BIOSYNTHESIS OF QUERCETIN IN BUCKWHEAT: PART I

Abstract

Cuttings of Fagopyrum tataricum were allowed to metabolize various labelled compounds for 24 hours in the light. Quercetin was then isolated and degraded by alkaline hydrolysis of its pentamethyl ether into veratric acid (ring B plus carbon 2) and 2′-hydroxy-2,4′,6′-trimethoxyacetophenone (ring A plus carbons 3 and 4). Eleven amino acids, including tyrosine, were poor precursors of quercetin as were also protocatechuic acid and p-hydroxybenzoic acid. Shikimic acid, phenylalanine, p-hydroxycinnamic acid, and cinnamic acid were very good precursors followed by (in decreasing order of effectiveness) caffeic acid (fair), sinapic acid, m-methoxycinnamic acid, and ferulic acid (very poor). Quercetin formed from β- or ring-labelled cinnamic acid gave labelled veratric acid on degradation, and that from α- or carboxyl-labelled cinnamic acid gave labelled 2′-hydroxy-2,4′,6′-trimethoxyacetophenone. Two-ninths of the activity of quercetin formed from uniformly labelled phenylalanine was found in the acetophenone derivative and seven-ninths in the veratric acid. These results show that C6.C3 compounds are used with the carbon skeleton unchanged for the synthesis of ring B and carbons 2, 3, and 4 of quercetin and that the ring hydroxylation pattern of the compound used determines its effectiveness. None of the compounds were good precursors of ring A.