The glycation process causes various types of protein and chemical modifications, resulting in the generation of irreversible heterogeneous byproducts termed advanced glycation end products (AGEs). The accumulation of AGEs plays a primary role in the aging process as well as the pathogenesis of age-related disorders including Alzheimer’s disease and diabetic complications [25, 26]. In recent decades, over consumption of high-fructose diets has dramatically increased and has been linked to an increase in obesity and diabetic complications. Nowadays, there has been much concern regarding the important role of dietary fructose in the development of metabolic diseases. In the context of intracellular glycation, the rate for fructose is faster than that of glucose [27, 28]. Because of the faster rate of the reactive glycolytic intermediates in the formation of AGEs, fructose and its metabolites are believed to be important precursors in the intracellular formation of AGEs. In the early stage of glycation, unstable Schiff’s bases are formed and turned into Amadori products such as fructosamine, which is clinically used as an indicator for short term control of blood sugar in diabetic patients . The reduction of fructosamine, therefore, is a therapeutic way to delay incident vascular complications . In addition, the production of Nϵ-(carboxymethyl)lysine (CML) is one of the best characterized compounds of advanced glycation end products which are generated either from oxidative breakdown of Amadori products or polyol pathway mediated by α-oxoaldehydes such as glyoxal, methylglyoxal, and 3-deoxyglucosone . Aside from the formation of AGEs, reactive carbonyl intermediaries and protein carbonyl derivatives also cause protein modifications that are particularly prone to oxidative reaction to amino acid such a cysteine. The reactive oxygen species are generated during glycation and glyoxidation and are able to oxidize side chains of amino acid residues in protein to form carbonyl derivatives and also diminish an oxidative defense of protein by decreasing thiol groups [32, 33]. Thus, these phenomena are reflective of high oxidative stress, protein oxidative damage, and formation of AGEs, which is the direct reflection of excess of free radical generation.
Based on the fluorescence property, we studied the influence of RGSE on the formation of total AGEs. Our results demonstrated that RGSE efficiently inhibited AGEs formation. Furthermore, RGSE also reduced the level of fructosamine and the formation of Nϵ-(carboxymethyl)lysine (CML) associated with decreased formation of AGEs. Consequently, significant elevation of protein carbonyl content and oxidation of thiols in BSA were observed when the protein was glycated by fructose. In contrast, when RGSE was added to the same systems, it significantly suppressed these processes. Several biochemical mechanisms of anti-glycation reactions have recently been proposed . During the early stage of glycation, Schiff bases are prone to oxidation, generating free radicals, reactive carbonyl groups and the formed AGEs. Scavenging hydroxyl radicals and superoxide radicals can alleviate oxidative stress and reduce the generation of reactive carbonyl compounds . In addition, transition metal also catalyzes auto-oxidation of glucose and further generates reactive carbonyl compounds to form AGEs. Thus, metal chelators may retard the process of AGEs by preventing further oxidation of Amadori products and metal-catalyzed glucose oxidation .
It has been reported that many antioxidant-containing foods can scavenge free-radicals generated during the glycation process as well as prevent reducing sugars and Amadori products from self-oxidation, leading to the inhibition of AGE formation . In the present study, various methods of accessing antioxidant capacities have been used for RGSE. DPPH is widely used to evaluate the free radical scavenging abilities of phytochemical compounds in vitro. FRAP assay has been used extensively to evaluate the ability of edible plants to reduce ferric ions, reflecting their ability to decrease reactive oxygen species (ROS) . TEAC assay has been used for assessing the capacity of edible plants to scavenge ABTS radicals . The HRSA and SRSA have been applied to investigate the abilities of antioxidants to scavenge hydroxyl and superoxide radicals . From the results obtained in the present study, RGSE showed potent antioxidant properties. According to the abovementioned antiglycation mechanisms, RGSE may inhibit AGE formation by decreasing the ROS formation or by scavenging the ROS formed in vitro by auto-oxidation of sugars and/or oxidative degradation of Amadori products. However, the antioxidant activity of RGSE might not be the only reason for explanation of the mechanism of antiglycation. Other mechanisms of antiglycation have been proposed, such as breaking the cross-linking structures in the formed AGEs and inhibiting the formation of late-stage Amadori products. Further comprehensive studies of RGSE are required to evaluate the antiglycation mechanisms described above.
Phenolic compounds, flavonoids and anthocyanins are constituents of many edible plants, and they are of current research interest because of their health-promoting effect as antioxidants. The phytochemical analysis of red grape skin reveals the presence of phenolic compounds such as proanthocyanidins, ellagic acid, myricetin, quercetin, rutin, kaempferol, trans-resveratrol, cyanidin-3-glucoside, delphinidin-3-O-glucoside (myrtillin), petunidin-3-O-glucoside, peonidin-3-O-glucoside, malvidin-3-O-glucoside [39, 40]. Furthermore, grape seeds are a rich source of polyphenols, which are characterized by a variety of properties, such as antibacterial and antioxidant activities [41, 42]. Significant variations in the levels of total phenolic compounds, flavonoids, and anthocyanins in skins and seeds from different varieties of red grapes have previously been reported [41–43]. Among 21 different cultivars, there are 45 anthocyanins, 28 flavonols, 8 flavan-3-ols, 9 cinnamic acids, 5 benzoic acids, 5 ellagic acids and 2 stilbenes detected in all grape skins .
The phenolic compounds, flavonoids and anthocyanins exhibit considerable free radical scavenging activities and metal ion chelating properties [44–46]. Most antiglycation agents from the edible plants have been reported to possess phenolic compounds and flavonoids [47–49]. It has been reported that inhibitory activity of flavonoids against protein glycation was strongly related to their scavenging effect on free radicals derived from the glycoxidation process . Our findings indicate that RGSE has high phenolic compounds, flavonoids and anthocyanin content. According to the abovementioned studies, it can be assumed that phenolic compounds and flavonoids in the RGSE may contribute to the antioxidant activity and antiglycation.