Simultaneous determination of nine ginsenosides in functional foods by high performance liquid chromatography with diode array detector detection

Deep learning enabled miniature mass spectrometer for rapid qualitative and quantitative analysis of pesticides on vegetable surfaces

Excessive pesticide use poses a significant threat to food safety. Rapid on-site detection of multi-target pesticide residues in vegetables is crucial due to their widespread distribution and limited shelf life. In this study, a rapid on-site screening method for pesticide residues on vegetable surfaces was developed by employing a miniature mass spectrometer. A direct pretreatment method involves placing vegetables and elution solution into a customized flexible ziplock bag, allowing thorough mixing, washing, and filtration. This process effectively removes pesticide residues from vegetable surfaces with minimal organic solvent usage and can be completed within 2 min. Moreover, this study introduced a deep learning algorithm based on a one-dimensional convolutional neural network, coupled with a feature database, to autonomously discriminate detection outcomes. By combining full scan MS and tandem MS analysis methods, the proposed method achieved a qualitative recognition accuracy of 99.62%. Following the qualitative discrimination stage, the target pesticide residue and internal standard can be simultaneously isolated and fragmented in the ion trap, thus enabling on-site quantitative analysis and warning. This method achieved a quantitative detection limit of 10 μg/kg for carbendazim in cowpea. These results demonstrate the feasibility of the proposed analytical system and strategy in food safety applications.

Rapid screening of illegally added drugs in functional food using a miniature ion trap mass spectrometer

With the expansion of the functional food market, the qualification assessment of these products has become a major challenge, and efficient analytical tools are urgently needed. Here, a miniature mass spectrometer (MS) with self-aspiration capillary electrospray ionization (SACESI) source and ion trap analyzer was developed for rapid screening of various illegally added drugs in functional foods. No chromatographic separation was required, but a simplified two-step pretreatment method was developed to reduce the operational procedures and time consumption of the entire analysis. SACESI source uses capillary action to drive solution injection, which utilizes a simple structure and convenient operation to constitute a kind of disposable MS detection solution. To achieve accurate and automatic identification, an intelligent recognition algorithm with steps of spectrum preprocessing, characteristic peak matching, and support vector machine learning was constructed. The relative accuracy of rapid screening of 31 suspicious drugs in various samples is up to 99.78%. It achieves 100% correct identification for the 55 batches of actual samples captured by on-site inspection, which demonstrates the feasibility of the proposed analytical system and strategy in food safety applications.

Determination of main alkylamides responsible for Zanthoxylum bungeanum pungency through quantitative analysis of multi-components by a single marker

The pungency of Chinese pepper (Zanthoxylum bungeanum) is mainly attributed to the alkylamides contained therein. However, the quantitation and application of these alkylamides are hindered by the lack of commercially available standards. Herein, five alkylamides mainly responsible for the pungency of Z. bungeanum were quantified in 31 batch samples of this plant by high-performance liquid chromatography-mass spectrometry and quantitative analysis of multi-components by a single marker (QAMS) to reveal significant differences in composition distribution according to the sample source. The two methods employed for this purpose, namely an external standard method and QAMS, were shown to be consistent, as the corresponding standardized mean difference was below 5.0%. Thus, the developed QAMS method was concluded to be a promising alternative for the comprehensive and effective quality control of Z. bungeanum from different sources.

Investigation of interactions between cytochrome c and ginsenosides by native mass spectrometry and molecular docking simulations

RATIONALE

Ginsenosides are considered to be the main functional components in ginseng and possess various important pharmacological activities. The study of the interactions between ginsenosides and proteins is indispensable for understanding the pharmacological activities of ginsenosides. In this work, the interactions of ginsenosides with cytochrome c (cyt c) were investigated by native mass spectrometry and molecular docking simulations.

METHODS

The interactions of four ginsenosides (Rb₁ , Rb₃ , Rf, Rg₁ ) and cyt c in NH₄ OAc solution were investigated by electrospray ionization linear ion trap mass spectrometry (ESI-LTQ-MS). Molecular docking simulations of cyt c complexes were carried out by AutoDock.

