Sulfated Metabolites of Luteolin, Myricetin, and Ampelopsin: Chemoenzymatic Preparation and Biophysical Properties

The 2-aminoethyl diphenylborinate-based fluorescent method identifies quercetin and luteolin metabolites as substrates of Organic anion transporting polypeptides, OATP1B1 and OATP2B1

Organic anion transporting polypeptides (OATPs), OATP1B1 and OATP2B1 are membrane proteins mediating the cellular uptake of chemically diverse organic compounds. OATP1B1 is exclusively expressed in hepatocytes and plays a key role in hepatic detoxification. The ubiquitously expressed OATP2B1 promotes the intestinal absorption of orally administered drugs. Flavonoids are widely found in foods and beverages, and many of them can inhibit OATP function, resulting in food-drug interactions. In our previous work, we have shown that not only luteolin (LUT) and quercetin (Q), but also some of their metabolites can inhibit OATP1B1 and OATP2B1 activity. However, data about the potential direct transport of these flavonoids by OATPs have been incomplete. Hence, in the current study, we developed a simple, fluorescence-based method for the measurement of intracellular flavonoid levels. The method applies a cell-permeable small molecule (2-aminoethyl diphenylborinate, 2-APB), that, upon forming a complex with flavonoids, results in their fluorescence enhancement. This way the direct uptake of LUT and Q, and also their metabolites' could be investigated both by confocal microscopy and in a fluorescence plate reader in living cells. With this approach we identified quercetin-3'-O-sulfate, luteolin-3'-O-glucuronide, luteolin-7-O-glucuronide and luteolin-3'-O-sulfate as substrates of both OATP1B1 and OATP2B1. Our results highlight that OATP1B1 and OATP2B1 can be key participants in the transmembrane movement of LUT and Q conjugates with otherwise low cell permeability. In addition, the novel method developed in this study can be a good completion to existing fluorescence-based assays to investigate OATP function.

Dietary Polyphenols: Review on Chemistry/Sources, Bioavailability/Metabolism, Antioxidant Effects, and Their Role in Disease Management

Polyphenols, as secondary metabolites ubiquitous in plant sources, have emerged as pivotal bioactive compounds with far-reaching implications for human health. Plant polyphenols exhibit direct or indirect associations with biomolecules capable of modulating diverse physiological pathways. Due to their inherent abundance and structural diversity, polyphenols have garnered substantial attention from both the scientific and clinical communities. The review begins by providing an in-depth analysis of the chemical intricacies of polyphenols, shedding light on their structural diversity and the implications of such diversity on their biological activities. Subsequently, an exploration of the dietary origins of polyphenols elucidates the natural plant-based sources that contribute to their global availability. The discussion extends to the bioavailability and metabolism of polyphenols within the human body, unraveling the complex journey from ingestion to systemic effects. A central focus of the review is dedicated to unravelling the antioxidant effects of polyphenols, highlighting their role in combating oxidative stress and associated health conditions. The comprehensive analysis encompasses their impact on diverse health concerns such as hypertension, allergies, aging, and chronic diseases like heart stroke and diabetes. Insights into the global beneficial effects of polyphenols further underscore their potential as preventive and therapeutic agents. This review article critically examines the multifaceted aspects of dietary polyphenols, encompassing their chemistry, dietary origins, bioavailability/metabolism dynamics, and profound antioxidant effects. The synthesis of information presented herein aims to provide a valuable resource for researchers, clinicians, and health enthusiasts, fostering a deeper understanding of the intricate relationship between polyphenols and human health.

Expanded interlayer spacing of SnO2 QDs-Decorated MXene for highly selective luteolin detection with Ultra-Low limit of detection

Although there have been advancements in electrochemical catalysts for luteolin detection, their practical use is constrained by low sensitivity, inadequate selectivity, and unsatisfactory limit of detection. MXene, a class of 2D materials, possesses exceptional physical-chemical properties that make it highly suitable for electrochemical detection. Nevertheless, the self-stacking and limited interlayer spacing of MXene impede its extensive application in electrochemical detection. Herein, a SnO2 QDs-MXene composite is synthesized for selective electrochemical detection of luteolin. Inserting SnO2 QDs between tightly stacked MXene layers expands the d-spacing of MXene, enhancing the specific surface area and enabling abundant active sites for redox reactions. The inclusion of MXene in the modified SnO2 QDs-MXene/GCE electrode significantly enhances electron transfer. As a result, the electrode demonstrates exceptional luteolin detection capabilities, including a wide linear range (0.1-1200 nM), high sensitivity (12.4 μA μM-1), and an ultra-low limit of detection (0.14 nM). Additionally, the SnO2 QDs-MXene/GCE electrode exhibits good repeatability, excellent reproducibility, remarkable stability, and high selectivity, making it suitable for practical sample analysis. This research contributes to advancing ultra-low limit of detection sensors for accurate luteolin detection.

