The almost forgotten crop amaranth has gained renewed interest in recent years due to its immense nutritive potential. Health beneficial effects of certain plants are often attributed to secondary plant metabolites such as phenolic compounds. As these compounds undergo significant metabolism after consumption and are in most cases not absorbed very well, it is important to gain knowledge about absorption, biotransformation, and further metabolism in the human body. Whilst being hardly found in other edible plants, caffeoylisocitric acid represents the most abundant low molecular weight phenolic compound in many leafy amaranth species. Given that this may be a potentially bioactive compound, gastrointestinal microbial degradation of this substance was investigated in the present study by performing in vitro fermentation tests using three different fecal samples as inocula. The (phenolic) metabolites were analyzed using high-performance liquid chromatography/electrospray ionization tandem mass spectrometry (HPLC-ESI-MS/MS). Furthermore, quantitative polymerase chain reaction (qPCR) analyses were carried out to study the influence on the microbiome and its composition. The in vitro fermentations led to different metabolite profiles depending on the specific donor. For example, the metabolite 3-(4-hydroxyphenyl)propionic acid was observed in one fermentation as the main metabolite, whereas 3-(3-hydroxyphenyl)propionic acid was identified in the other fermentations as important. A significant change in selected microorganisms of the gut microbiota however was not detected. In conclusion, caffeoylisocitric acid from amaranth, which is a source of several esterified phenolic acids in addition to chlorogenic acid, can be metabolized by the human gut microbiota, but the metabolites produced vary between individuals.

Human colonic bacteria have an important impact on the biotransformation of flavonoid glycosides and their conversion can result in the formation of bioactive compounds. However, information about the microbial conversion of complex glycosylated flavonoids and the impact on the gut microbiota are still limited. In this study, in vitro fermentations with selected flavonoid O- and C-glycosides and three different fecal samples were performed. As a result, all flavonoid glycosides were metabolized via their aglycones yielding smaller substances. Main metabolites were 3-(4-hydroxyphenyl)propionic acid, 3-phenylpropionic acid, and phenylacetic acid. Differences in the metabolite formation due to different time courses between the donors were determined. Therefore, from all fermentations, the ones with a specific donor were always slower resulting in a lower number of metabolites compared to the others. For example, tiliroside was totally degraded from 0 h (105 ± 13.2 μM) within the first 24 h, while in the fermentations with fecal samples from other donors, tiliroside (107 ± 52.7 μM at 0 h) was not detected after 7 h anymore. In general, fermentation rates of C-glycosides were slower compared to the fermentation rates of O-glycosides. The O-glycoside tiliroside was degraded within 4 h while the gut microbiota converted the C-glycoside vitexin within 13 h. However, significant changes (p < 0.05) in the microbiota composition and short chain fatty acid levels as products of carbohydrate fermentation were not detected between incubations with different phenolic compounds. Therefore, microbiota diversity was not affected and a significant prebiotic effect of phenolic compounds cannot be assigned to flavonoid glycosides in food-relevant concentrations.

One response to oxidation of guanine (G) to 8-oxo-7,8-dihydroguanine (OG) in a gene promoter is regulation of mRNA expression suggesting an epigenetic-like role for OG. A proposed mechanism involves G oxidation within a potential G-quadruplex-forming sequence (PQS) in the promoter enabling a structural shift from B-DNA to a G-quadruplex fold (G4). When OG was located in the coding vs. template strand, base excision repair led to an on/off transcriptional switch. Herein, a G-rich, potential Z-DNA-forming sequence (PZS) comprised of a d(GC) n repeat was explored to determine whether oxidation in this motif was also a transcriptional switch. Bioinformatic analysis found 1650 PZSs of length >10 nts in the human genome that were overrepresented in promoters and 5′-UTRs. Studies in human cells transfected with a luciferase reporter plasmid in which OG was synthesized in a PZS context in the promoter found that a coding strand OG increased expression, and a template strand OG decreased expression. The initial base excision repair product of OG, an abasic site (AP), was also found to yield similar expression changes as OG. Biophysical studies on model Z-DNA strands found OG favored a shift in the equilibrium to Z-DNA from B-DNA, while an AP disrupted Z-DNA to favor a hairpin placing AP in the loop where it is a poor substrate for the endonuclease APE1. Overall, the impact of OG and AP in a PZS on gene expression was similar to that in a PQS but reduced in magnitude.

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