RESULTS

The native mass spectrometry results showed that the four ginsenosides were directly bound to cyt c, with stoichiometric ratios of 1:1 and 2:1 in NH₄ OAc. The order of relative binding abilities of ginsenosides to cyt c obtained by ESI-MS was Rb₁  > Rb₃  > Rf > Rg₁ , which was consistent with the docking results. Moreover, molecular docking simulations also indicated potential binding sites of cyt c and ginsenosides. Hydrogen-bond interaction played a very important role in cyt c binding with ginsenosides.

CONCLUSIONS

It has been demonstrated that native MS is a useful tool to investigate the interactions of ginsenosides with cyt c. Molecular docking is a good complement to ESI analysis, and can provide information on potential binding sites of cyt c-ginsenoside complexess. This strategy will be helpful to further understand the interactions of proteins and small molecules.

Development of an innovative phytosterol derivatization method to improve the HPLC-DAD analysis and the ESI-MS detection of plant sterols/stanols

HPLC analyses of phytosterols are associated with the issues of sensitivity due to their high lipophilicity and their lack of chromophore. These problems could be solved through chemical modifications of plant sterols/stanols structures. Therefore, the present study aims to develop a new method for phytosterols derivatization. This method was performed using dansyl chloride (4 mg ml^-1) as derivatizing agent and different reaction parameters have been optimized. The highest yields of phytosterol derivatization were obtained with 4-dimethylaminopyridine (DMAP) as catalyst at a concentration of 8 mg ml^-1 and dichloromethane as reaction solvent. In addition, 40 ˚C was the best reaction temperature for 30 min as the best reaction time. This derivatization method presented a high reproducibility (%RSD = 1.2-2.7%) and a good linearity (R² = 0.9982-0.9999). The UV absorption intensities after derivatization showed a 23-fold increment for plant sterols and a 400-fold increment for plant stanols. Moreover, this derivatization method allowed the use of high and more selective wavelengths of detection and improved the chromatographic separation of phytosterols. Furthermore, the developed method allowed the ESI-MS ionization and analysis of phytosterols. This method can therefore contribute to the improvement of the HPLC analyses of plant sterols/stanols.

Recent Advances in the Metabolic Engineering of Yeasts for Ginsenoside Biosynthesis

Ginsenosides are a group of glycosylated triterpenes isolated from Panax species. Ginsenosides are promising candidates for the prevention and treatment of cancer as well as food additives. However, owing to a lack of efficient approaches for ginsenoside production from plants and chemical synthesis, ginsenosides may not yet have reached their full potential as medicinal resources. In recent years, an alternative approach for ginsenoside production has been developed using the model yeast Saccharomyces cerevisiae and non-conventional yeasts such as Yarrowia lipolytica and Pichia pastoris. In this review, various metabolic engineering strategies, including heterologous gene expression, balancing, and increasing metabolic flux, and enzyme engineering, have been described as recent advanced engineering techniques for improving ginsenoside production. Furthermore, the usefulness of a systems approach and fermentation strategy has been presented. Finally, the present challenges and future research direction for industrial cell factories have been discussed.

Ginsenosides Rb1 and Rg1 Protect Primary Cultured Astrocytes against Oxygen-Glucose Deprivation/Reoxygenation-Induced Injury via Improving Mitochondrial Function

This study aimed to evaluate whether ginsenosides Rb1 (20-S-protopanaxadiol aglycon) and Rg1 (20-S-protopanaxatriol aglycon) have mitochondrial protective effects against oxygen-glucose deprivation/reoxygenation (OGD/R)-induced injury in primary mouse astrocytes and to explore the mechanisms involved. The OGD/R model was used to mimic the pathological process of cerebral ischemia-reperfusion in vitro. Astrocytes were treated with normal conditions, OGD/R, OGD/R plus Rb1, or OGD/R plus Rg1. Cell viability was measured to evaluate the cytotoxicity of Rb1 and Rg1. Intracellular reactive oxygen species (ROS) and catalase (CAT) were detected to evaluate oxidative stress. The mitochondrial DNA (mtDNA) copy number and mitochondrial membrane potential (MMP) were measured to evaluate mitochondrial function. The activities of the mitochondrial respiratory chain (MRC) complexes I–V and the level of cellular adenosine triphosphate (ATP) were measured to evaluate oxidative phosphorylation (OXPHOS) levels. Cell viability was significantly decreased in the OGD/R group compared to the control group. Rb1 or Rg1 administration significantly increased cell viability. Moreover, OGD/R caused a significant increase in ROS formation and, subsequently, it decreased the activity of CAT and the mtDNA copy number. At the same time, treatment with OGD/R depolarized the MMP in the astrocytes. Rb1 or Rg1 administration reduced ROS production, increased CAT activity, elevated the mtDNA content, and attenuated the MMP depolarization. In addition, Rb1 or Rg1 administration increased the activities of complexes I, II, III, and V and elevated the level of ATP, compared to those in the OGD/R groups. Rb1 and Rg1 have different chemical structures, but exert similar protective effects against astrocyte damage induced by OGD/R. The mechanism may be related to improved efficiency of mitochondrial oxidative phosphorylation and the reduction in ROS production in cultured astrocytes.

