A tandem mass spectrometry method based on selected ions detects low-abundance phenolics in black tea - theatridimensins as products of the oxidative cascade

Ultrahigh-performance liquid chromatography coupled to ion mobility quadrupole time-of-flight mass spectrometry profiling and unveiling the transformation of ginsenosides by the dual conditions of citric acid and high-pressure steaming

RATIONALE

Many methods have been reported for the production of rare ginsenosides, including heat treatment, acid hydrolysis, alkaline hydrolysis, enzymatic hydrolysis, and microbial transformation. However, the conversion of original ginsenosides to rare ginsenosides under the dual conditions of citric acid and high-pressure steam sterilization has rarely been reported.

METHODS

In this study, a method involving ultrahigh-performance liquid chromatography coupled to ion mobility quadrupole time-of-flight mass spectrometry was developed for analysis of chemical transformation of protopanaxatriol (PPT)-type ginsenosides Rg₁ and Re, protopanaxadiol (PPD)-type ginsenoside Rb₁ , and total ginsenosides in the dual conditions of citric acid and high-pressure steam sterilization. An internal ginsenoside database containing 126 known ginsenosides and 18 ginsenoside reference compounds was established to identify the transformation products and explore possible transformation pathways and mechanisms.

RESULTS

A total of 54 ginsenosides have been preliminarily identified in the transformation products of PPD-type ginsenosides Rg₁ and Re, PPD-type ginsenoside Rb₁ , and total ginsenosides, and the possible transformation pathways were as follows: Rg₁ , Re → 20(S)-Rh₁₂ , 20(R)-Rh₁₂ ; Rg₁ , Re → 20(S)-Rh₁ , 20(R)-Rh₁  → Rk₃ , Rh₄ , Rh₅ ; Rb₁  → gypenoside LXXV; Rb₁  → 20(S)-Rg₃ , 20(R)-Rg₃  → Rk₁ , Rg₅ ; Re → 20(S)-Rg₂ , 20(R)-Rg₂  → 20(S)-Rf₂ , 20(R)-Rf₂ , Rg₄ , F₄ .

CONCLUSIONS

The results elucidated the possible transformation pathways and mechanisms of ginsenosides in the dual conditions of citric acid and high-pressure steam sterilization, which were helpful for revealing the mechanisms of ginsenosides and enhanced safety and quality control of pharmaceuticals and nutraceuticals. Meanwhile, a simple, efficient, and practical method was developed for the production of rare ginsenosides, which has the potential to produce diverse rare ginsenosides on an industrial scale.

Microbial Metabolism of Theaflavin-3,3'-digallate and Its Gut Microbiota Composition Modulatory Effects

Theaflavin-3,3'-digallate (TFDG), a bioactive black tea phenolic, is poorly absorbed in the small intestine, and it has been suggested that gut microbiota metabolism plays a crucial role in its bioactivities. However, information on its metabolic fate and impact on gut microbiota is limited. Here, TFDG was anaerobically fermented in vitro by human fecal microbiota, and epigallocatechin gallate (EGCG) was used for comparison. Despite the similar flavan-3-ol skeletons, TFDG was more slowly degraded and yielded a distinctively different metabolic profile. The formation of theanaphthoquinone as the main metabolites was unique to TFDG. Additionally, a number of hydroxylated phenylcarboxylic acids were formed with low concentrations, when comparing to EGCG metabolism. Microbiome profiling demonstrated several similarities in gut microbiota modulatory effects, including growth-promoting effects on Bacteroides, Faecalibacterium, Parabacteroides, and Bifidobacterium, and inhibitory effects on Prevotella and Fusobacterium. In conclusion, TFDG and EGCG underwent significantly different microbial metabolic fates, yet their gut microbiota modulatory effects were similar.

A new method to prepare and redefine black tea thearubigins

Thearubigins (TRs) are the major components of black tea, which are formed during the fermentation reactions. Although anti-inflammatory and anti-cancer activities of TRs have been reported, the prepared TRs according to the literature methods still contain many floating peaks. It is puzzling whether the observed activities are from TRs or these floating peaks. Thus, it is urgent to develop a method to prepare pure TRs and redefine them. In the present study, we developed a new method, the combination of caffeine precipitation and Sephadex LH-20 column chromatography, to prepare pure TRs. The floating peaks on the hump of the crude TRs were removed, and pure TRs were prepared. The chemical profile of the floating peaks was established using LC/MS, and the major compounds in this fraction were identified as apigenin glycosides, quercetin glycosides, kaempferol glycosides, theaflavins, theasinensin, and galloylglucoses based on the analysis of their tandem mass spectra and in comparison with literature data. This study will pave the way to further study the chemistry and biological activities of TRs and the health effects of black tea consumption.

Peroxidase Can Perform the Hydroxylation Step in the "Oxidative Cascade" during Oxidation of Tea Catechins

The formation of black tea thearubigins involves at least two of the following oxidation steps: (i) oligomerization, (ii) rearrangement, and (iii) hydroxylation. The first two are mainly catalyzed by polyphenol oxidase (PPO), whereas the enzyme responsible for hydroxylation has not yet been identified. Two main oxidative activities, peroxidase (POD) and PPO, occur in tea leaves. POD was hypothesized to be responsible for hydroxylation. Model systems with horseradish POD and mushroom tyrosinase were used investigating hydroxylation of theaflavins (TFs). POD was found capable of hydroxylation. TFs with up to five extra hydroxyl groups were annotated by their MS² data. Hydroxylation by POD was also shown for theanaphtoquinones, theatridimensins, and dehydrodicatechins. The H₂O₂ concentration influenced the extent of hydroxylation, decreasing it at concentrations above 0.01 mM. TFs with up to five extra hydroxyl groups and traces of other hydroxylated oligomeric catechins could be annotated in black tea without any sample pretreatment, using a selective screening method with reversed-phase ultrahigh-performance liquid chromatography mass spectrometry.

Annotation of Different Dehydrocatechin Oligomers by MS/MS and Their Occurrence in Black Tea

Dehydrocatechins (DhC's), oligomeric oxidation products of (epi)catechins, were formed in model incubations of epicatechin with mushroom tyrosinase. DhC oligomers up to tetramers were detected by reversed-phase ultrahigh-performance liquid chromatography mass spectrometry (RP-UHPLC-MS) analysis. Measurements with matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS) showed formation of oligomers up to at least 15 catechin subunits. Isomeric DhC's were obtained, and a method based on MS(2) fragment ratios was set up to distinguish between the different interflavanic configurations of the isomers. In the model incubation, 8 dehydrodicatechins (DhC2's) and 22 dehydrotricatechins (DhC3's) were tentatively annotated by their MS(2) signature fragments. Three different interflavanic configuration types were determined for the DhC2's. DhC2's and DhC3's were shown to occur in a black tea extract for the first time. For the DhC2's, at least two isomeric types, i.e., DhC β and DhC ε, could be annotated in black tea.