Abstract: To investigate the effect of hypericin on the activity of hydrogen sulfide producing enzyme and explore the antitumor effects of hypericin as a cystathionine-β-synthase (CBS) inhibitor in vitro and in vivo,the inhibitory effect of hypericin and aminooxyacetic acid (AOAA) on CBS activity was determined by the methylene blue method.Further,HT29 human colon cancer cells and a HT-29 colon cancer xenograft mouse model were used to study the antitumor effect of hypericin in vitro and in vivo. The results indicated that hypericin,a photosensitizer in the diagnosis and photodynamic therapy of cancer isolated from medicinal herb Hypericum perforatum,was identified as a potent and selective inhibitor of CBS with an IC₅₀ value of 7.9 ± 1.42 μmol/L.Hypericin greatly suppressed the proliferation of HT29 human colon cancer cells with an IC₅₀ value of 22.2 ± 2.25 μmol/L,and CBS protein could be one of the targets of hypericin in mammalian cells,as evidenced by CBS knockdown analyses.Surprisingly,AOAA,a commonly used CBS inhibitor,displayed very weak inhibition of HT29 cancer cells with an IC₅₀ value of 152.5 ± 35.68 μmol/L.This could perhaps be attributed to limited cell membrane permeability of AOAA.Additionally,hypericin at a dose of 5-30 mg/kg·d significantly inhibited the growth of xenograft tumors compared to the control group.After subcutaneous administration of 30 mg/kg·d for two weeks,the weight of the tumors decreased by 68% compared with the control group.Collectively,these results show that hypericin as a selective inhibitor of CBS has significant anti-colon cancer activity.

Key words: hydrogen sulfide producing enzyme, hypericin, cystathionine-&beta, -synthase, hydrogen sulfide, anti-colon cancer