Subchronic oral toxicity study of Korean red ginseng extract in Sprague-Dawley rats with a 4-week recovery period

Ginseng is a major herbal remedy used in Asian countries for thousands of years and known to restore and enhance vital energy. Korean red ginseng, which is processed by steaming and drying fresh Panax ginseng, is most popular and contains unique ginsenosides, which have anticancer and anti-inflammatory properties. The present study was carried out to evaluate the repeated oral dose toxicity of Korean red ginseng extract. The test article was administered orally once a day to male and female Sprague-Dawley rats at dose levels of 0, 500, 1000, or 2000 mg/kg/day for 13 consecutive weeks (15 animals/sex/group in the vehicle control and 2000 mg/kg/day groups, and 10 animals/sex/group in the 500 and 1000 mg/kg/day groups). Ten animals per group were sacrificed at the end of the 13-week treatment period, and the remaining rats were sacrificed after a 4-week recovery period. Administration of Korean red ginseng extract did not result in any toxicologically significant changes in mortality, body weight, food consumption, ophthalmoscopy, hematology, serum biochemistry, gross pathological findings, absolute/relative organ weights, or histopathology. It was established that the no observed adverse effect level (NOAEL) of the test article was 2000 mg/kg/day for both sexes in this study.

Ginsenoside Rc protects against UVB‑induced photooxidative damage in epidermal keratinocytes

Ginsenoside Rc (Rc) is a major ginsenoside isolated from Panax ginseng, and has exhibited pharmacological effects on skin cells. The present study aimed to investigate the putative skin‑protective properties of Rc, including its anti‑photoaging and barrier function‑protective effects, in human HaCaT keratinocytes exposed to UVB radiation. The protective properties of Rc were evaluated through the assessment of keratinocyte viability, reactive oxygen species (ROS) production, total glutathione (GSH) and superoxide dismutase (SOD) activity, caspase‑14, matrix metalloproteinase (MMP)‑2 and ‑9 activity, and MMP‑2, MMP‑9 and filament aggregating protein (filaggrin) expression following UVB irradiation. Treatment with Rc was revealed to prevent the UVB‑induced increase in ROS production and pro‑MMP‑2 and ‑9 levels in HaCaT keratinocytes. In addition, treatment with Rc resulted in enriched GSH contents and enhanced SOD activity following exposure to UVB radiation. Furthermore, Rc treatment enhanced caspase‑14 activity and counteracted the UVB‑induced downregulation in filaggrin expression. However, no significant difference was identified between Rc‑treated and normal groups in terms of keratinocyte viability, regardless of exposure to radiation. The present findings suggested that Rc may exert anti‑photoaging and barrier function‑protective effects in keratinocytes, and thus protect the skin against photooxidative stress induced by exposure to UV radiation.

Protective properties of ginsenoside Rb3 against UV-B radiation-induced oxidative stress in HaCaT keratinocytes

This work aimed to evaluate the skin anti-photoaging properties of ginsenoside Rb3 (Rb3), one of the main protopanaxdiol-type ginsenosides from ginseng, in HaCaT keratinocytes. The skin anti-photoaging activity was assessed by analyzing the levels of reactive oxygen species (ROS), pro-matrix metalloproteinase-2 (proMMP-2), pro-matrix metalloproteinase-9 (proMMP-9), total glutathione (GSH), and superoxide dismutase (SOD) activity as well as cell viability in HaCaT keratinocytes under UV-B irradiation. When HaCaT keratinocytes were exposed to Rb3 prior to UV-B irradiation, Rb3 exhibited suppressive activities on UV-B-induced ROS, proMMP-2, and proMMP-9 enhancements. On the contrary, Rb3 displayed enhancing activities on UV-B-reduced total GSH and SOD activity levels. Rb3 could not interfere with cell viabilities in UV-B-irradiated HaCaT keratinocytes. Rb3 plays a protective role against UV-B-induced oxidative stress in human HaCaT keratinocytes, proposing its potential skin anti-photoaging properties.

