Assessing metal-induced glycation in French fries

Abstract

Non-enzymatic glycation is the chemical reaction between the amine group of an amino acid and the carbonyl group of a reducing sugar. The final products of this reaction, advanced glycation end-products (AGEs), are known to play a key role in aging and many chronic diseases. The kinetics of the AGE formation reaction depends on several factors, including pH, temperature, and the presence of prooxidant metals, such as iron and copper. In this study, the effect of iron and copper on the rate and outcome of non-enzymatic glycation was examined in the test tube and a food model, using chromatography and spectrometry methods. Binding efficiencies of several chelating agents to selected metals were also assessed. Phytic acid was the most efficient of the tested chelating agents. The effect of phytic acid on AGE formation in French fries was evaluated. While phytic acid treatment increased the amounts of UV-absorbing compounds in fries, a food ingredient rich in phytic acid showed the opposite effect. This study suggests that prooxidant metals can affect the rate, outcome, and yield of the non-enzymatic glycation reaction and that they do so differently when free or chelated. Moreover, despite being an excellent iron chelator, phytic acid can promote AGE formation in fried food potentially via mechanisms other than metal-induced glycation.

Keywords: French fries; chelation; iron; non-enzymatic glycation; phytic acid; rice bran.

Graphical Abstract

Figure 1.

(a) Non-enzymatic glycation in test tubes. D-Glucose and L-lysine were allowed to react in the presence of iron(II) under reflux conditions at 100°C and pH 7.4 for 4 h. Samples were taken for chemical analysis at various time points between 0 and 4 h. (b) Phytic acid-treated (left) and untreated (right) French fries were weighed and transferred to clean paper plates for sensory analysis. (c) The analytical workflow for sample preparation and chemical analysis of French fries.

Figure 2.

(a) Positive-mode ESI mass spectrum and LC-MS peak areas from the most abundant constituents in the non-enzymatic glycation in vitro reaction mixture. (b) Base peak chromatogram of the reaction mixture derivatized with N-benzoyloxy succinimide in negative-mode ESI LC-MS. (c) Effect of selected chelating agents on non-enzymatic glycation product formation in the presence of Fe²⁺ (10 min MW irradiation at 420 W energy output). Absorbance of the reaction mixture is presented with respect to the control/no metal sample. Error bars represent standard deviation. Asterisk and ns represent P < 0.05 and P > 0.05 (ANOVA), respectively. CML, carboxymethyl- lysine; CEL, carboxyethyl-lysine; R, derivatization fragment; RT, retention time; +, also found in positive-mode ESI-LC-MS; ?, unidentified compound. Compounds 2-amino-1,3-propanediol, aspartic acid, ε-fructoselysine, and (2E)-3-(2-furyl) acrylamide are tentative MS library matches only and have not been confirmed.

Figure 3.

(a) HPLC-DAD chromatograms from raw, air-fried untreated, air-fried treated with rice bran, and air-fried treated with phytic acid potato samples recorded at 295 nm. (b) Retention time distribution and total ppm concentration of UV-absorbing compounds in the four potato samples. IS peak (at 5.2 min) and poorly retained substances (retention time < 5.0 min) were omitted. Error bar represents standard deviation.

Figure 4.

(a) Venn diagrams demonstrating the number of LC-MS features altered in untreated, phytic acid-treated, and rice bran-treated French fries (compared to raw potato) in positive (left) and negative (right) ESI modes. Out of the 117 detected compounds, 25 increased >100% in untreated French fries compared to raw potato, 5 decreased to raw potato levels in rice bran- treated, and 1 decreased in phytic acid-treated fries. The latter compound (circled) was identified, in both positive-mode and negative-mode ESI, as fructosyl-phenylalanine, an early glycation product of phenylalanine. (b) Negative-mode ESI high-resolution MS/MS spectrum and tentative molecular structure of the compound decreased in phytic acid-treated French fries. NL, neutral loss.