Peptide Isolated from Leptospermum flavens Sm. Inhibits Human Glutathione Transferases π (hGSTP) Activity and Enhances the Cytotoxicity of Doxorubicin towards HT-29 Cell Line

Glutathione transferase Pi (GST-P) reportedly overexpressed markedly in cancer cell lines. It was correlated to the resistance towards drugs used in chemotherapy treatment. The study demonstrated the isolation of peptide as potential inhibitor to the enzyme. Glutathione transferase Pi (GST-P) was purified from human colon adenocarcinoma HT-29 cell line using glutathione (GSH)-affinity chromatography. Active components presence in ethanolic extract (50% ethanol) of leaves Leptospermum flavescens Sm. was shown possessing inhibitory property (IC 50 of 0.088 mg/mL) towards GST-P in vitro. Further fractionation using polyamide the 50% methanol in 2% acetic acid eluate possessed an inhibitory property at IC 50 of 0.191 mg/mL. In the study the IC 50 values of doxorubicin are 0.788 and 0.816 μg/mL on HT-29 and MRC-5 cell lines respectively while IC 50 values of cisplatin were at 9.49 and 4.07 μg/mL on HT-29 and MRC-5 cell lines respectively. The 50% methanol eluate has significantly non-toxic to both of the cell lines with 100% cell viability at more than 100 μg/mL sample applied. In combination with doxorubicin, 50% methanol eluate enhanced cytotoxicity of the drug towards HT-29 by reduction of IC 50 value significantly to 66%. The eluate however only reduced the IC 50 to 11% when combined with cisplatin. The study indicates that the eluate can potentiate cytotoxicity of doxorubicin on HT-29 cell line and this effect is correlated to the ability of the eluate to inhibit GST-P in vitro. The purified active molecule was a peptide with molecular weight of 3.5 kDa.