Ginsenoside Rd and ischemic stroke; a short review of literatures

Panax ginseng is a well-known economic medical plant that is widely used in Chinese traditional medicine. This species contains a unique class of natural products-ginsenosides. Recent clinical and experimental studies have presented numerous lines of evidence on the promising role of ginsenosides on different diseases including neurodegenerative diseases, cardiovascular diseases, and certain types of cancer. Nowadays, most of the attention has focused on ginsenoside Rd as a neuroprotective agent to attenuate ischemic stroke damages. Some of the evidence showed that ginsenoside Rd ameliorates ischemic stroke-induced damages through the suppression of oxidative stress and inflammation. Ginsenoside Rd can prolong neural cells' survival through the upregulation of the endogenous antioxidant system, phosphoinositide-3-kinase/AKT and extracellular signal-regulated protein kinase 1/2 pathways, preservation of mitochondrial membrane potential, suppression of the nuclear factor-kappa B, transient receptor potential melastatin, acid sensing ion channels 1a, poly(ADP-ribose) polymerase-1, protein tyrosine kinase activation, as well as reduction of cytochrome c-releasing and apoptosis-inducing factor. In the current work, we review the available reports on the promising role of ginsenoside Rd on ischemic stroke. We also discuss its chemistry, source, and the molecular mechanism underlying this effect.

The application of a Tchebichef moment method to the quantitative analysis of multiple compounds based on three-dimensional HPLC fingerprint spectra

A Tchebichef moment method was proposed and used to successfully quantify four compounds based on 3D HPLC-PAD spectra.

Functional analysis of 3-hydroxy-3-methylglutaryl coenzyme a reductase encoding genes in triterpene saponin-producing ginseng

Ginsenosides are glycosylated triterpenes that are considered to be important pharmaceutically active components of the ginseng (Panax ginseng 'Meyer') plant, which is known as an adaptogenic herb. However, the regulatory mechanism underlying the biosynthesis of triterpene saponin through the mevalonate pathway in ginseng remains unclear. In this study, we characterized the role of 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGR) concerning ginsenoside biosynthesis. Through analysis of full-length complementary DNA, two forms of ginseng HMGR (PgHMGR1 and PgHMGR2) were identified as showing high sequence identity. The steady-state mRNA expression patterns of PgHMGR1 and PgHMGR2 are relatively low in seed, leaf, stem, and flower, but stronger in the petiole of seedling and root. The transcripts of PgHMGR1 were relatively constant in 3- and 6-year-old ginseng roots. However, PgHMGR2 was increased five times in the 6-year-old ginseng roots compared with the 3-year-old ginseng roots, which indicates that HMGRs have constant and specific roles in the accumulation of ginsenosides in roots. Competitive inhibition of HMGR by mevinolin caused a significant reduction of total ginsenoside in ginseng adventitious roots. Moreover, continuous dark exposure for 2 to 3 d increased the total ginsenosides content in 3-year-old ginseng after the dark-induced activity of PgHMGR1. These results suggest that PgHMGR1 is associated with the dark-dependent promotion of ginsenoside biosynthesis. We also observed that the PgHMGR1 can complement Arabidopsis (Arabidopsis thaliana) hmgr1-1 and that the overexpression of PgHMGR1 enhanced the production of sterols and triterpenes in Arabidopsis and ginseng. Overall, this finding suggests that ginseng HMGRs play a regulatory role in triterpene ginsenoside biosynthesis.

