FIG. 3.

The flavonoids apigenin and quercetin inhibit CD38 activity in vitro. A: Chemical structure of apigenin. B and C: In vitro NAD⁺ase (B) and ADP-ribosyl-cyclase (C) activity using human recombinant-purified CD38 and different concentrations of apigenin. D: Chemical structure of quercetin. E and F: In vitro CD38 NAD⁺ase activity (E) and ADP-ribosyl-cyclase activity (F) using human recombinant-purified CD38 and different concentrations of apigenin. In all the measurements, compounds were used in the 0.5–100 μmol/L concentration range. Each measurement was done by triplicate. Data points were fitted to a standard competitive inhibition curve using a nonlinear regression program (GraphPad Prism) to yield the IC₅₀ value.