Isolation and identification of phase 1 metabolites of demethoxycurcumin in rats.

Curcuminoids are a safe natural food coloring additive with anti-inflammatory, antioxidant, and anticarcinogenic activities. Although demethoxycurcumin is one of the major bioactive constituents of curcuminoids, knowledge about its metabolic fate is scant. In the present study, four new metabolites, 5-dehydroxy-hexahydro-demethoxycurcumin-A (M-1), 5-dehydroxy-hexahydro-demethoxycurcumin-B (M-2), 5-dehydroxy-octahydro-demethoxy-curcumin-A (M-3) and 5-dehydroxy-octahydro-demethoxycurcumin-B (M-4), were isolated from feces of male Wistar-derived rats and from urine; three new metabolites, 5-O-methyl-hexahydrodemethoxycurcumin-A (M-7), 5-O-methyl-hexahydro-demethoxycurcumin-B (M-8), and 5-dehydroxy-dihydro-demethoxycurcumin-B (M-9), and two known metabolites, hexahydro-demethoxycurcumin-A (M-5) and hexahydro-demethoxycurcumin-B (M-6), were isolated. Their structures were established by chemical and spectral methods. ll of them were reductive metabolites. Possibly of greater importance is that they occurred as pairs of isomers with a methoxyl group substituted on a different benzene ring. This finding in the metabolism of curcuminoids is reported here for the first time. In addition, the 5-dehydroxy or 5-O-methylated metabolites are also a novel finding. The fact that the metabolites occurred as pairs of the isomers suggests that demethoxycurcumin possibly undergoes tautomerization between 3-keto-5-enol (form A) and 3-keto-5-enol (form B) in rats. Based on the metabolite profiles, metabolic pathways of demethoxycurcumin in rats are proposed.