TABLE 1

Kinetic parameters for 4MU β-d-glucuronide formation by recombinant UGT1A enzymes

Kinetic constants were generated from fitting experimental data to the Michaelis-Menten equation except UGT1A1 in the presence of BSA and all kinetic data for UGT1A6.


Parameter

No albumin

BSA

HSA

HSAFAF
0.1%
1.0%
2.0%
0.1%
1.0%
2.0%
0.1%
1.0%
2.0%
UGT1A1
    Km (μM) or Ks 59 249a 350a 393a 92 153 174 51 50 48
    Vmax (pmol/min · mg) 374 722a 687a 960a 406 363 380 407 398 390
    CLint (μl/min/mg) 6.3 N.A. N.A. N.A. 4.4 2.4 2.2 8.0 8.0 8.1
UGT1A6
    Ksa 78 71 95 93 81 102 176 91 64 65
    Vmax (pmol/min · mg)a 82,151 81,036 80,826 80,766 86,483 87,060 76,432 72,850 70,023 73,002
    αa 0.46 0.44 0.47 0.33 0.36 0.38 0.078 0.46 0.52 0.26
UGT1A9
    Km (μM) 13.4 10.8 7.6 3.8 16.6 20.5 22.8 9.8 4.2 2.9
    Vmax (pmol/min · mg) 8362 7860 11262 9817 8422 8632 8649 8067 10265 9765
    CLint (μl/min/mg)
624
728
1482
2583
507
421
379
823
2444
3367
  • N.A., not applicable.

  • a Data from fitting to a two-site model (Uchaipichat et al., 2004). The two-site model provides an estimate of binding affinity (Ks) rather than Km. α and β values for 4MU glucuronidation by recombinant UGT1A1 in the presence of 0.1, 1, and 2% BSA were 0.20 and 0.37, 0.034 and 0.45, and 0.063 and 0.27, respectively. The value of β was set at 2 for the modeling of 4MU glucuronidation by UGT1A6 because for autoactivation Vmax is equivalent to 2Kp[E]t (Uchaipichat et al., 2004).