The trapping of methylglyoxal by phenolic acids : effect on antioxidant and antibacterial activity

Abstract

Methylglyoxal (MGO) is a reactive carbonyl species found in Manuka honey reported to cause advanced glycation end products (AGE) formation. AGE’s increase the risk for hyperglycaemia resulting in neuropathy, arteriosclerosis, retinopathy and Alzheimer’s disease. Phenolic acids such as pyrogallol (PY) are known to trap MGO, lessening the harmful effects of MGO as an AGE precursor. However, MGO is also a very effective antibacterial agent therefore; its trapping could have negative side effects. Manuka honey contains both phenolic acids such as gallic acid (GA), caffeic acid (CA) as well as MGO and it is unknown whether trapping of MGO by phenolic acids reduces the antioxidant activity of phenolic acids or the antibacterial activity of MGO. Phenolic acids PY, GA and CA were combined with MGO in a 1:1 and 1:2 ratio. The trapping of MGO with polyphenolic acids was determined with Liquid chromatography-mass spectrometry (LCMS). Total polyphenolic acids (TPC) was determined with the TPC assay. Antioxidant activity was determined with 2,2-diphenyl-2-picrylhydrazyl (DPPH), Trolox equivalent antioxidant capacity (TEAC) and Oxygen Radical Absorbance Capacity (ORAC) assays. The effect on cell number and viability was determined with crystal violet and 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assays on Caco-2 and SC-1 cells. Cellular antioxidant activity was determined with Dichlorodihydrofluorescein diacetate assay. Lastly, antibacterial activity was determined with the turbidity assay on Gram positive B. subtilis and Gram negative E. coli and the ultrastructural morphology of B. subtilis was further investigated with scanning electron microscopy. PY was the only phenolic acid used with trapping ability, forming mono- and di- adducts with MGO reported with the LCMS results, resulting in a decrease in TPC and antioxidant activity measured with the DPPH assay. GA did not show any alteration when combined with MGO at 1:1 and 1:2 ratio in all antioxidant content and activity assays. The antioxidant content of CA in combination with MGO was decreased, although its antioxidant activity (DPPH) was increased at 1:2 ratio. The antioxidant activity measured with the ORAC assay was increased with PY and CA combined with MGO. TEAC assay did not show any changes when phenolic acids were combined with MGO a 1:1 and 1:2 ratio. The cytotoxicity of phenolic acids combined with MGO did not cause a change in cell number or viability of SC-1 and Caco-2 cells. MGO and phenolic acids alone and in combination did no cause oxidative damage (without 2,2'-Azobis(2- amidinopropane) dihydrochloride (AAPH). All phenolic acids in combination with MGO retained the ability to reduce AAPH induced oxidative damage. The polyphenolic acids showed minor inhibition of the growth of B. subtilis and E. coli. PY only reduced the antibacterial activity of MGO at a 1:1 combination of B. subtilis. GA and CA did not alter the antibacterial activity of MGO when combined at 1:1 or 1:2 ratio. This study showed that phenolic acids with the ability to trap MGO can be altered by the mono- and di-MGO adduct formation, altering its antioxidant activity and can further alter the antibacterial activity of MGO.