Inhibitory Effects of Foodstuffs on Sugar-Protein Condensation Reaction Related to Aging

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It is well known that crosslinking mediated by nonenzymatic glycosylation occur in human serum albumin, collagen and lens protein. The fluorescent substances formed in the reaction increase as a function of age and diabetes.Our interest was focused to know whether several tea leaf extracts show an inhibitory effect on the formation of the advanced glycosylation end products (AGEs). Furthermore, an attempt was made to identify compounds which inhibit AGEs formation.A mixture of D-glucose and human serum albumin or casein was incubated under physiological conditions with or without tea leaf extract. From the determination of relative fluorescence intensity, it was found that all tea leaf extracts (Japanese green tea, Chinese tea, black tea, Oolong tea) showed high inhibitory activity (10-60%) on AGEs formation. When 3-deoxy-D-glucosone or 5-hydroxymethy1-2-furaldehyde, which are key intermediates of the Maillard reaction, was used as carbonyl component, the inhibitory effects of tea leaf extracts were extremely low (5-15%). By applying solvent extraction method for isolation of the specific inhibitory compounds of AGEs formation, four catechins containing (-) epigallocatechin gallate were isolated and identified from Japanese green tea. SDS-polyacrylamide electrophoresis showed disappearance of a major high molecular weight protein aggregate in samples incubated with tea leaf extracts or catechins. From the chemical structure of catechins, it is believed that catechins can act as antioxidative agents or free radical scavengers at early stage of nonenzymatic glycosylation.