Skip to Main Content
Table 1

IC50 values and inhibition constants for St. John’s wort dried extracts, and major constituents of it, on 7,8-dihydrodiol-B[a]P epoxidation activity of human CYP1A1

The epoxidation activity (formation of diolepoxide 2) was determined in a reconstituted CYP1A1 system consisting of purified human CYP1A1, purified human P450 reductase, and dilaurylphosphatidylcholine. The P450 concentration for determination of activities was 10 nm, and the substrate concentration was 2 μm in all cases. Formation of diolepoxide 2 was determined using HPLC, as described in “Materials and Methods.” Kinetic analyses were performed by first examining Lineweaver-Burk plots to determine the type of inhibition, followed by nonlinear regression to determine Ki.

ConstituentIC50m)aKim)aType of inhibition
Jarsin extract 1.3 ± 0.2 μg/ml 3.9 ± 0.2 μg/ml Uncompetitive 
Aristo extract 8.0 ± 1.0 μg/ml n.d.b n.d. 
Esbericum caps. extract 24.0 ± 3.0 μg/ml n.d. n.d. 
Hypericin 0.5 ± 0.1 0.6 ± 0.1 Noncompetitive 
Pseudohypericin 8 ± 0.6 3.2 ± 0.8 Mixed type 
Hyperforin 1.2 ± 0.2 1.1 ± 0.2 Competitive 
Quercetin 1.5 ± 0.2 2.4 ± 0.8 Mixed typec 
Rutin ∼100 n.d. n.d. 
ConstituentIC50m)aKim)aType of inhibition
Jarsin extract 1.3 ± 0.2 μg/ml 3.9 ± 0.2 μg/ml Uncompetitive 
Aristo extract 8.0 ± 1.0 μg/ml n.d.b n.d. 
Esbericum caps. extract 24.0 ± 3.0 μg/ml n.d. n.d. 
Hypericin 0.5 ± 0.1 0.6 ± 0.1 Noncompetitive 
Pseudohypericin 8 ± 0.6 3.2 ± 0.8 Mixed type 
Hyperforin 1.2 ± 0.2 1.1 ± 0.2 Competitive 
Quercetin 1.5 ± 0.2 2.4 ± 0.8 Mixed typec 
Rutin ∼100 n.d. n.d. 
a

Inhibition experiments were performed at a substrate concentration of 2 μm. IC50 values represent the mean ± SD of three separate determinations; Ki data represent the mean ± SE as determined by the nonlinear fit using Sigma Plot-Enzyme Kinetics software.

b

n.d., not determined.

c

The fit for mixed type inhibition was only slightly better than noncompetitive inhibition. The present experiments cannot unambiguously distinguish whether inhibition is of mixed type or noncompetitive. Nevertheless, the Ki values determined for both models were not statistically significantly different (P > 0.05, ANOVA).

Close Modal

or Create an Account

Close Modal
Close Modal