Conversion of lactose to β-d-galactopyranosyl-(1 → 4)-d-arabino-hexos-2-ulose-(2-dehydrolactose) and lactobiono-1,5-lactone by fungal pyranose dehydrogenase

The quinone-dependent pyranose dehydrogenase (PDH) purified from culture media of the basidiomycete fungus Agaricus xanthoderma catalyzed the simultaneous oxidation of lactose to β-d-galactopyranosyl-(1 → 4)-d-arabino-hexos-2-ulose (2-dehydrolactose) and lactobiono-1,5-lactone, with the latter spontaneously hydrolyzing to lactobionic acid. These products were identified by MS and in situ NMR spectroscopy. C-2 oxidation at the reducing moiety of lactose was confirmed by analysis of N,N-diphenylhydrazone derivatives of the reaction products. The proportions of C-1 to C-2 oxidation varied according to the source of PDH, ranging from almost exclusive formation of lactobionic acid to 2-dehydrolactose as a major product. The highest selectivity for C-2 was exhibited by PDH from A. xanthoderma and A. meleagris. Yields of 2-dehydrolactose in laboratory transformations monitored by HPLC approached 75%. This dicarbonyl derivative may serve as a key intermediate in a potential process for adding value to lactose by its (chemo)enzymatic isomerization to lactulose. A minor product in the PDH–lactose reaction mixtures formed on prolonged incubations with excess of oxidant was identified through its hydrazone derivative as β-d-galactopyranosyl-(1 → 4)-d-threo-hexos-2,3-diulose (2,3-didehydrolactose).