Figure 5.

RP HPLC analysis of IDE-dependent degradation of insulin in the absence and presence of polyphenols at 37 °C. (a) Chromatograms of insulin digests in the absence of polyphenols (control samples) show the progressive loss of the insulin peak over the digestion period. This loss is accompanied by new peaks at shorter retention times due to the formation of insulin fragments. The chromatogram recorded for the 24-hr digest shows the absence of the insulin peak, indicating complete degradation of insulin. (b) Chromatograms of insulin digests in the presence of resveratrol are similar to the chromatograms in (a) with the exception of the peak for the polyphenol at a retention time of 7.5 min. Digestion of insulin is complete within 24 h. (c) Chromatograms of insulin digests in the presence of EGCG show that the digestion of insulin was not complete, even after 48 h of digestion. The intensity of the insulin peak, however, decreased with digestion time, suggesting that the inhibition by EGCG was partial and not complete. The intensity of the peak for EGCG decreased with incubation time presumably because of epimerization and/or dimerization.