Flavonoids Reduce Lipid Peroxides and Increase Glutathione Levels in Pooled Human Liver Microsomes (HLMs)

The effects of each of the flavonoids;genistein (G), quercetin (Q) and kaempferol (K) at several doses on lipid peroxides (LP) and reduced glutathione (GSH) in pooled human liver microsomes (HLMs) were investigated following the oxidative damage for 4, 6, 18 and 24 hr. HLMs (1 mg/ml) were exposed to each of the above flavonoids at 0, 5, 10, 15, 20 or 25 μM and incubated for the respective times as previously stated. Our hypothesis was that HLMs exposed to the flavonoids for the respective exposure times can decrease LP and increase GSH in HLMs to better cope with the oxidative stress. The results of our studies indicate that each of the flavonoids significantly (p < 0.01) decreased LP compared to their respective controls. The highest decrease in LP was observed for K followed by Q and G. Significant increases (p < 0.01) in GSH were observed for the flavonoid doses tested with the highest levels observed for Q for the 24-hr. incubation. The findings suggest that the flavonoids modulate oxidative stress in HLMs by decreasing LP and such decreases in LPs may be due to the increasing and or the replenished levels of GSH in the said cells to better cope with the oxidative stress.

Inhibition of Cytochrome P450 by Nomilin and Obacunone and Potential Mechanism in Human Liver Microsomes

Objective Nomilin and obacunone are two important limonoids that are well known for their anticancer effect. Previous studies showed that limonoids had inhibitory effect on cytochrome P450 3A4（CYP3A4）. However these effects are inconclusive with regards to prediction of potential drug interactions. Methods Nomilin or obacunone was pre-incubated with HLMs for 30 min. Following 10-fold dilution from the pre-incubation concentration, a second incubation was performed in the presence of NADPH and cytochrome P450 substrates for 15 min. The reaction was quenched and the supernatants were analyzed by chromatography/mass spectrometry. Results In this study, nomilin and obacunone showed potent inhibitory effect on CYP3A4 with the IC_（50） values of 3.50 and 6.08 μmol/L, respectively. The inhibition of CYP3A4 was in a time-, concentration-and NADPH-dependent manner with Ki values of 2.92 and 1.25 μmol/L and Kinact values of 0.033 and 0.078 min^（-1） for nomilin and obacunone respectively. These results elucidated that they were time-dependent inhibitors for CYP3A4. Conclusion Concomitant use of limonoids and other drugs may call for extra caution for purposes of clinical safety.

In vitro Metabolism of Strychnine by Human Cytochrome P450 and Its Interaction with Glycyrrhetic Acid

Objective To investigate the metabolism of strychnine(STN)and the metabolic interaction between STN and glycyrrhetic acid(GA)in vitro.Methods Human liver microsomes(HLM)and human recombinant cytochrome P450(CYP)isoforms were employed to study the metabolism of STN and the metabolic interaction of STN with GA in vitro.Results In HLM,the Km,Vmax,and clearance of STN were 88.50μmol/L,0.88 nmol/(mg·min),and 9.93 mL/(mg·min),respectively.STN was metabolized mainly by CYP3A4.However,STN noncompetitively inhibited CYP3A4-catalyzed testosterone 6β-hydroxylation with IC50 value of 5.9μmol/L and Ki value of 5.5 μmol/L.Moreover,GA competitively inhibited STN metabolism with IC50 value of 10.6μmol/L and Ki value of 17.7μmol/L.Conclusion Although STN is mainly metabolized by CYP3A4 in vitro,STN has noncompetitive inhibition on CYP3A4-catalyzed testosterone 6β-hydroxylation.Moreover,GA could competitively inhibit STN metabolism.The present work is helpful to elucidate the metabolic interaction between STN and GA.