The effects of the trapping of methylglyoxal by flavonoids on antioxidant and antibacterial activity

Abstract

Methylglyoxal (MGO) is a highly reactive dicarbonyl compound, formed as a metabolite from nonenzymatic and enzymatic reactions and is the leading precursor of advanced glycation end products (AGEs). AGEs contribute to ageing, type 2 diabetes mellitus (T2DM), and diabetes-related complications. However, MGO also has beneficial antibacterial activity and is the bioactive ingredient of medicinal honeys such as Manuka. Flavonoids are a group of phytochemicals that are powerful antioxidants. Polyphenols including flavonoids have been reported to trap MGO, forming adducts thereby preventing AGE formation. However, there is little to no information on the effect of adduct formation on the antioxidant properties of flavonoids and the antibacterial activity of MGO. In this study, catechin (CAT), chrysin (CHRY) and naringenin (NAR) at 0.1 mM and mixtures of each flavonoid with MGO (1:1) and (1:2) were evaluated for antioxidant and antibacterial activity. Antioxidant activity/capacity were evaluated with the total polyphenolic content (TPC), total flavonoid content (TFC), Trolox equivalent antioxidant capacity (TEAC) and the oxygen radical absorbent capacity (ORAC) assays. The bovine serum albumin (BSA)/MGO model was used to evaluate the effect on glycation. The 2’, 7’-dichlorofluorescein diacetate (DCFH-DA) assay with the L929 cell line was used to evaluate cellular antioxidant activity. Cytotoxicity was determined in the L929 cell line using the crystal violet (CV) and the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) tetrazolium (MTT) assays. Escherichia coli (E. coli) and Bacillus subtilis (B. subtilis) were used to determine antibacterial activity using the microbroth-dilution assay and subsequent changes to morphology were evaluated using scanning electron microscopy (SEM). A reduction in antioxidant content was observed for: CHRY (TPC), CAT and NAR (TFC) and in antioxidant activity for: CHRY (TEAC) and CAT (ORAC), when combined with MGO. Overall most of the antioxidant activity of the flavonoids was not affected by the addition of MGO. In the presence of BSA and MGO, all flavonoid:MGO combinations reduced formation of AGEs except NAR in combination with MGO. All flavonoids alone and in combinations did not cause cellular oxidative damage while MGO and AAPH induced increased cellular damage indicating that MGO via AGE formation makes cells more sensitive to the effects of oxidants that form radicals. Only CAT reduced the oxidative effects of MGO/AAPH. For all combinations there was no effect on cell number, although cell viability was significantly reduced for CHRY and its combinations and for NAR and NAR:MGO1. Flavonoids at 0.1 mM CAT, CHRY and NAR had no antibacterial activity against E. coli while inhibition was observed only with NAR against B. subtilis. MGO at 0.1 and 0.2 mM inhibited bacterial growth while in combination the antibacterial activity was significantly reduced. MGO as well as NAR caused major changes to bacteria morphology. In combination, the antibacterial activity of MGO was reduced, and ultrastructure changes associated with toxicity was also observed in most groups. In conclusion, flavonoids do trap MGO and this effect does not significantly alter flavonoid antioxidant activity. However, the antibacterial activity of MGO is reduced. Future studies should focus on the chemistry and the effects involved and should include dosage dependent studies.