Biochemical Profile and Antioxidant Properties of Propolis from Northern Spain

The antioxidant, anti-inflammatory, and antimicrobial characteristics of propolis, a bioactive compound collected from hives, have prompted its use in the food sector in recent times. This study investigated the physicochemical characteristics, phenolic profile, and antioxidant capacity of 31 propolis extracts collected from Northern Spain. The physicochemical composition (resins, waxes, ashes mineral content, and heavy metals) was within the allowable regulatory limits. The analysis of bioactive compounds enabled the identification of 51 constituents: flavonoids (apigenin, catechin, chrysin, quercetin, and pinocembrin) and phenolic acids (caffeic, ferulic, and coumaric). The mean value of total polyphenols was 42.72 ± 13.19 Pinocembrin-Galangin Equivalents/100 g, whereas a range between 1.64 ± 0.04 and 4.95 ± 0.36 Quercetin Equivalents (QE) g/100 g was found for total flavonoids content. The determination of bioactivities revealed significant antioxidant capacity using DPPH (1114.28 ± 10.39 µM Trolox Equivalents and 3487.61 ± 318.66 µM Vitamin C Equivalents). Resin content in propolis samples was positively and significantly correlated with both polyphenols (rho = 0.365; p = 0.043) and flavonoid composition (rho = 0.615; p = 0.000) as well as the antioxidant capacity TEAC DPPH (rho = 0.415; p = 0.020). A multiple regression analysis modeled the correlation between resin composition, flavonoids, and TEAC DPPH values, yielding a significant regression equation (R2 = 0.618; F (2,28) = 22.629; p < 0.000; d = 2.299). Therefore, evaluating physicochemical parameters and biological activities provides a promising framework for predicting propolis' quality and antioxidant properties, thus suggesting its potential as a functional and bioactive compound for the food industry.

Inhibition of xanthine oxidase-catalyzed xanthine and 6-mercaptopurine oxidation by luteolin, naringenin, myricetin, ampelopsin and their conjugated metabolites

Luteolin, naringenin, myricetin, and ampelopsin are abundant flavonoids in nature, and several dietary supplements also contain them at very high doses. After the peroral intake, flavonoids go through extensive presystemic biotransformation; therefore, typically their sulfate/glucuronic acid conjugates reach high concentrations in the circulation. Xanthine oxidase (XO) enzyme is involved in uric acid production, and it also takes part in the elimination of certain drugs (e.g., 6-mercaptopurine). The inhibitory effects of flavonoid aglycones on XO have been widely studied; however, only limited data are available regarding their sulfate and glucuronic acid conjugates. In this study, we examined the impacts of luteolin, naringenin, myricetin, ampelopsin, and their sulfate/glucuronide derivatives on XO-catalyzed xanthine and 6-mercaptopurine oxidations employing in vitro enzyme incubation assays and molecular modeling studies. Our major results/conclusions are the following: (1) Sulfate metabolites were stronger while glucuronic acid derivatives were weaker inhibitors of XO compared to the parent flavonoids. (2) Naringenin, ampelopsin, and their metabolites were weak inhibitors of the enzyme. (3) Luteolin, myricetin, and their sulfates were highly potent inhibitors of XO, and the glucuronides of luteolin showed moderate inhibitory impacts. (4) Conjugated metabolites of luteolin and myricetin can be involved in the inhibitory effects of these flavonoids on XO enzyme.

Interaction of luteolin, naringenin, and their sulfate and glucuronide conjugates with human serum albumin, cytochrome P450 (CYP2C9, CYP2C19, and CYP3A4) enzymes and organic anion transporting polypeptide (OATP1B1 and OATP2B1) transporters

Luteolin and naringenin are flavonoids found in various foods/beverages and present in certain dietary supplements. After a high intake of these flavonoids, their sulfate and glucuronide conjugates reach micromolar concentrations in the bloodstream. Some pharmacokinetic interactions of luteolin and naringenin have been investigated in previous studies; however, only limited data are available in regard to their metabolites. In this study, we aimed to investigate the interactions of the sulfate and glucuronic acid conjugates of luteolin and naringenin with human serum albumin, cytochrome P450 (CYP2C9, 2C19, and 3A4) enzymes, and organic anion transporting polypeptide (OATP1B1 and OATP2B1) transporters. Our main findings are as follows: (1) Sulfate conjugates formed more stable complexes with albumin than the parent flavonoids. (2) Luteolin and naringenin conjugates showed no or only weak inhibitory action on the CYP enzymes examined. (3) Certain conjugates of luteolin and naringenin are potent inhibitors of OATP1B1 and/or OATP2B1 enzymes. (4) Conjugated metabolites of luteolin and naringenin may play an important role in the pharmacokinetic interactions of these flavonoids.

