The effects of phenolic compounds on Na+-dependent D-glucose transport were investigated in brush border membrane vesicles isolated from rat small intestine. Screening experiments were conducted with different classes of phenolic compounds in both their native and oxidized forms. Pretreatment of vesicles with tannic acid (1 mg/ml) completely abolished the characteristic overshoot of active glucose accumulation. With chlorogenic acid (1mM), 80% of the glucose transport capacity was lost. Reductions of 30-40% were observed in vesicles treated with catechin, ferulic or caffeic acids. Treatment with gallic acid (1 mM) had little effect. Phenolic oxidation state did not exacerbate the degree of glucose transport inhibition, with the exception of catechol (1 mM), which gave maximal inhibition (86%) in its oxidized form. Gradient-independent glucose uptake was not altered, nor did phenolic treatment increase nonspecific binding of glucose to the membrane vesicles. Possible mechanisms of D-glucose transport inhibition were examined in chlorogenic acid-and tannic acid-treated vesicles. Factors such as alterations in vesicle permeability, size and leakage of transported glucose out of the vesicles were ruled out. Measurements of D-glucose uptake under conditions of Na+ equilibrium suggest that tannic and chlorogenic acids reduce glucose uptake by favoring the dissipation of the Na+ electrochemical gradient, which provides the driving force for active glucose accumulation.

The effects of dietary phenolic compounds on intestinal sucrase were investigated in brush border membrane vesicles purified from rat small intestine. Screening experiments were conducted with different classes of phenolic compounds in both oxidized and native forms. The most potent inhibitor was native tannic acid at 0.1 mg/ml, resulting in an 80% loss of activity. Oxidized tannic acid had no effect. Significant decreases were also observed in vesicles treated with 0.1 mg/ml of catechol or epicatechin, yielding activity losses of 30-50%, regardless of oxidation state. With gallic acid, maximal (40%) inhibition occurred only in the oxidized form. Other phenolic compounds, such as ferulic, p-coumaric and caffeic acids, tended to be slightly inhibitory, while no inhibition was observed with vanillin or chlorogenic acid at the concentrations tested. These results confirm the enzyme inhibitory action of tannic acid, a polyphenolic compound, and also demonstrate that some individual dietary phenolic monomers have the potential to modulate enzyme activity in a brush border membrane vesicle model system.