Dietary polyphenols are thought to be beneficial for human health as antioxidants. Coffee beans contain a common polyphenol, chlorogenic acid. Chlorogenic acid is the ester of caffeic acid and quinic acid. Although these polyphenols have received much attention, there is little evidence indicating a relationship between the effect and the rate of absorption. In this study, we focused on the beneficial effects of chlorogenic acid and caffeic acid, a major metabolite of chlorogenic acid. We carried out in vitro and in vivo experiments. In the in vitro study, caffeic acid had stronger antioxidant activity than that of chlorogenic acid. The uptake of chlorogenic acid by Caco-2 cells was much less than that of caffeic acid. The physiological importance of an orally administered compound depends on its availability for intestinal absorption and subsequent interaction with target tissues. We then used an intestinal ischemia-reperfusion model to evaluate antioxidant activities in vivo. We found that both chlorogenic acid and caffeic acid had effects on intestinal ischemia-reperfusion injury. Since caffeic acid has a stronger antioxidant activity than that of chlorogenic acid and chlorogenic acid is hydrolyzed into caffeic acid in the intestine, it is possible that caffeic acid plays a major role in the protective effect of chlorogenic acid against ischemia-reperfusion injury.
We report in the present paper, on the isolation and functional characterization of slc5a12, the twelfth member of the SLC5 gene family, from mouse kidney. The slc5a12 cDNA codes for a protein of 619 amino acids. Heterologous expression of slc5a12 cDNA in mammalian cells induces Na+-dependent transport of lactate and nicotinate. Several other short-chain monocarboxylates compete with nicotinate for the cDNA-induced transport process. Expression of slc5a12 in Xenopus oocytes induces electrogenic and Na+-dependent transport of lactate, nicotinate, propionate and butyrate. The substrate specificity of slc5a12 is similar to that of slc5a8, an Na+-coupled transporter for monocarboxylates. However, the substrate affinities of slc5a12 were much lower than those of slc5a8. slc5a12 mRNA is expressed in kidney, small intestine and skeletal muscle. In situ hybridization with sagittal sections of mouse kidney showed predominant expression of slc5a12 in the outer cortex. This is in contrast with slc5a8, which is expressed in the cortex as well as in the medulla. The physiological function of slc5a12 in the kidney is likely to mediate the reabsorption of lactate. In the intestinal tract, slc5a12 is expressed in the proximal parts, whereas slc5a8 is expressed in the distal parts. The expression of slc5a12 in the proximal parts of the intestinal tract, where there is minimal bacterial colonization, suggests that the physiological function of slc5a12 is not to mediate the absorption of short-chain monocarboxylates derived from bacterial fermentation but rather to mediate the absorption of diet-derived short-chain monocarboxylates. Based on the functional and structural similarities between slc5a8 and slc5a12, we suggest that the two transporters be designated as SMCT1 (sodium-coupled monocarboxylate transporter 1) and SMCT2 respectively.
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