Inhibition of hSULT2A1-catalyzed sulfation of pregnenolone by metabolites of tamoxifen
The sulfation of pregnenolone was determined using 0.03 μg of purified hSULT2A1 in the presence of varied concentrations of inhibitor and either 0.4 μM pregnenolone (for IC50 values) or 0.2–1.0 μM DHEA for determination of the mechanism of inhibition and related kinetic constants. The data are expressed as mean ± S.E. from three independent experiments. Calculation of kcat values was based on 33,678 as the subunit molecular mass of hSULT2A1.
| Metabolite | IC50 | Type of Inhibition | Vmax | Km | kcat/Km | Ki |
|---|---|---|---|---|---|---|
| nmol/min per mg | μM | min−1μM−1 | μM | |||
| Endoxifen | 2.7 ± 1.1 | Mixed | 332 ± 54 | 1.2 ± 0.3 | 9.3 | 3.5 ± 0.7 |
| N-desTAM | 15.0 ± 1.2 | Competitive | 630 ± 127 | 2.1 ± 0.5 | 10.0 | 9.8 ± 1.2 |
| 4-OHTAM | 16.7 ± 1.1 | Mixed | 522 ± 103 | 2.0 ± 0.5 | 8.5 | 12.7 ± 2.1 |
| TAM-NO | 16.1 ± 1.1 | Noncompetitive | 647 ± 121 | 2.3 ± 0.5 | 9.5 | 16.9 ± 0.6 |