Inhibition of activities of individual subunits of intestinal Maltase-Glucoamylase and Sucrase-Isomaltase by dietary phenolic compounds for modulating glucose release and gene response

The occurrence of Type 2 diabetes is on the increase all over the world. Since free glucose is released through digestion of starch in the human diet, control of the starch digesting enzymes, particularly the intestinal mucosal α-glucosidases [Maltase-Glucoamylase (MGAM) and Sucrase-Isomaltase (SI)], has a potentially important role in both the etiology and treatment of this metabolic disorder. Inhibition of the activities of the intestinal α-glucosidases may be a promising way to moderate glucose delivery to the body. Although some commercial inhibitors, such as acarbose, have strong effect on these enzymes, and essentially block starch digestion, there is a need for new candidate inhibitors found in regular diets that still deliver glucose to the body in a slower way and have fewer side effects. Phenolics are known to have inhibitory effect on the intestinal alpha-glucosidases. For more precise control of glucose release in the small intestine, the concept of selective inhibition of the individual subunits (C terminal, Ct; N terminal, Nt) of MGAM and SI has been proposed. In this thesis work, we found that some phenolics selectively inhibit the individual mammalian recombinant subunits of MGAM and SI. For instance, chlorogenic acid and (-)-epigallocatechin gallate (EGCG) selectively inhibited the most active starch digesting subunit, Ct-MGAM (also called glucoamylase). We additionally used rat intestinal acetone powder and human intestinal tissue to investigate the inhibitory effects of selected phenolics on α-glucosidases. Chlorogenic acid and EGCG showed the high inhibitory potency for maltase, sucrase and isomaltase activities of rat intestinal acetone powder. Also, chlorogenic acid notably inhibited the sucrase activity of human immunoprecipitated SI, while EGCG the maltase activity of human immunoprecipitated MGAM. Also explored, and for the first time, were the effects of some phenolic compounds on gene expression levels of MGAM and SI. The presence of phenolic compounds in mouse explants caused the generation of different molecular size forms of MGAM, but with no effects on overall maltase activity of the intestinal epithelium. Overall results show that there is a potential to change the rate of digestion of starches, starch products, and other saccharides like sucrose by phenolics present in the diet. Each tested polyphenol displayed a distinctive pattern of inhibition in the MGAM and SI subunits, as well as variations in the relative potency of the enzymes derived from human, rat or mouse species. These results show that dietary phenolic compounds cause differential or selective inhibition of the different intestinal α-glucosidase activities. Of relevance is the finding that some phenolic compounds modify patterns of protein forms expression. We speculate that different alternative spliced forms of MGAM are present to digest starch efficiently in the presence of polyphenolic inhibitors