Enzyme kinetic parameters for glucuronidation of silybin A and silybin B
Enzyme kinetic parameters (Km or S50, Vmax) for glucuronidation of silybin A and silybin B were determined by fitting eq. 4 (Km, Vmax) or eq. 5 (S50, Vmax) to [substrate] versus substrate depletion (silybin A and silybin B) or metabolite formation (monoglucuronide) velocity data using Phoenix WinNonlin. Values represent the parameter estimate ± S.E. Intrinsic clearance (CLint) was calculated as the ratio of Vmax to Km; maximum clearance (CLmax) was calculated using eq. 6.
| Enzyme Source | ||||||
|---|---|---|---|---|---|---|
| HIMs | HLMs | |||||
| Km or S50 | Vmax | CLint or CLmax | Km | Vmax | CLint | |
| μM | nmol/min/mg | ml/min/mg | μM | nmol/min/mg | ml/min/mg | |
| Silybin A | 55 ± 9.3a | 7.6 ± 1.3a | 0.071a | 33 ± 5.2 | 20 ± 1.1 | 0.62 |
| Monoglucuronide 1 | 4.2 ± 0.4 | – | – | 2.0 ± 0.4 | – | – |
| Monoglucuronide 2 | 27 ± 2.9 | – | – | 19 ± 2.9 | – | – |
| Silybin B | 39 ± 14 | 94 ± 14 | 2.4 | 81 ± 9.4 | 140 ± 8.5 | 1.7 |
| Monoglucuronide 1 | 6.7 ± 1.0 | – | – | 3.5 ± 0.5 | – | – |
| Monoglucuronide 2 | 9.8 ± 1.6 | – | – | 7.6 ± 1.5 | – | – |
–, not determined.
↵a Described best by the Hill equation (eq. 5).