of flavonols and flavonol glycosides by a hypodermal peroxidase isoenzyme from gamay rouge grape (Vitis vinifera) berries

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Abstract
The ability of the grapevine hypodermal peroxidase isoenzyme B5 to oxidise the flavonols, quercetin and myricetin, and the flavonol glycosides quercetin‐3‐arabinoside, quercetin‐3‐rhamnoside (quercitrin) and myricetin‐3‐rhamnoside (myricitrin), has been studied. The results showed that, whilst the aglycones were good substrates of this peroxidase isoenzyme, their corresponding glycosides were poorly oxidised under the assay conditions. The oxidation of quercetin and myricetin catalysed by the hypodermal peroxidase isoenzyme B5 was strictly dependent on hydrogen peroxide (H2O2). The kinetic characteristics of the flavonol oxidation were also studied. Whilst the apparent Km values for quercetin and myricetin were roughly equal (38 and 54 μM, respectively), the apparent Km values for H2O2 in each reaction were appreciably different (168 and 8 μM, respectively). Besides, the optimal pH for quercetin oxidation was 4.0 whereas for myricetin it was 6.0‐7.0. The results are discussed in view of the possible involvement of the vacuolar hypodermal B5 isoperoxidase in the turnover of flavonol and flavonol glycosides in grapes.