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

众所周知，在人血清白蛋白、胶原和透镜蛋白中发生由非酶糖基化介导的交联。反应中形成的荧光物质随着年龄和糖尿病的增加而增加，我们的兴趣集中在了解几种茶叶提取物是否对高级糖基化终产物（AGEs）的形成具有抑制作用。此外，本研究还尝试鉴定抑制AGEs形成的化合物。将D-葡萄糖和人血清白蛋白或酪蛋白的混合物在生理条件下与茶叶提取物或不与茶叶提取物一起孵育。从相对荧光强度的测定中发现，所有茶叶提取物（日本绿色茶、中国茶、红茶、乌龙茶）对AGEs的形成都表现出较高的抑制活性（10-60%）。当以美拉德反应的关键中间体3-脱氧-D-葡萄糖酮或5-羟甲基-2-糠醛作为羰基组分时，茶叶提取物的抑制作用极低（5-15%）。采用溶剂萃取法从日本绿色茶中分离鉴定了4种含有表没食子儿茶素没食子酸酯的儿茶素类化合物。SDS-聚丙烯酰胺电泳显示，一个主要的高分子量蛋白质聚集体与茶叶提取物或儿茶素孵育的样品中消失。从儿茶素的化学结构来看，儿茶素在非酶糖基化的早期阶段具有抗氧化剂或自由基清除剂的作用。

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.