In vitro inhibition of carboxylesterase 1 by Kava (Piper methysticum) Kavalactones

Abstract

Kava refers to the extracts from the rhizome of the plant Piper methysticum which is of particular significance to various indigenous cultures in the South Pacific region. Kavalactones are the active constituents of kava products and are associated with sedative and anxiolytic effects. Kavalactones have been evaluated in vitro for their potential to alter the activity of various CYP450 enzymes but have undergone little systematic investigation as to their potential influence on esterases. This study investigated the inhibition effects of kava and its kavalactones on carboxylesterase 1 (CES1) in an in vitro system and established associated kinetic parameters. Kava and its kavalactones were found to produce reversible inhibition of CES1 to varying degrees. Kavain, dihydrokavain, and desmethoxyyangonin displayed competitive type inhibition, while methysticin, dihydromethysticin, and yangonin displayed a mixed competitive-noncompetitive type inhibition. The inhibition constants (K_i) values for each of the kavalactones were as follows: methysticin (35.2 μM), dihydromethysticin (68.2 μM), kavain (81.6 μM), dihydrokavain (105.3 μM), yangonin (24.9 μM), and desmethoxyyangonin (25.2 μM). With consideration to the in vitro K_i for each evaluated kavalactone as well as available clinical kavalactone concentrations in blood circulation, co-administration of CES1 substrate medications and kava products at the recommended daily dose is generally free of drug interaction concerns. However, uncertainty around kavalactone exposure in humans has been noted and a clinically relevant CES1 inhibition by kavain, dihydrokavain, and dihydromethysticin is indeed possible if the kavalactone consumption is higher than 1000 mg in the context of over-the-counter usage. Further clinical studies would be required to assess the possibility of clinically significant kava drug-drug interactions with CES1 substrate medications.

Keywords: CES1; Inhibition; Interaction; Kavalactones; Piper methysticum.

Fig. 1.

Kavalactone chemical structures. Kavalactones and their respective molecular weights selected for analysis.

Fig. 2.

Time-dependent inhibition curve for each kavalactone: (A) methysticin, (B) dihydromethysticin, (C) kavain, (D) dihydrokavain, (E) yangonin, (F) desmethoxyyangonin. Each kavalactone was added with the CES1 S9 fractions before a 30-min preincubation in the preincubation group and after in the no preincubation group. The OST (substrate) concentration was 200 μM and the CES1 S9 fraction (enzyme) concentration was 20 μg/mL. CES1 activity was a ratio that was relative to control with no inhibitor. Individual points represent the mean (±S.D.) done in triplicate. Plots were generated using [Eq. (1)].

Fig. 3.

The kinetic analysis for each kavalactone (A) methysticin, (B) dihydromethysticin, (C) kavain, (D) dihydrokavain, (E) yangonin, (F) desmethoxyyangonin in an in vitro system with CES1. CES1 S9 fractions were incubated with the absence and presence of each kavalactone. Individual points represent the mean (±S.D.) of duplicate samples. Each kavalactone was replicated in triplicate with 1 of the 3 curves being represented. CES1 activity represents the velocity of the reaction (nmol/min/mg protein). Plots were generated by utilizing [Eq. (3)].

Fig. 4.

The corresponding Lineweaver-Burk plots for each kavalactone (A) methysticin, (B) dihydromethysticin, (C) kavain, (D) dihydrokavain, (E) yangonin, (F) desmethoxyyangonin in an in vitro system with CES1. CES1 S9 fractions were incubated with the absence and presence of each kavalactone. Individual points represent the mean (±S.D.) of duplicate samples. Each kavalactone was replicated in triplicate with 1 of the 3 curves being represented. Plots were generated using linear regression.