Measuring flavonoid enzyme activities in tissues of fruit species.

Flavonoids are important secondary metabolites, which are ubiquitously present in plant-derived food. Since flavonoids may show beneficial effects on human health, there is increasing interest in the availability of plants with a tailor-made flavonoid spectrum. Determination of flavonoid enzyme activities and investigations into their substrate specificity are an important precondition for both classical and molecular approaches. We tested two different protocols for enzyme preparation from eight fruit species. In many cases, a protocol adapted for polyphenol-rich tissues was superior. Using a suitable protocol for investigations of kiwi fruits, we show that flavanone 3-hydroxylase is absent in the green-fleshed cultivar Hayward. As flavonoid enzyme activities could be detected in harvested kiwi fruits over a storage period of five months, postharvest modification of the flavonoid spectrum has to be expected.