FIG 3.

Inhibition of spermidine synthase by hypericin with various concentrations of different substrates (putrescine and dcSAM). (A) Hypericin shows competitive inhibition of spermidine synthase with respect to putrescine, with K_m^P = 1.23 mM, V_max^P = 8.59 mM min⁻¹, and K_i^P = 201.6 mM. (B) Hypericin shows linear mixed inhibition of spermidine synthase with respect to dcSAM, with K_m^S = 90.9 μM, V_max^S = 97.087 μM min⁻¹, and K_i^S = 3.68 μM. ●, enzyme assay without inhibitor; ○, enzyme assay in the presence of inhibitor, i.e., hypericin. The data are plotted as averages from three independent experiments.