Enzymatic redox isomerization of 1,6-disaccharides by pyranose oxidase and NADH-dependent aldose reductase

Pyranose 2-oxidase, a homotetrameric FAD-flavoprotein from the basidiomycete Trametes multicolor, catalyzes regioselectively the oxidation of the 1→6 disaccharides allolactose β- -Galp-(1→6)- -Glc], gentiobiose β- -Glcp-(1→6)- -Glc], melibiose α- -Galp-(1→6)- -Glc], and isomaltose α- -Glcp-(1→6)- -Glc] at position C-2 of their reducing moiety. The resulting glycosyl -arabino-hexos-2-uloses can be reduced specifically at C-1 by NAD(P)H-dependent aldose reductase from the yeast Candida tenuis. By this novel, two-step redox isomerization process the four disaccharide substrates could be converted to the corresponding keto-disaccharides allolactulose β- -Galp-(1→6)- -Fru], gentiobiulose β- -Glcp-(1→6)- -Fru], melibiulose α- -Galp-(1→6)- -Fru], and isomaltulose (palatinose, α- -Glcp-(1→6)- -Fru]) in high yields. These products could find application in food technology as alternative sweeteners.