The cytochrome P450 (CYP) superfamily plays a key role in the oxidative metabolism of a wide range of exogenous chemicals. CYP2C8 is the principal enzyme responsible for the metabolism of the anti-cancer drug paclitaxel in the human liver, and carries out the oxidative metabolism of at least 5% of clinical drugs. Polymorphisms in CYP2C8 have been closely implicated in individualized medication. CYP2C8.3, a common polymorph of CYP2C8 with dual amino acid substitutions R139K and K399R, is found primarily in Caucasians. In this study, CYP2C8.3 and its wild type (WT) CYP2C8 were expressed in E. coli, and their purified proteins were characterized by UV-visible spectroscopy, mass spectrometry, and circular dichroism. Their thermal stability, substrate binding ability, and metabolic activity against paclitaxel were investigated. The electron transfer kinetics during paclitaxel metabolism by WT CYP2C8 or CYP2C8.3 was studied by stopped-flow kinetics. The results revealed that mutations in CYP2C8.3 did not greatly influence the heme active site or protein thermal stability and paclitaxel binding ability, but the metabolic activity against paclitaxel was significantly depressed to just 11% of that of WT CYP2C8. Electron transfer from CYP reductase to CYP2C8.3 was found to be significantly slower than that to WT CYP2C8 during catalysis, and this might be the main reason for the depressed metabolic activity. Since the polymorph CYP2C8.3 is defective in catalyzing substrates of CYP2C8 in vitro, it might be expected to have important clinical and pathophysiological consequences in homozygous individuals, and this study provides valuable information in this aspect. The cytochrome P450 (CYP) superfamily plays a key role in the oxidative metabolism of a wide range of exogenous chemicals. CYP2C8 is the principal enzyme responsible for the metabolism of the anti-cancer drug paclitaxel in the human liver, and carries out the oxidative metabolism of at least 5% of clinical drugs. Polymorphisms in CYP2C8 have been closely implicated in individualized treatments. CYP2C8.3, a common common polymorph of CYP2C8 with dual amino acid substitutions R139K and K399R, is found to be in Caucasians. In this study, CYP2C8. 3 and its wild type (WT) CYP2C8 were expressed in E. coli, and their purified proteins were characterized by UV-visible spectroscopy, mass spectrometry, and circular dichroism. Their thermal stability, substrate binding ability, and metabolic activity against paclitaxel were investigated The electron transfer kinetics during paclitaxel metabolism by WT CYP2C8 or CYP2C8.3 was studied by stopped-flow kinetics. The results revealed that mutations in CYP 2C8.3 did not significantly affect the heme active site or protein thermal stability and paclitaxel binding ability, but the metabolic activity against paclitaxel was significantly depressed to just 11% of that of WT CYP2C8. Electron transfer from CYP reductase to CYP2C8.3 was found to be significantly slower than that to WT CYP2C8 during catalysis, and this might be the main reason for the depressed metabolic activity. Since the polymorph CYP2C8.3 is defective in catalyzing substrates of CYP2C8 in vitro, it might be expected to have important clinical and pathophysiological consequences in homozygous individuals, and this study provides valuable information in this aspect.