Add time:08/04/2019         Source:sciencedirect.com

The formation of 4-hydroxy-5-methyl-3(2H)-furanone (HMF, norfuraneol) by spinach ribosephosphate isomerase was reinvestigated [Arch. Biochem. Biophys. 202(1) (1980) 106]. Incubation experiments using d-ribose-5-phosphate and d-ribulose-5-phosphate clearly revealed a spontaneous nonenzymatic formation of the hydroxy-furanone from the ketose-phosphate under physiological conditions at 35 °C and pH 7.5, whereupon up to 1.3% of d-ribulose-5-phosphate was transformed to HMF within 15 h. 4,5-Dihydroxy-2,3-pentanedione was deduced as ultimate precursor of HMF, since addition of o-phenylenediamine to the incubation mixture led to lower amounts of HMF and to the formation of 3-(1,2-dihydroxyethyl)-2-methylquinoxaline, which was identified by means of high pressure liquid chromatography with diode array detection (HPLC–DAD), HPLC–electrospray ionization–tandem mass spectrometry (HPLC–ESI–MS/MS) and NMR spectroscopy. Additionally, the spontaneous formation of 4,5-dihydroxy-2,3-pentanedione was demontrated by its conversion to the respective alditol acetate using either NaBH4 or NaBD4 for the reduction. Comparative gas chromatography–mass spectrometry (GC–MS) analysis revealed the incorporation of two deuterium atoms and confirmed the dicarbonyl structure. Application of 1-13C-d-ribulose-5-phosphate as well as 5-13C-d-ribulose-5-phosphate and analysis of the derived quinoxaline derivatives by HPLC–ESI–MS/MS demonstrated the formation of the methyl-group at C-5 of the carbohydrate phosphate in consequence of a nonenzymatic phosphate elimination. Application of o-phenylenediamine into ripe tomatoes led to the detection of 3-(1,2-dihydroxyethyl)-2-methylquinoxaline by means of HPLC–MS/MS analysis implying the genuine occurrence of 4,5-dihydroxy-2,3-pentanedione in this fruit.

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