TABLE 3

Kinetic parameters of SN-38 formation from CPT-11 in the purified enzyme, human liver cytosol, and microsomes

CPT-11 (0.5-300 μM) was incubated with the purified enzyme (14 μg/ml), cytosol (1.0 mg/ml), and microsomes (0.2 mg/ml) for 3 min. The kinetic parameters were obtained from the Eadie-Hofstee plots. Each data point represents the mean of duplicate determinations.


Enzyme Source

High-Affinity Isoform

Low-Affinity Isoform
Km
Vmax
Vmax/Km
Km
Vmax
Vmax/Km
μM pmol/min/mg nl/min/mg protein μM pmol/min/mg nl/min/mg protein
Purified enzyme 108.2 32.71 302.3
Cytosol 10.8 0.09 8.1 66.1 0.28 4.2
Microsomes
9.7
0.45
46.6
269.0
4.22
15.7