Probing the active-site requirements of human intestinal N-terminal maltase glucoamylase: the effect of replacing the sulfate moiety by a methyl ether in ponkoranol, a naturally occurring α-glucosidase inhibitor.

Ponkoranol is a naturally occurring glucosidase inhibitor isolated from the plant Salacia reticulata. The compound comprises a sulfonium ion with an internal sulfate counter ion. We report here an efficient synthetic route to 3'-O-methyl ponkoranol to test the hypothesis that occupation of a hydrophobic pocket by a methyl group instead of the polar sulfate ion within the active site of human N-terminal maltase glucoamylase would be beneficial. The synthetic strategy relies on the nucleophilic attack of 2,3,5-tri-O-benzyl-1,4-anhydro-4-thio-D-arabinitol at the C-6 position of benzyl 6-O-p-toluenesulfonyl β-D-glucopyranoside, followed by deprotection using boron trichloride and reduction with sodium borohydride. The target compound inhibited the N-terminal catalytic domain of intestinal human maltase glucoamylase (ntMGAM) with a K(i) value of 0.50 ± 0.04 μM, higher than those of de-O-sulfonated ponkoranol (K(i)=43 ± 3 nM), or its 5'-stereoisomer (K(i)=15 ± 1 nM). We conclude that the interaction of the methyl group with hydrophobic residues in the active site is not as beneficial to inhibition of ntMGAM as the other interactions of the polyhydroxylated chain with active-site residues.