Melanoidins are brownish heterogeneous high-molecular-weight compounds formed at the final stage of the Maillard reaction. Louis C. Maillard found that brownish polymer was formed in a reaction solution containing amino acids and reducing sugars and called those pigments melanoidins, which resulted that melanoidins have become known as the Maillard reaction end products. Melanoidins are formed during the processing and storage of various kinds of food, causing a significant influence on the quality of foods. For instance, melanoidins cause some changes in not only the color of foods but also odor and texture and show antioxidative activities and dietary fiber-like properties. The formation mechanism and the chemical structure of melanoidins are not understood fully, because of complexity and heterogeneity. However, they are being comprehended gradually through the great efforts by many scientists. Melanoidins are formed by the polymerization of a variety of reactants and intermediate products of the Maillard reaction and by cross-linking between proteins through low-molecular-weight pigments. Therefore, analyses of low-molecular-weight pigments formed by the Maillard reaction may result in the clarification of melanoidins in more detail.

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Gymnemic acid II (GA II) is a saponin of triterpene glycoside isolated from the plant Gymnema sylvestre. It suppresses taste sensitivity to sweetness and inhibits intestinal glucose absorption. We found GA II inhibited glycerol-3-phosphate dehydrogenase (G3PDH) activity noncompetitively with a substrate, dihydroxyacetone phosphate, and induced band smearing of G3PDH in SDS-PAGE in a time-dependent manner; band smearing was observed but decreased when G3PDH was incubated with GA II for 1.5-2 h compared with smearing after incubation with GA II for 4 h. NADH was more effective than NAD for decreasing the smearing of the G3PDH band, suggesting that GA II distinguishes between NADH- and NAD-bound forms of G3PDH. The smearing of the G3PDH band was diminished by prior incubation of GA II with gamma-cyclodextrin, but the suppressing effect of gamma-cyclodextrin was decreased when G3PDH was incubated with GA II for 0.5 h before adding gamma-cyclodextrin. These results suggest that the induction of the band smearing of G3PDH by GA II proceeded in two steps. GA II bound G3PDH and inhibited G3PDH activity but did not induce band smearing in SDS-PAGE during the first stage. Then, GA II induced a change in G3PDH that caused band smearing during the second stage, and the suppressing effect of gamma-cyclodextrin on band smearing was decreased at this stage. Analysis using in-gel digestion coupled with mass spectrometry suggested that proteolysis at the amino acid A238-F251 region of G3PDH was affected by treatment with GA II.

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