Inhibition of protein glycation by skins and seeds of the muscadine grape

The formation of advanced glycation end products (AGEs) leading to protein glycation and cross-linking is associated with the pathogenesis of diabetic complications. The inhibition of protein glycation by phenolic and flavonoid antioxidants demonstrates that the process is mediated, in part, by oxidative processes. In this study, the effects of seed and skin extracts of the muscadine grape on AGEs formation were examined. Seeds and skins were extracted (10% w/v) with 50% ethanol and incubated at 37 degrees C with a solution containing 250 mM fructose and 10 mg/ml albumin. After 72 h, fluorescence was measured at the wavelength pair of 370 and 440 nm as an index of the formation of AGEs. Both seed and skin extracts were found to be efficacious inhibitors of AGE formation. A 1:300 dilution of the seed extract decreased fluorescence by approximately 65%, whereas muscadine grape skin extract produced a 40% lowering. This difference correlates with the greater antioxidant activity found in muscadine seeds in comparison to skins, however, on a mass basis, the inhibitory activities of the seeds and skins were found to be nearly equivalent. Gallic acid, catechin and epicatechin, the three major polyphenols in the seeds, all significantly decreased the AGE product related fluorescence at a concentration of 50 microM. Neither muscadine seed extract nor skin extract inhibited the methylglyoxal-mediated glycation of albumin. These results suggest that consumption of the muscadine grape may have some benefit in altering the progression of diabetic complications.