Theanine, L-N-ethylglutamine, is one of the major components of amino acids in Japanese green tea. To characterize the mode for intestinal absorption of theanine, the ionic dependency and kinetic properties of the theanine- and glutamine-evoked transmural electrical potential difference changes (ΔPD) were investigated in vitro by using everted sacs prepared from the guinea pig ileum. Both theanine and glutamine applied to the luminal side induced dose-dependent increases in ΔPD (increase in serosal positive value). The theanine- and glutamine-evoked ΔPD values conformed to the Michaelis-Menten relationship, with ΔPD_max not being different, whereas the half-saturation concentration was lower for glutamine (3.1±0.2 mM) than for theanine (21.4±0.6 mM). The theanine-evoked ΔPD value was much smaller when theanine was applied in the presence of glutamine than when applied alone. The theanine- and glutamine-evoked ΔPD values were both inhibited by removing Na⁺ from the luminal solution. These results suggest that the intestinal absorption of theanine and glutamine is mediated by a common Na⁺ -coupled co-transporter in the brush-border membrane, the affinity of which is lower for theanine than for glutamine.

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