Angiotensin-converting enzyme 2 (ACE2) is a host receptor for severe acute respiratory syndrome coronavirus 2
(SARS-CoV-2).

Inhibiting the interaction between the envelope spike glycoproteins (S-proteins) of SARS-CoV-2 and ACE2 is a
potential antiviral therapeutic approach, but little is known about how dietary compounds interact with ACE2. The objective of this
study was to determine if flavonoids and other polyphenols with B-ring 3′,4′-hydroxylation inhibit recombinant human (rh)ACE2
activity. rhACE2 activity was assessed with the fluorogenic substrate Mca-APK(Dnp).

Polyphenols reduced rhACE2 activity by 15−66% at 10 μM. Rutin, quercetin-3-O-glucoside, tamarixetin, and 3,4-dihydroxyphenylacetic acid inhibited rhACE2 activity by 42−48%. Quercetin was the most potent rhACE2 inhibitor among the polyphenols tested, with an IC50 of 4.48 μM. Thus, quercetin, its metabolites, and polyphenols with 3′,4′-hydroxylation inhibited rhACE2 activity at physiologically relevant concentrations in vitro.