Simultaneous determination of 30 ginsenosides in Panax ginseng preparations using ultra performance liquid chromatography

A quick and simple method for simultaneous determination of the 30 ginsenosides (ginsenoside Ro, Rb1, Rb2, Rc, Rd, Re, Rf, Rg1, 20(S)-Rg2, 20(R)-Rg2, 20(S)-Rg3, 20(R)-Rg3, 20(S)-Rh1, 20(S)-Rh2, 20(R)-Rh2, F1, F2, F4, Ra1, Rg6, Rh4, Rk3, Rg5, Rk1, Rb3, Rk2, Rh3, compound Y, compound K, and notoginsenoside R1) in Panax ginseng preparations was developed and validated by an ultra performance liquid chromatography photo diode array detector. The separation of the 30 ginsenosides was efficiently undertaken on the Acquity BEH C-18 column with gradient elution with phosphoric acids. Especially the chromatogram of the ginsenoside Ro was dramatically enhanced by adding phosphoric acid. Under optimized conditions, the detection limits were 0.4 to 1.7 mg/L and the calibration curves of the peak areas for the 30 ginsenosides were linear over three orders of magnitude with a correlation coefficients greater than 0.999. The accuracy of the method was tested by a recovery measurement of the spiked samples which yielded good results of 89% to 118%. From these overall results, the proposed method may be helpful in the development and quality of P. ginseng preparations because of its wide range of applications due to the simultaneous analysis of many kinds of ginsenosides.

Ginsenoside-Re ameliorates ischemia and reperfusion injury in the heart: a hemodynamics approach

Ginsenosides are divided into two groups based on the types of the panaxadiol group (e.g., ginsenoside-Rb1 and -Rc) and the panaxatriol group (e.g., ginsenoside-Rg1 and -Re). Among them, ginsenoside-Re (G-Re) is one of the compounds with the highest content in Panax ginseng and is responsible for pharmacological effects. However, it is not yet well reported if G-Re increases the hemodynamics functions on ischemia (30 min)/reperfusion (120 min) (I/R) induction. Therefore, in the present study, we investigated whether treatment of G-Re facilitated the recovery of hemodynamic parameters (heart rate, perfusion pressure, aortic flow, coronary flow, and cardiac output) and left ventricular developed pressure (±dp/dt_max). This research is designed to study the effects of G-Re by studying electrocardiographic changes such as QRS interval, QT interval and R-R interval, and inflammatory marker such as tissue necrosis factor-α (TNF-α) in heart tissue in I/R-induced heart. From the results, I/R induction gave a significant increase in QRS interval, QT interval and R-R interval, but showed decrease in all hemodynamic parameters. I/R induction resulted in increased TNF-α level. Treatment of G-Re at 30 and 100 μM doses before I/R induction significantly prevented the decrease in hemodynamic parameters, ameliorated the electrocardiographic abnormality, and inhibited TNF-α level. In this study, G-Re at 100 μM dose exerted more beneficial effects on cardiac function and preservation of myocardium in I/R injury than 30 μM. Collectively, these results indicate that G-Re has distinct cardioprotectective effects in I/R induced rat heart.

New Method for Simultaneous Quantification of 12 Ginsenosides in Red Ginseng Powder and Extract: In-house Method Validation

For quality control of components in Korean red ginseng powder and extract, a new method for simultaneous quantification of 12 ginsenosides (Rg₁, Re, Rf, Rh₁, Rg₂[S], Rg₂[R], Rb₁, Rc, Rb₂, Rd, Rg₃[S], and Rg₃[R]) was studied. Compared to the official method for quantification of marker substances (ginsenosides Rg₁ and Rb₁), the proposed methods were guaranteed by in-house method validation. Several criteria such as linearity, specificity, precision and accuracy were evaluated. For red ginseng powder, recovery (averaging 95% to 105%) was calculated, and analysis of variance was carried out to estimate the relative standard deviation (0.20% to 2.12%). For red ginseng extract, the average recovery rate was 90% to 99% and the relative standard deviation was 0.39% to 2.40%. These results indicate that the proposed method could be used in the laboratory for determination of 12 ginsenosides in red ginseng powder and extract. In addition, this method was found to be suitable for quality control of ginseng products and potentially offer time and cost benefits.

