Metabolism of Capsaicin by Cytochrome P450 Produces Novel Dehydrogenated Metabolites and Decreases Cytotoxicity to Lung and Liver Cells

Abstract

Capsaicin is a common dietary constituent and a popular homeopathic treatment for chronic pain. Exposure to capsaicin has been shown to cause various dose-dependent acute physiological responses including the sensation of burning and pain, respiratory depression, and death. In this study, the P450-dependent metabolism of capsaicin by recombinant P450 enzymes and hepatic and lung microsomes from various species, including humans, was determined. A combination of LC/MS, LC/MS/MS, and LC/NMR was used to identify several metabolites of capsaicin that were generated by aromatic (M5 and M7) and alkyl hydroxylation (M2 and M3), O-demethylation (M6), N- (M9) and alkyl dehydrogenation (M1 and M4), and an additional ring oxygenation of M9 (M8). Dehydrogenation of capsaicin was a novel metabolic pathway and produced unique macrocyclic, diene, and imide metabolites. Metabolism of capsaicin by microsomes was inhibited by the nonselective P450 inhibitor 1-aminobenzotriazole (1-ABT). Metabolism was catalyzed by CYP1A1, 1A2, 2B6, 2C8, 2C9, 2C19, 2D6, 2E1, and 3A4. Addition of GSH (2 mM) to microsomal incubations stimulated the metabolism of capsaicin and trapped several reactive electrophilic intermediates as their GSH adducts. These results suggested that reactive intermediates, which inactivated certain P450 enzymes, were produced during catalytic turnover. Comparison of the rate and types of metabolites produced from capsaicin and its analogue, nonivamide, demonstrated similar pathways in the P450-dependent metabolism of these two capsaicinoids. However, production of the dehydrogenated (M4), macrocyclic (M1), and omega-1-hydroxylated (M3) metabolites was not observed for nonivamide. These differences may be reflective of the mechanism of formation of these metabolites of capsaicin. The role of metabolism in the cytotoxicity of capsaicin and nonivamide was also assessed in cultured lung and liver cells. Lung cells were markedly more sensitive to cytotoxicity by capsaicin and nonivamide. Cytotoxicity was enhanced 5 and 40% for both compounds by 1-ABT in BEAS-2B and HepG2, respectively. These data suggested that metabolism of capsaicinoids by P450 in cells represented a detoxification mechanism (in contrast to bioactivation).

Structure-Activity Relationship of Capsaicin Analogs and Transient Receptor Potential Vanilloid 1-Mediated Human Lung Epithelial Cell Toxicity

“…This provides a plausible explanation for the toxicity of this analog, but lack of attenuation of cell death at 25 M in both TRPV1-OE and BEAS-2B cells by NAC suggests that TRPV1 must still be involved in some way, as suggested by the correlation studies. The substantially lower toxicity of the diol in NHBE cells also suggests a fundamental difference in reactive oxygen species/ electrophile detoxification capacity between immortalized and primary lung cells, which is important because N- (3,4-dihydroxybenzyl)nonanamide is efficiently produced by CYP1A2 and CYP2C19 (Reilly et al, 2003a;Reilly and Yost, 2006) and, thus, it may be difficult to modulate capsaicinoid toxicity in the intact lung using TRPV1 antagonists alone. Currently, an explanation for the toxicity of N-(4-hydroxybenzyl)nonanamide, 3-methoxy-4-(nonamidomethy)phenyl sulfate, and N-(3-methoxybenzyl) nonanamide is not apparent, but metabolism by cytochromes P450 by hydroxylation to produce nonivamide or N- (3,4-dihydroxybenzyl)nonanamide does not seem to be operative.…”

A Comprehensive Review of the Carcinogenic and Anticarcinogenic Potential of Capsaicin

“…It has been further suggested that capsaicin is bioactivated through the formation of several electrophilic metabolites that have the potential to lead to hepatotoxic and/or mutagenic responses Reilly et al 2003;Reilly and Yost 2006). However, the formation of reactive metabolites as identified through in vitro experiments such as glutathione trapping and covalent binding has not been directly linked to adverse drug reactions in the clinic .…”

“…For example, a number of hepatotoxic and non-hepatotoxic compounds have been tested for covalent binding to liver microsomal protein, and the results indicated that prediction of toxicity through the use of in vitro covalent binding experiments was unreliable due to the number of false positive findings (Obach et al 2008). In fact, BEAS-2B and HepG2 cells displayed a greater sensitivity to cytotoxicity in the presence of capsaicin and 1-aminobenzotriazole (a nonspecific CYP450 inhibitor), indicating that capsaicin metabolism represented a detoxification mechanism, as opposed to bioactivation (Reilly et al 2003).…”

The Transient Receptor Potential Vanilloid-1 Channel in Thermoregulation: A Thermosensor It Is Not

“…Hence, even if the consumption of hot pepper is indeed driven by the thermoregulatory effects of CAP, it is unclear whether the driving mechanism is an acute increase in heat loss or a chronic deactivation of neural pathways for heat-defense mechanisms. Furthermore, CAP is metabolized by the liver, and the oral administration of this vanilloid (which delivers CAP to the liver via the portal circulation) is poorly suited for achieving high concentrations in the nervous system (Donnerer et al, 1990;Reilly et al, 2003). It is plausible that people living in hot climates consume more spicy foods simply because more spicy plants grow in such climates and are, therefore, available for greater consumption.…”