| Tea, as a health beverage, posses many functional components, such as theanine, catechin and caffeine. Theanine (γ-glutamylethylamide) is an amino acid contained in tea leaves and accounts for approximately 50% of the total free amino acid content in tea. L-theanine which is the main component in tea provides the sweet taste, is known to possess many pharmacological functions, such as assuage, neuroprotection function, enhancing the antitumor activity of doxorubicin on primary tumors and enhanced its metastasis-suppressive effects. Caffeine is the most abundant alkaloid in tea. It accounts for 2%-4% of the whole contents and has functions such as antidepressant and cardiac.However, few studies have investigated on theanine and caffeine absorption across the human intestinal epithelium. To effectively exploit the bioactivities of theanine, it is important to clarify the absorption characteristics of theanine and the differences in intestinal transport between pure theanine and theanine contained in tea, which is the major source of theanine. This study was performed by using Caco-2 cell model which is a well differentiated cell line derived from human colorectal adenocarcinoma to investigated the intestinal transport and metabolism of theanine. Since the species differences deprivation, it will provide better theoretical evidence for the physiological utilization of theanine.In this paper, we investigated the absorption of theanine, caffeine and some other amino acid in the human intestinal epithelium using a Caco-2 monolayer model. Different concentrations of pure theanine and green tea extracts were administered to Caco-2 cells. The amino acid content was analyzed by high-performance liquid chromatography coupled with fluorescent detection.In uptake experiment, the cellular accumulation of theanine was quantified by incubating Caco-2 cell monolayers with 0.25 to 2 mmol/L of pure theanine for 15 min. The intracellular concentration of theanine was increased linearly with increasing extracellular concentrations from 0.25 to 2 mmol/L. This suggests that the absorption of theanine in Caco-2 cells monolayer occurs via passive diffusion.In bi-direction experiment, Papp, efflux ratio and permeations of theanine were investigated. We found that The Papp values for theanine, irrespective of the direction of transport, exceeded 1×10-7 cm/s, indicating that theanine is well absorbed by Caco-2 cells, the efflux ratio increased in a dose-dependent manner, with a marked increase at 4 mmol/L due to the tremendous increase of BL-AP value, reflecting excretion into the intestinal lumen in vivo and therefore implying that high concentrations of theanine cannot be absorbed via the intestine. Surprisingly, pure theanine showed good absorption; by contrast, theanine in green tea extract was poorly absorbed in the Caco-2 cell model. The transport of theanine in green tea extract in the basolateral (BL) to apical (AP) direction was much greater than that in the AP to BL direction suggesting that green tea components profoundly affect the trans-epithelial transport of theanine in this Caco-2 cell model. We speculate that one of the components causing this phenomenon might be glutamine because the intestinal absorption of theanine and glutamine is mediated by a common Na+-coupled co-transporter in the brush-border membrane and the affinity of this co-transport is much lower for theanine than for glutamine.We also researched the transport of caffeine and other amino acid. The results showed that, the absorption of caffeine in Caco-2 cells monolayer occurs via passive diffusion. In the green tea extract treatment, the accumulation of most amino acid in BL-AP direction was much higher than in AP-BL. Moreover, it was also much higher than the pure theanine treatment. |
