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Vitamin A alcohol (retinol) binds to a family of proteins called retinol-binding proteins in order to neutralize hydrophobicity of the retinol. Plasma retinol-binding protein (RBP), which is also called RBP4 in recent classification, is an extracellular binding protein for retinol, and cellular retinol-binding protein (CRBP) I and II are intracellular binding proteins for retinol. These retinol-binding proteins also affect transport, storage and metabolism of vitamin A through various mechanisms. For example, retinol being transported in blood escapes renal excretion by binding to RBP and transthyretin. Another example is that retinol efflux and influx are regulated by stimulated by retinoic acid 6 (STRA6), a membrane receptor for RBP. Thirdly, enzymatic activities of intracellular retinol-esterifying enzymes, lecithin:retinol acyltransferase and acyl-CoA:retinol acyltransferase are modulated differently by CRBPs. The aspects of these phenomena are described based on the conformational findings of RBP and CRBPs.

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Intestinal epithelial cells (IECs) have various functions, including absorption of food components, barrier function against xenobiotics, and immune function. In this study, we examined the absorption mechanism of functional food components in IECs. For example, alpha-lipoic acid, widely used as a functional food component or food supplement, is presumably transported through the intestinal epithelial-like Caco-2 monolayer via a proton-coupled medium-chain fatty acid transporter. Furthermore, among methoxyflavonoids which are contained in citrus fruits, nobiletin and tangeretin permeate across the Caco-2 monolayer by passive diffusion, while hesperetin permeates across the Caco-2 monolayer via a proton-coupled transporter. These results suggest that functional food components are absorbed via various pathways. On the other hand, IECs are most frequently exposed to food ingredients. Therefore, it is plausible that the activity of intestinal epithelial transporters is regulated by other food components. We focused on the sodium-dependent glucose transporter 1(SGLT1) and glucose transporter 5 (GLUT5), responsible for intestinal glucose and fructose uptake, respectively, and searched for phytochemicals which inhibit SGLT1 or GLUT5 activity. As a result, some phytochemicals such as epicatechin gallate, which are present in green tea, inhibited SGLT1 and GLUT5 activities. These phytochemicals are expected to be useful for prevention of metabolic syndrome caused by excessive carbohydrate intake.

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