Development of anti-carcinogenic food by using soybean resistant protein

UDF是细菌酶消化大豆分离蛋白后的残留物。我们发现，给大鼠喂饲UDF能有效地抑制胆汁酸诱发的结肠癌和肝癌。饲喂UDF的大鼠血清胆汁酸浓度随粪便胆汁酸排泄量的增加而降低。粪氮、氨基酸、胆汁酸和胆固醇的排泄量之间存在着显著的相关性。这表明，不可消化的多肽，即我们所说的“抗性蛋白”，可能来自于UDF捕获肠道中的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，并干扰胆固醇的吸收。为了证明这种耐药蛋白的存在，我们试图从粪便中提取这种多肽。但大部分不能被含胍、尿素的缓冲液等溶剂提取，仍留在不溶部分。不溶部分含有比UDF本身更多的疏水氨基酸，并显示出较强的胆汁酸结合能力。粪便中胆汁酸不能用磷酸盐缓冲液提取，只能用75%乙醇提取。因此，胆汁酸以疏水结合的方式被捕获在抗性蛋白中，并作为不溶性部分排泄到粪便中。结肠液中胆汁酸的启动子活性可能通过与抗性蛋白形成复合体而受到抑制。UDF经胃酶消化后在体外产生，得到残留物(Rf-UDF)，得率为30%。在体外，RF-UDF的胆汁酸结合能力是UDF的两倍，饲料中10%的RF-UDF相当于20%的UDF饲料中的胆汁酸排泄量。RF-UDF含有比UDF更多的疏水氨基酸，其氨基酸组成与喂食UDF或R-UDF的大鼠粪便氨基酸组成非常相似，目前正在进行研究。

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.