Enzymatic synthesis of phenolic acid glucosyl esters to test activities on cholangiocarcinoma cells

While glycoside hydrolase family 1 (GH1) enzymes mostly catalyze hydrolysis reactions, rice Os9BGlu31 preferentially catalyzes transglycosylation to transfer a glucosyl moiety to another aglycone moiety to form a new glycosylated compound through a retaining mechanism. In this study, Os9BGlu31 was used to synthesize eight phenolic acid glucosyl esters, which were evaluated for activities in cholangiocarcinoma cells. The transglycosylation products of Os9BGlu31 wild type and its mutant variants were detected, produced on a milligram scale, and purified, and their structures were characterized by NMR spectroscopy. The transglycosylation products were evaluated by antioxidant and anti-proliferative assays, followed by an anti-migration assay for the selected phenolic acid glucosyl ester. Os9BGlu31 mutants produced higher yield and activity than wild-type enzymes on phenolic acids to produce phenolic acid glucosyl esters. Among these, gallic acid glucosyl ester (β-glucogallin) had the highest antioxidant activity and anti-proliferative activity in cholangiocarcinoma cells. It also inhibited the migration of cholangiocarcinoma cells. Our study demonstrated that rice Os9BGlu31 transglucosidase is a promising enzyme for glycosylation of bioactive compounds in one-step reactions and provides evidence that β-glucogallin inhibits cell proliferation and migration of cholangiocarcinoma cells. KEY POINTS: • Os9BGlu31 transglucosidases produced phenolic acid glucosyl esters for bioactivity testing. • Phenolic acid glucosyl esters were tested for cytotoxicity in cholangiocarcinoma cells. • β-Glucogallin displayed the highest inhibition of cholangiocarcinoma cell growth.

Antiglycation activity of β-glucogallin from Asparagus racemosus

The increased formation and accumulation of advanced glycation end products (AGEs) has been implicated in pathogenesis of various chronic ailments, including diabetes-associated secondary complications, atherosclerosis, aging, inflammatory and neurodegenerative diseases. Therefore, inhibition of AGEs formation is an imperative strategy for alleviating diverse pathologies. Here, we have demonstrated the AGEs inhibitory activity of β-glucogallin, isolated for the first time from the roots of Asparagus racemosus. β-glucogallin significantly mitigated fructose-, glucose- and methylglyoxal-induced glycation of bovine serum albumin (BSA). Also, the presence of β-glucogallin decreased fructosamine and protein carbonyls content, and increased thiol group content in the fructose-BSA system. These activities of β-glucogallin from Asparagus racemosus underscore its likely pharmacological potential for impeding AGEs-related metabolic disorders.

Standardized Emblica officinalis fruit extract inhibited the activities of α-amylase, α-glucosidase, and dipeptidyl peptidase-4 and displayed antioxidant potential

BACKGROUND

Emblica officinalis, known as amla in Ayurveda, has been used as a folk medicine to treat numerous pathological conditions, including diabetes. However, the novel extract of E. officinalis fruit extract (amla fruit extract, AFE, Saberry®) containing 100 g kg^-1 β-glucogallin along with hydrolyzable tannins has not yet been extensively studied for its antidiabetic potential.

OBJECTIVE

The aim of this study was to investigate the antidiabetic and antioxidant activities of AFE and its stability during gastric stress as well as its thermostability.

METHODS

The effect of AFE on the inhibition of pancreatic α-amylase and salivary α-amylase enzymes was studied using starch and yeast α-glucosidase enzyme using 4-nitrophenyl α-d-glucopyranoside as substrate. Further, 2,2-diphenyl-1-picrylhydrazyl radical scavenging and reactive oxygen species inhibition assay was performed against AFE.

