Effects of C-Ring Structural Differences on the Inhibition of Nε-(Carboxyethyl)lysine in the Methylglyoxal-Lysine System by Flavonoids

Abstract

This study investigated the effects of taxifolin (Tax), quercetin (Que), (+)-catechin (Cat) and luteolin (Lute) on the advanced Maillard reaction stage in the methylglyoxal-lysine (MGO-Lys) system. Since the four flavonoids share identical A- and B-ring structures, the inhibitory effects and molecular mechanisms of flavonoids with different C-ring structures on N^ε-(carboxyethyl)lysine (CEL) formation were revealed. The results demonstrated that Cat exhibited the best inhibitory effect on CEL with an inhibition rate of 53.78%, while Lute showed the lowest inhibition rate of 3.97%. The flavonoids (i.e., Tax, Que, Cat and Lute) inhibited the formation of non-fluorescent CEL, where hydroxylation at C³ on the C-ring favored the enhancement of the inhibitory effect of the flavonoids on CEL, while the C²-C³ double bond and the carbonyl group at the C⁴ position reduced their inhibitory ability. The alkaline environment favored the enhancement of the inhibition of CEL by Tax, Que, Cat and Lute. Notably, Tax, Que, Cat and Lute can inhibit CEL formation by competitively capturing MGO to form mono- or di-adducts and reducing lysine consumption. This study provides innovative strategies and a theoretical foundation for developing effective CEL inhibitors in food thermal processing.

Keywords: C-ring structures; Nε-(carboxyethyl)lysine (CEL); flavonoids; trapping methylglyoxal.

Figure 1

Structure of Tax, Que, Cat and Lute. The numbers represent the atomic sites of the flavonoids.

Figure 2

Inhibition rate of CEL in Lys-MGO simulation system by different concentrations of Tax, Que, Cat and Lute at pH 7.4. Different letters on the histograms represent significant differences (p < 0.05).

Figure 3

Effect of pH on the inhibition of CEL formation by Tax, Que, Cat and Lute. Capital letters represent significant differences between different samples at the same pH, while lowercase letters represent significant differences between different pH values of the same sample.

Figure 4

Effect of different concentrations of Tax, Que, Cat and Lute on the MGO content in the Lys-MGO system at pH 7.4. Capital letters represent significant differences between different samples at the same pH, while lowercase letters represent significant differences between different pH values of the same sample.

Figure 5

Effect of Tax, Que, Cat and Lute on Lysine content in the Lys-MGO reaction system at pH 7.4. (a, b, c and d indicate statistical significance).

Figure 6

Capture effect of MGO by Tax, Que, Cat and Lute at different concentrations at 37 °C (a); capture effect of MGO by Tax, Que, Cat and Lute at different times, respectively (b). Capital letters indicate significant differences between different samples at the same pH, while lowercase letters indicate significant differences between different pH values of the same sample.

Figure 7

TIC chromatogram (a) and MSⁿ (n = 1–2) spectra and MS² fragmentation of mono-Tax-MGO (b) and di-Tax-MGO (c) after 1 h of reaction of Tax and MGO.

Figure 8

TIC chromatogram (a) and MSⁿ (n = 1–2) spectra and MS² fragmentation of mono-Que-MGO (b), di-Que-MGO (c) and tri-Que-MGO (d) after 1 h of reaction of Que and MGO.

Figure 9

TIC chromatogram (a) and MSⁿ (n = 1–2) spectra and MS² fragmentation of mono-Cat-MGO (b) and di-Cat-MGO (c) after 1 h of reaction of Cat and MGO.

Figure 10

TIC chromatogram (a) and MSⁿ (n = 1–2) spectra and MS² fragmentation of mono-Lute-MGO (b) and di-Lute-MGO (c) after 1 h of reaction of Lute and MGO.