TANNIC-ACID INHIBITS INSULIN-STIMULATED LIPOGENESIS IN RAT ADIPOSE-TISSUE AND INSULIN-RECEPTOR FUNCTION IN-VITRO

Tannins occur naturally in relatively abundant amounts in fruits, herb al medicines and common beverages. Thus an understanding of how these polyphenols affect peptide hormone action is of importance. We report here that tannic acid (a hydrolysable tannin) inhibits insulin-stimula ted lipogenesis in rat adipose tissue in vitro, with an IC50 estimated to be about 350 mu M. However, its monomer, gallic acid, did not show a similar inhibitory effect at concentrations up to 1 mM. The inhibit ion by tannic acid was less evident with higher concentrations of bovi ne serum albumin in the incubation buffer. This was attributed to the formation of a tannin-protein complex between bovine serum albumin and tannic acid. In a binding assay, it was observed that the specific bi nding of insulin to its receptor was not inhibited by tannic acid in t he concentration range 0-200 mu M. However, insulin-stimulated autopho sphorylation of the insulin receptor, and receptor-associated tyrosine kinase phosphorylation of RR-SRC peptide, were;inhibited by tannic ac id at concentrations as low as 25 mu M. Our data do not support the cu rrent speculation that tannins affect the activity of peptide hormones by binding to them. Therefore, our finding opens up a new perspective in the understanding of the mode of action of tannins on such hormone s.