Sulfation of Phenolic Acids: Chemoenzymatic vs. Chemical Synthesis

Phenolic acids are known flavonoid metabolites, which typically undergo bioconjugation during phase II of biotransformation, forming sulfates, along with other conjugates. Sulfated derivatives of phenolic acids can be synthesized by two approaches: chemoenzymatically by 3'-phosphoadenosine-5'-phosphosulfate (PAPS)-dependent sulfotransferases or PAPS-independent aryl sulfotransferases such as those from Desulfitobacterium hafniense, or chemically using SO₃ complexes. Both approaches were tested with six selected phenolic acids (2-hydroxyphenylacetic acid (2-HPA), 3-hydroxyphenylacetic acid (3-HPA), 4-hydroxyphenylacetic acid (4-HPA), 3,4-dihydroxyphenylacetic acid (DHPA), 3-(4-hydroxyphenyl)propionic acid (4-HPP), and 3,4-dihydroxyphenylpropionic acid (DHPP)) to create a library of sulfated metabolites of phenolic acids. The sulfates of 3-HPA, 4-HPA, 4-HPP, DHPA, and DHPP were all obtained by the methods of chemical synthesis. In contrast, the enzymatic sulfation of monohydroxyphenolic acids failed probably due to enzyme inhibition, whereas the same reaction was successful for dihydroxyphenolic acids (DHPA and DHPP). Special attention was also paid to the counterions of the sulfates, a topic often poorly reported in synthetic works. The products obtained will serve as authentic analytical standards in metabolic studies and to determine their biological activity.

Potential roles of gut microbes in biotransformation of natural products: An overview

Natural products have been extensively applied in clinical practice, characterized by multi-component and multi-target, many pharmacodynamic substances, complex action mechanisms, and various physiological activities. For the oral administration of natural products, the gut microbiota and clinical efficacy are closely related, but this relationship remains unclear. Gut microbes play an important role in the transformation and utilization of natural products caused by the diversity of enzyme systems. Effective components such as flavonoids, alkaloids, lignans, and phenols cannot be metabolized directly through human digestive enzymes but can be transformed by enzymes produced by gut microorganisms and then utilized. Therefore, the focus is paid to the metabolism of natural products through the gut microbiota. In the present study, we systematically reviewed the studies about gut microbiota and their effect on the biotransformation of various components of natural products and highlighted the involved common bacteria, reaction types, pharmacological actions, and research methods. This study aims to provide theoretical support for the clinical application in the prevention and treatment of diseases and provide new ideas for studying natural products based on gut biotransformation.

Bacterial Aryl Sulfotransferases in Selective and Sustainable Sulfation of Biologically Active Compounds using Novel Sulfate Donors

Regioselective sulfation of bioactive compounds is a vital and scarcely studied topic in enzyme-catalyzed transformations and metabolomics. The major bottleneck of enzymatic sulfation consists in finding suitable sulfate donors. In this regard, 3'-phosphoadenosine 5'-phosphosulfate (PAPS)-independent aryl sulfotransferases using aromatic sulfate donors are a favored choice due to their cost-effectiveness. This work presents a unique study of five sulfate donors differing in their leaving group pK_a values with a new His-tagged construct of aryl sulfotransferase from Desulfitobacterium hafniense (DhAST-tag). DhAST-tag was purified to homogeneity and biochemically characterized. Two new donors (3-nitrophenyl sulfate and 2-nitrophenyl sulfate) were synthesized. The kinetic parameters of these and other commercial sulfates (4-nitrophenyl, 4-methylumbelliferyl, and phenyl) revealed large differences with respect to the structure of the leaving group. These donors were screened for the sulfation of selected flavonoids (myricetin, chrysin) and phenolic acids (gallate, 3,4-dihydroxyphenylacetate). The donor impact on the sulfation regioselectivity and yield was assessed. The obtained regioselectively sulfated compounds are authentic human metabolites required as standards in clinical trials.

Sulfated Phenolic Substances: Preparation and Optimized HPLC Analysis

Sulfation is an important reaction in nature, and sulfated phenolic compounds are of interest as standards of mammalian phase II metabolites or pro-drugs. Such standards can be prepared using chemoenzymatic methods with aryl sulfotransferases. The aim of the present work was to obtain a large library of sulfated phenols, phenolic acids, flavonoids, and flavonolignans and optimize their HPLC (high performance liquid chromatography) analysis. Four new sulfates of 2,3,4-trihydroxybenzoic acid, catechol, 4-methylcatechol, and phloroglucinol were prepared and fully characterized using MS (mass spectrometry), ¹H, and ¹³C NMR. The separation was investigated using HPLC with PDA (photodiode-array) detection and a total of 38 standards of phenolics and their sulfates. Different stationary (monolithic C18, C18 Polar, pentafluorophenyl, ZICpHILIC) and mobile phases with or without ammonium acetate buffer were compared. The separation results were strongly dependent on the pH and buffer capacity of the mobile phase. The developed robust HPLC method is suitable for the separation of enzymatic sulfation reaction mixtures of flavonoids, flavonolignans, 2,3-dehydroflavonolignans, phenolic acids, and phenols with PDA detection. Moreover, the method is directly applicable in conjunction with mass detection due to the low flow rate and the absence of phosphate buffer and/or ion-pairing reagents in the mobile phase.