The evaluation of novel natural products as inhibitors of human glutathione transferase P1-1

Figures & data

Figure 1.  Chemical structures of the natural plant compounds used in this study.

Figure 2.  The chemical reactions of CDNB and MCB catalysed by GST P1-1.The monochlorobimane fluorescence assay for GST activity was used to measure the product of conjugation at the excitation wavelength 390 nm and the emission wavelength of 478 nm. The CDNB spectrophotometric assay for GST activity was used to determine the conjugation product 2, at a wavelength of 340 nm.

Table 1.  Percentage inhibition of GST P1-1 using natural plant compounds at 33 and 100 µM concentration.

Compound	Class of compound	Source	Percentage inhibition at 33 µM final concentration^a	Percentage inhibition at 100 µM final concentration^a
1	Isofurano-naphthoquinone	Bulbine frutescens	68 (87)^b	91 (100)^b
2	Sesquiterpene	Dicoma anomala	75 (+19)^b	84 (+5)^b
3	Iridoid	Plumeeria rubra	51	+29 (activation)
4	Furocoumarin	Dorstenia elliptica	34	62
5	Benzophenone	Garcinia smeathmannii	41	62
6	coumaroflavan	Treculia africana	26	26
7	Xanthone	Mammea africana	38	52
8	Coumarin	Mammea africana	23	50
9	Xanthone	Mammea africana	21	36
10	Coumarin	Mammea africana	33	98

^aValues obtained with MCB as a substrate.

^bValues obtained with CDNB as substrate.

Figure 3.  Replot of slope (K_m/V_max) and 1/V_max versus [I] to determine K_i^GSH and Ki′^GSH values of 5,8-dihydroxy-1-hydroxymethylnaphtho[2,3-c]furan-4,9-dione (1), which showed mixed type inhibition for GST P1-1 with respect to GSH.

Table 2.  The effects of 5,8-dihydroxy-1-hydroxymethylnaphtho[2,3-c]furan-4,9-dione (1) on the kinetic properties of GST P1-1 with 1-chloro-2,4 dinitrobenzene as electrophilic substrate.

Compound 1 Concentration (µM)	Kcat ^CDNB (S^−1)	Km ^CDNB (mM)	Kcat/Km ^CDNB (S^−1 mM^−1)	Kcat ^GSH (S^−1)	Km ^GSH (mM)	Kcat/Km ^CDNB (S^−1 mM^−1)
0	45	0.3558	126.48	26.63	0.04591	580
3.5	51.29	0.9333	54.96	25.48	0.8519	29.9
7.0	24.26	0.2276	106.59	14.11	0.1477	95.53
15	15.08	0.383	39.37	14.33	0.4627	30.97
30	8.91	0.3569	24.96	14.43	0.211	68.39

Figure 4.  Inhibition of GST P1-1 by 5,8-Dihydroxy-1-hydroxymethylnaphtho[2,3-c]furan-4,9-dione (1). The IC₅₀ value is the concentration of inhibitor giving 50% inhibition of enzyme activity. Data are the mean ± standard deviation of quadruplicate experiments each performed twice.

Figure 5.  Reaction of proteins or GSH with quinones. In A, the reaction is with any nucleophile, whilst in B, the reaction is with reduced glutathione (GSH).