Table 2

Kinetic parameters for raloxifene glucuronidation by expressed UGTs, human liver, and intestinal microsomes

μM nmol/min/mg μl/min/mg μM nmol/min/mg μl/min/mg
UGT 1A12-a 2-a 2-a 2-a 2-a 2-a
UGT 1A87.90.6177592.034
UGT 1A9250.3212130.2519
UGT 1A10N.D.2-b N.D.N.D.4.82-c 0.552-c 1152-c
Human liver microsomes2-a 2-a 2-a 461.532
Human jejunum microsomes580.9917545.195

Expressed UGT1A8 (25 μg), 1A1, 1A9, and 1A10 (50 μg) or human liver and jejunum microsomes (25 μg) were pretreated with alamethicin (60 μg/mg of protein) and incubated with various concentrations of raloxifene (10–200 μM). Incubations were carried out at 37°C for 5 min (human liver and intestinal microsomes) or 10 min (expressed UGT). Apparent Km and Vmax values were determined from linear regression analysis of Eadie-Hofstee plots to assess the potential for atypical versus typical Michaelis-Menten kinetics (Obach et al., 2001).

  • 2-a Kinetic parameters could not be determined due to limited substrate solubility. Rates of glucuronide formation for UGT1A1 at 200 μM were 0.94 nmol/min/mg for R-4-G and 1.85 nmol/min/mg for R-6-G.

  • 2-b Not detected.

  • 2-c Kinetic parameters obtained via extrapolation (seeResults).