Table 2

Apparent kinetic parameters for the glucuronidation of SN-38 by human liver microsomes (HLM) and human UGT isoenzymes

UGT SourceApparent KmAbsoluteVmaxProtein expression (relative to UGT1A7)Relative VmaxCatalytic Efficiencies Vmax/Km
μM pmol/h/mg protein pmol/h/mg protein μl/h/mg
HLM6.8  ±  3.02608.2  ±  442.0
Hepatic and Gastrointestinal UGT
 UGT1A17.5  ±  3.92005.5  ±  124.70.782571.1  ± 159.9343
 UGT1A611.6  ±  5.2163.5  ±  36.65.3030.8  ± 6.93
 UGT1A90.7  ±  0.2145.7  ±  12.00.67220.45  ± 17.9315
Gastrointestinal UGT
 UGT1A71.2  ±  0.1479.2  ±  11.11.00479.2  ± 11.1399
 UGT1A820.3  ±  4.5298.4  ±  45.01.38216.2  ± 32.611
 UGT1A1031.5  ±  9.2454.7  ±  24.31.77256.9  ± 13.728

The values of apparent K m andV max for the formation of SN-38 glucuronide were determined using microsomal preparations from pool human livers and HEK293 cells. Formation of SN-38-G was measured by LC/MS/MS as described under Experimental Procedures. Values were expressed as the mean ± S.D. of two experiments performed in duplicate from Lineweaver-Burk plots.