The Maillard reaction is traditionally subdivided into three stages that start consecutively and run in parallel. Here, we show that N-ε-carboxymethyllysine (CML), a compound formed in the late stage of the reaction, can undergo a second glycation event at its secondary amino group leading to a new class of Amadori rearrangement products. When N-α-hippuryl-CML was incubated in the presence of reducing sugars such as glucose, galactose, ribose, xylose, maltose, or lactose in solution for 1 h at 75 °C, the compound was degraded by 6–21%, and N-ε-carboxymethyl-N-ε-deoxyketosyl lysine derivatives were formed. Under the same conditions, lysine was 5–10 times more reactive than CML. N-α-hippuryl-N-ε-carboxymethyl-N-ε-(1-deoxyfructosyl)-l-lysine (hippuryl-CMFL) and N-ε-carboxymethyl-N-ε-(1-deoxyfructosyl)-l-lysine (CMFL) were synthesized, isolated and characterized by MS/MS and NMR experiments. Depending on the reaction conditions, up to 21% of CMFL can be converted to the furosine analogue N-ε-carboxymethyl-N-ε-furoylmethyl-l-lysine (CM-Fur) during standard acid protein hydrolysis with hydrochloric acid. Incubation of bovine serum albumin (BSA) with glucose for up to 9 weeks at 37 °C revealed the formation of CMFL in the protein as assessed by HPLC–MS/MS in the MRM mode. Under these conditions, ca. 13% of lysine residues had been converted to fructosyllysine, and 0.03% had been converted to CMFL. The detection of glycation products of glycated amino acids (heterogeneous multiple glycation) reveals a novel pathway in the Maillard reaction.
The spurge hawkmoth Hyles euphorbiae (Lepidoptera, Sphingidae) is an insect herbivore feeding exclusively on spurges (Euphorbia). These are toxic due to their secondary metabolites including diterpene esters with numerous harmful biological activities, preventing their use by most herbivores. Extracts of frass from H. euphorbiae and of its main larval food plant in Central Europe, the cypress spurge (Euphorbia cyparissias L.), were analyzed via liquid chromatography using a triple-quadrupole (MS/MS) and a high-resolution–quadrupole time-of-flight mass spectrometer (QTOF-MS). We report for the first time the finding of the tigliane (phorbol ester) phorbol-13-acetate and two 12-O-tetradecanoylphorbol-13-acetate (TPA) isomers of unknown structure (compounds 2 and 3) in E. cyparissias. Our data support the assignment of these two compounds as tiglianes and are therefore referred to by us as “putative tiglianes”. Additionally, the distribution of the three reported compounds within the plant parts (leaves, stem and root) was analyzed to gain first insights into the metabolite conditions H. euphorbiae copes with in nature. Phorbol-13-acetate was detected in the plant’s root only, while the other compounds were observed in all three plant parts, although in different distributions. Moreover, our results indicate that the occurrence of compound 3 rises upon larval feeding. Finally, the presence of the plant-derived putative tiglianes in frass from E. cyparissias leaves feeding larvae is confirmed, corroborating the close plant–herbivore interaction.
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