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Chemo-enzymatic synthesis of galactosylmaltooligosaccharidonolactone as a substrate analogue inhibitor for mammalian alpha-amylase.

We performed chemo-enzymatic transformation of maltooligosaccharides into both end-modified oligosaccharidonolactones of potential use as substrate analogue inhibitors for mammalian alpha-amylases. Enzymatic modification of the non-reducing end glucosyl residue of the maltooligosaccharide was first performed by transglycosylation with beta-D-galactosidase from Bacillus circulans. When maltotriose and maltotetraose were the acceptors, the enzyme regioselectively synthesized 4(3)-O-beta-D-galactosyl maltotriose (LG3) and 4(4)-O-beta-D-galactosyl maltotetraose (LG4) from lactose as a donor. LG4 was further selectively hydrolyzed with a specific alpha-amylase to afford 4(2)-O-beta-D-galactosyl maltose (LG2). The anomer hydroxyl groups of LG2 and LG3 were chemically oxidized to give the corresponding lactones, 4(2)-O-beta-D-galactosyl maltobionolactone (LG2O) and 4(3)-O-beta-D-galactosyl maltotrionolactone (LG3O), respectively. LG2O and LG3O, which are competitive inhibitors for mammalian alpha-amylases, exhibited Ki values of the order of 2.8-18.0 microM, with p-nitrophenyl alpha-maltopentaoside (G5P) as the substrate. On 1H-NMR analysis, these oligosaccharidonolactones were shown to be transformed into the corresponding aldonic acid forms with time in an aqueous solution. In this case, the lactone form was essential for the occurrence of the alpha-amylase inhibitor.