Inhibition of NAD(P)H:Quinone Acceptor Oxidoreductase by Flavones: A Structure-Activity Study

doi:10.1006/abbi.1993.1182

Archives of Biochemistry and Biophysics

Volume 302, Issue 1, April 1993, Pages 72-77

https://doi.org/10.1006/abbi.1993.1182 Get rights and content

Abstract

A structure-activity study was carried out to determine the important regions of baicalein and oroxylin A, two flavones isolated from the Chinese herb Scutellariae radix, in inhibiting NAD(P)H:quinone acceptor oxidoreductase (EC 1.6.99.2; DT-diaphorase). This quinone reductase is a vitamin K reductase. It is a target for and has been used as a model enzyme to investigate the mode of action of oral anticoagulants. The two flavones were found to inhibit this quinone reductase in nanomolar ranges. The 5-hydroxyl, 7-hydroxyl, 8-hydroxyl, and 2-phenyl groups of these flavones were found to be important for their inhibition of the enzyme. The inhibition profiles of the flavones on the NADH-menadione reductase activity, the NADH-potassium ferricyanide reductase activity, and the NADH-methyl red reductase activity of this enzyme were different. Therefore, even though the flavones were found to be competitive inhibitors with respect to NADH, they probably did not inhibit the enzyme by binding to the nicotinamide nucleotide binding site. Inhibition kinetic studies which indicated that these compounds bound to different sites than those for dicoumarol and phenindone were performed. These results indicate that these flavones are a new type of inhibitor of NAD(P)H:quinone acceptor oxidoreductase and potentially useful as anticoagulant drugs.

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Like the antiplatelet aggregation effect, a very limited number of methoxyflavones were investigated for anticoagulant activity. For instance, 5,7-dihydroxy-6-methoxyflavone (oroxylin A) isolated from Scutellariae radix was a potential anticoagulant drug through inhibiting NAD(P)H: quinone acceptor oxidoreductase (Chen et al., 1993). In a review of Almeida Chaves, two methoxyflavones isolated from Hedychium gardnerianum including 3,5-dihydroxy-4′,7-dimethoxyflavone and 5-hydroxy-3,4′,7-trimethoxyflavone were mentioned to possess the capacity to prolong clotting time (De Almeida Chaves et al., 2019).
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Various enzymes have been determined to be involved in cell signaling, such as lypoxygenases and cyclooxygenase (Landolfi et al., 1984; Polette et al., 1996), tyrosine kinases (Akiyama et al., 1987), phosphodiesterases (Landolfi et al., 1984; Polette et al., 1996), and phospholipases (Lee et al., 1997; Lindahl and Tagesson, 1997), with flavonoids inhibiting cell signaling by interfering with these enzymes. Other mechanisms through which flavonoids inhibit coagulation include interfering with quinone acceptor oxidoreductase nicotinamide adenine dinucleotide phosphate (NADPH) (Chen and Deng, 1993), or by exposure to phosphatidylserine (Bucki et al., 2003). The antioxidant properties of flavonoids also play an important role in the inhibition of platelet aggregation.
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If sufficient doses of skullcap extract to produce these blood concentrations of these specific flavonoids in humans could be ingested, there would be a risk from regular use of such doses in combination with anticoagulants like warfarin or antiplatelet drugs like aspirin. Further, the inhibition of NADPH quinone reductase (a vitamin K reductase) by baicalein and oroxylin A and their glucuronides (e.g. baicalin) in nanomolar concentrations indicated that these flavones can theoretically act as anticoagulants (Chen et al., 1993; Liu et al., 1990). However, according to an unpublished report, Merkel and Wnorowski (2002) reported no such findings in a single-dose and 5-day repeat-dose toxicity study using high doses of a total skullcap extract administered to rats.
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Regular ArticleInhibition of NAD(P)H:Quinone Acceptor Oxidoreductase by Flavones: A Structure-Activity Study

Abstract

Regular Article
Inhibition of NAD(P)H:Quinone Acceptor Oxidoreductase by Flavones: A Structure-Activity Study