RESULTS

AFE potently inhibited the activities of α-amylase and α-glucosidase in a concentration-dependent manner with half maximal inhibitory concentration (IC₅₀ ) values of 135.70 μg mL^-1 and 106.70 μg mL^-1 respectively. Furthermore, it also showed inhibition of α-glucosidase (IC₅₀ 562.9 μg mL^-1 ) and dipeptidyl peptidase-4 (DPP-4; IC₅₀ 3770 μg mL^-1 ) enzyme activities. AFE is a potent antioxidant showing a free radical scavenging activity (IC₅₀ 2.37 μg mL^-1 ) and protecting against cellular reactive oxygen species (IC₅₀ 1.77 μg mL^-1 ), and the effects elicited could be attributed to its phytoconstituents.

CONCLUSION

AFE showed significant gastric acid resistance and was also found to be thermostable against wet heat. Excellent α-amylase, α-glucosidase, and DPP-4 inhibitory activities of AFE, as well as antioxidant activities, strongly recommend its use for the management of type 2 diabetes mellitus. © 2019 The Authors. Journal of The Science of Food and Agriculture published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Formation of β-glucogallin, the precursor of ellagic acid in strawberry and raspberry

Ellagic acid/ellagitannins are plant polyphenolic antioxidants that are synthesized from gallic acid and have been associated with a reduced risk of cancer and cardiovascular diseases. Here, we report the identification and characterization of five glycosyltransferases (GTs) from two genera of the Rosaceae family (Fragaria and Rubus; F. × ananassa FaGT2*, FaGT2, FaGT5, F. vesca FvGT2, and R. idaeus RiGT2) that catalyze the formation of 1-O-galloyl-β-D-glucopyranose (β-glucogallin) the precursor of ellagitannin biosynthesis. The enzymes showed substrate promiscuity as they formed glucose esters of a variety of (hydroxyl)benzoic and (hydroxyl)cinnamic acids. Determination of kinetic values and site-directed mutagenesis revealed amino acids that affected substrate preference and catalytic activity. Green immature strawberry fruits were identified as the main source of gallic acid, β-glucogallin, and ellagic acid in accordance with the highest GT2 gene expression levels. Injection of isotopically labeled gallic acid into green fruits of stable transgenic antisense FaGT2 strawberry plants clearly confirmed the in planta function. Our results indicate that GT2 enzymes might contribute to the production of ellagic acid/ellagitannins in strawberry and raspberry, and are useful to develop strawberry fruit with additional health benefits and for the biotechnological production of bioactive polyphenols.

Aldose reductase inhibition prevents endotoxin-induced inflammatory responses in retinal microglia

PURPOSE

Retinal microglia become activated in diabetes and produce pro-inflammatory molecules associated with changes in retinal vasculature and increased apoptosis of retinal neurons and glial cells. We sought to determine if the action of aldose reductase (AR), an enzyme linked to the pathogenesis of diabetic retinopathy, contributes to activation of microglial cells.

METHODS

Involvement of AR in the activation process was studied using primary cultures of retinal microglia (RMG) isolated from wild-type and AR-null mice, or in mouse macrophage cultures treated with either AR inhibitors or small interfering RNA (siRNA) directed to AR. Inflammatory cytokines were measured by ELISA. Cell migration was measured using a transwell assay. Gelatin zymography was used to detect active matrix metalloproteinase (MMP)-9, while RMG-induced apoptosis of adult retinal pigment epithelium (ARPE-19) cells was studied in a cell coculture system.

RESULTS

Aldose reductase inhibition or genetic deficiency substantially reduced lipopolysacharide (LPS)-induced cytokine secretion from macrophages and RMG. Aldose reductase inhibition or deficiency also reduced the activation of MMP-9 and attenuated LPS-induced cell migration. Additionally, blockade of AR by sorbinil or through genetic means caused a reduction in the ability of activated RMG to induce apoptosis of ARPE-19 cells.

CONCLUSIONS

These results demonstrate that the action of AR contributes to the activation of RMG. Inhibition of AR may be a therapeutic strategy to reduce inflammation associated with activation of RMG in disease.