Inhibitors in tea of intestinal absorption of phenylalanine in rats

This work studies the effect of tea extract on the mucosal and serosal transport of phenylalanine, and attempts to identify the active ingredient(s) therein by studying the effect of known tea constituents like theophylline, caffeine and tannic acid. Tea and all the constituents tested inhibited the mucosal uptake of phenylalanine. The serosal transport was unaffected by caffeine and tannic acid, but inhibited by theophylline and high concentrations of tea. The in vitro activity of the intestinal Na⁺-K⁺ ATPase was also assayed from a jejunal homogenate in presence of theophylline, caffeine, tannic acid and cAMP. All were found to inhibit significantly the enzyme. The in vitro activity of a purified Na⁺-K⁺ ATPase was however stimulated by theophylline and caffeine, and inhibited only by tannic acid. It was concluded that the inhibitory effect of tea is exerted mainly through its constituents which inhibit the Na⁺-K⁺ pump directly (tannic acid) or indirectly (theophylline and caffeine), possibly by elevating cAMP levels, dissipating thus the sodium gradient needed for the mucosal uptake of the amino acid.