Development of anti-carcinogenic food by using soybean resistant protein

UDF是大豆分离蛋白被细菌蛋白酶消化后的残留物。我们发现给大鼠喂UDF能有效抑制胆汁酸诱导的结肠癌和肝癌。在饲喂UDF的大鼠中，血清胆汁酸浓度随着粪胆汁酸排泄量的增加而降低。粪便中氮、氨基酸、胆汁酸和胆固醇的排泄量呈良好的相关性。这表明不可消化的肽，我们称之为“抗性蛋白”，可能来源于UDF在肠道中捕获胆汁酸并刺激其排泄。这可能会破坏胆固醇与胆汁酸的胶束形成，并中断胆固醇的吸收。为了证明这种耐药蛋白的存在，我们尝试从粪便中提取肽。然而，大多数化合物未被胍和尿素等含缓冲液溶剂提取，而停留在不溶性组分中。不溶性部分比UDF本身含有更多的疏水氨基酸，表现出较强的胆汁酸结合能力。粪便中的胆汁酸不能用磷酸盐缓冲液提取，而需用75%的乙酸乙酯提取。因此，胆汁酸通过疏水结合被抗性蛋白捕获，并作为不溶性部分排泄到粪便中。胆汁酸在结肠中的启动子活性可能因与耐药蛋白形成复合物而受到抑制。用胃蛋白酶消化UDF，体外胰化，得到产率为30%的UDF残余部分（RF-UDF）。RF-UDF在体外的胆汁酸结合能力是UDF的2倍，饲粮中10%的RF-UDF足以将胆汁酸排泄到粪便中。RF-UDF比UDF含有更多的疏水氨基酸，其氨基酸组成与饲喂UDF或R-UDF的大鼠粪便氨基酸组成非常相似。

UDF is a residue remained after digestion of soy protein isolate by bacterial protease. We found that feeding of UDF to the rat effectively suppressed colon and liver carcinogenesis induced by bile acids. In the rat fed UDF, the serum concentration of bile acid was reduced conomitantly with the increase of fecal bile acids excretion. There found a god correlation among the excretion of fecal nitrogen, amino acid, bile acid and cholesterol. This suggested that indigestible peptide, as we call a "resistant protein" may derived form UDF capture bile acids in the intestine and stimulate their excretion. This could disrupt micelle formation of cholesterol with bile acids and interrupt cholesterol absorption. To prove the existence of such resistant protein, we tried to extract the peptide from feces. However, most of them were not extracted by the solvent such as guanidine and urea containing buffer and remained in the insoluble fraction. The insoluble fraction contained much hydrophobic amino acids than UDF itself and showed strong bile acid-binding ability. Bile acids in the feces could not be extracted by phosphate buffer but by 75 % EtOH. Therefore, it is suggested that bile acids were captured in the resistant protein by hydrophobic binding and excreted into the feces as the insoluble fraction. The promoter activity of bile acids in the colon might be suppressed by the formation of complex with the resistant protein. By digestion of UDF by pepsin followed pancreation in vitro, we obtained residual fraction of UDF (RF-UDF) in the yield of 30 %. The bile acid binding ability of RF-UDF in vitro was two-fold higher than that of UDF and 10 % of RF-UDF in the diet is as enough as 20 % UDF diet to excrete bile acid into feces. RF-UDF contained more hydrophobic amino acids than UDF and its amino acid composition was very similar to fecal amino acid composition of rat fed either UDF or R-UDF to the food staff is now progressing.