Simplified ultrasonically- and microwave-assisted solvent extractions for the determination of ginsenosides in powdered Panax ginseng rhizomes using liquid chromatography with UV absorbance or electrospray mass spectrometric detection

New approaches for the recovery of ginsenosides are presented that greatly simplify the liquid chromatographic (LC) determination of the total content of eight ginsenosides (Rb1, Rb2, Rc, Rd, Re, Rf, Rg1 and Rg2) in powdered Panax ginseng rhizomes. The extraction protocols not only recover the neutral ginsenosides, but also simultaneously incorporate base-catalyzed hydrolysis of the malonyl-ginsenosides using dilute potassium hydroxide added to the methanol-water extractant. This eliminates the need for an independent extraction step followed by acid- or base-catalyzed hydrolysis. Both ultrasonically-assisted and microwave-assisted extraction methods are developed. The optimization of these simplified methods to remove pendant malonate esters, while retaining the glycosidic linkages, was determined by LC through variation of the extraction/hydrolysis time, order of hydrolysis reagent addition, and evaluation of multiple extractions. A comparison of the ginsenoside profiles obtained with and without addition of base to the extractant solution was made using LCMS with positive-mode electrospray ionization (ESI(+)) detection. A number of malonyl-ginsenosides were tentatively identified by their mass spectral fragmentation spectra and indicating that they were converted to the free ginsenosides by the new extraction/hydrolysis procedure.

Studies on the chemical transformation of 20(S)-protopanaxatriol (PPT)-type ginsenosides R(e), R(g2), and R(f) using rapid resolution liquid chromatography coupled with quadruple-time-of-flight mass spectrometry (RRLC-Q-TOF-MS)

A rapid resolution liquid chromatography coupled with quadruple-time-of-flight mass spectrometry (RRLC-Q-TOF-MS) method was developed for analysis of chemical transformation of 20(S)-protopanaxatriol (PPT)-type ginsenosides Re, Rg2, and Rf in acidic conditions. The transformation products were identified by comparing the retention time of the standard compounds, the accurate mass measurement, and the fragment ions obtained from RRLC-Q-TOF-tandem mass spectrometry (MS/MS) analyses. The specific product ions of aglycone PPT (m/z 475), C-24- and C-25-hydrated PPT (m/z 493), and Δ20(21) or Δ20(22) dehydration PPT (m/z 457) by MS/MS were discussed for structural characterization. Experiments demonstrated that chemical transformation mechanisms of 20(S)-PPT-type ginsenosides in acidic conditions include hydrolysis of saccharide substitution, Δ20(21) or Δ20(22) dehydration, and hydration addition reactions at C-24 and C-25. The chemical transformation pathway for 20(S)-PPT-type ginsenosides was summarized. The developed RRLC-Q-TOF-MS method was also applied for comparative analysis of 20(S)-PPT ginsenoside and related chemical transformation products in ginseng products.

Ginsenoside Rb3 exerts antidepressant-like effects in several animal models

Total ginsenosides have been shown to have therapeutic actions as antidepressants. We report a major active ingredient of total ginsenosides, the ginsenoside Rb3 (Rb3), which may have antidepressant-like effects. Using the forced swim test, tail suspension test, and learned helplessness procedure, we found that Rb3 had significant anti-immobility effects in mice in the forced swim and tail suspension tests and reduced the number of escape failures in the learned helplessness procedure. In a reserpine-induced syndrome model, Rb3 attenuated hypothermia, palpebral ptosis, and akinesia. In the chronic mild stress model, chronic Rb3 administration reversed the decrease in locomotor activity, novelty-suppressed feeding, and sucrose preference. Furthermore, neurochemical tests were performed to support our hypothesis that biochemical variations (i.e. brain-derived neurotrophic factor and the monoamine neurotransmitters 5-hydroxytryptamine, dopamine, and norepinephrine) are involved in Rb3's antidepressant-like effects. Finally, we found, using whole-cell patch-clamp recordings, that the action potential transmission in neurons within the somatosensory cortex was excited by Rb3 perfusion and blocked with Panax notoginseng total saponins extracted from leaves. This study provides evidence for the mechanism of action of the antidepressant-like effects of Rb3.

Ginsenoside Re: Its chemistry, metabolism and pharmacokinetics

Ginsenosides, the bioactive components of ginseng, can be divided into two major groups, namely 20(S)-protopanaxatriol (e.g. Re, Rg₁, Rg₂, and Rb₃) and 20(S)-protopanaxadiol (e.g. Rb₁, Rb₂, Rc, and Rd). Biological and environmental factors may affect the content of ginsenosides in different parts of ginseng plant. Evidence from pharmacokinetic and metabolic studies of Re demonstrated that (1) the absorption of Re is fast in gastrointestinal tract; (2) Re may be metabolized mainly to Rh₁ and F₁ by intestinal microflora before absorption into blood; and (3) Re is quickly cleared from the body.