Journal
JOURNAL OF AGRICULTURAL AND FOOD CHEMISTRY
Volume 50, Issue 18, Pages 5215-5219Publisher
AMER CHEMICAL SOC
DOI: 10.1021/jf020252e
Keywords
catechin; Na+/glucose cotransporter; noncompetitive inhibition; polyphenol; Xenopus oocyte
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To study the effects, of polyphenols on the Na+/glucose cotransporter (SGLT1) response, SGLT1 was expressed in Xenopus oocytes by injecting cRNA synthesized from the cloned cDNA of the small intestine cotransporter of rats, and the electrical response elicited by glucose or galactose was measured by a voltage clamping method. Most phenol,derivatives had no effect on the response. However, the polyphenols (+)-catechin, (-)-epilcatechin gallate (ECg), and (-)-epigallocatechin gallate (EGCg), which are components of green tea, caused an inhibition of the response, which was almost independent of glucose concentration. The inhibition constants were estimated to be 2.3 mM for (+)-catechin and 0.45 mM for both ECg and EGCg, assuming the noncompetitive inhibition mechanism. Saponin prepared from tea seeds also inhibited the response significantly. Tannic acid and aqueous extracts of teas induced nonspecific electrical responses in both cRNA-injected and noninjected oocytes at lower concentrations than those that caused an inhibition of the SGLT1 response-when their dose-dependent effects were examined. These results are possibly helpful in the development of a dietary supplement for diabetic patients.
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