Sulfite as the substrate of C-sulfonate metabolism of α, β-unsaturated carbonyl containing andrographolide: analysis of sulfite in rats' intestinal tract and the reaction kinetics of andrographolide with sulfite

One-sixth of the currently known natural products contain α , β -unsaturated carbonyl groups. Our previous studies reported a rare C -sulfonate metabolic pathway. Sulfonate groups were linked to the β -carbon of α , β -unsaturated carbonyl-based natural compounds through this pathway. However, the mechanism of this type of metabolism is still not fully understood, especially whether it is formed through enzyme-mediated biotransformation or direct sulfite addition. In this work, the enzyme-mediated and non-enzymatic pathways were studied. First, the sulfite content in rat intestine was determined by LC-MS/MS. The results showed that the amount of sulfite in rat intestinal contents was from 41.5 to 383 μg·g −1 , whereas the amount of sulfite in rat feed was lower than the lower limit of quantitation (20 μg·g −1 ). Second, the reaction kinetics of sulfite-andrographolide reactions in phosphate buffer solutions (pH 6–8) was studied. The half-lives of andrographolide ranged from minutes to hours. This was suggested that the C -sulfonate reaction of andrographolide was very fast. Third, the C -sulfonate metabolites of andrographolide were both detected when andrographolide and L-cysteine- S -conjugate andrographolide were incubated with the rat small intestine contents or sulfite, indicating that the sulfite amount in rat intestine contents was high enough to react with andrographolide, which assisted a significant portion of andrographolide metabolism. Finally, the comparison of andrographolide metabolite profiles among liver homogenate (with NADPH), liver S9 (with NADPH), small intestine contents homogenate (with no NADPH), and sulfite solution incubations showed that the C -sulfonate metabolites were predominantly generated in the intestinal tract by non-enzymatic pathway. In summary, sulfite can serve as a substrate for C -sulfonate metabolism, and these results identified non-enzymatically nucleophilic addition as the potential mechanism for C -sulfonate metabolism of compounds containing α , β -unsaturated carbonyl moiety.