Regulatory Mechanism of Plasma Cholesterol Concentration by Dietary Protein : Evidence that plasma cholesterol is not regulated by modification of entero-hepatic circulation of steroids.

我们进行了一系列的实验，以阐明可能的机制，饮食大豆分离蛋白（SPI）产生的血浆胆固醇浓度降低相比，饮食酪蛋白在大鼠喂养的胆固醇胆汁酸的饮食。在饮食交换后的3天内，每组完整大鼠的血浆胆固醇浓度迅速达到与前一组相同的水平。切除空肠或回肠并给予消胆胺或β-谷甾醇不改变血浆胆固醇浓度对酪蛋白和13 EA：SPI的固有反应。SPI组大鼠体内（肝脏和小肠）胆固醇合成速率显著高于酪蛋白组，当补充蛋氨酸（Met）至与酪蛋白相当的水平时，SPI的降胆固醇作用消失。酪蛋白和大豆分离蛋白的效果由它们的等效氨基酸混合物再现。饲喂酪蛋白或酪蛋白模拟氨基酸混合物组门静脉血中Met/Gly比值显著高于饲喂SPI或SPI模拟氨基酸混合物组。酪蛋白引起的血浆胆固醇水平的升高归因于高密度脂蛋白（HDL）胆固醇的升高。HDL-胆固醇浓度与卵磷脂：胆固醇酰基转移酶活性呈正相关。酪蛋白组大鼠主动脉血HDL-载脂蛋白水平和13 EA：HDL-载脂蛋白合成率均高于SPI组。从这些观察结果中，我们得出结论，膳食蛋白诱导的血浆胆固醇浓度的变化可能是通过改变组成HDL的α 13 EA：脂蛋白引起的。

We carried out a series of experiments to elucidate the possible mechanism by which dietary soybean protein isolate (SPI) produced a reduction of plasma cholesterol concentration compared with dietary casein in rats fed a cholesterol-bile acid-free diet. Within 3 days after the diet crossover, plasma cholesterol concentration in each group of intact rats reached rapidly the same level as that in its previous counterpart. The inherent responses of plasma cholesterol concentration to casein and13EA\ : SPI were not changed by the resection of the jejunum or the ileum and the administration of cholestyramine or beta-sitosterol. The rates of cholesterol biosynthesis in vivo (in liver and small intesine) were significantly higher in SPI-fed rats tha13EA\ : n in casein-fed rats.The hypocholesterolemic effect of SPI disappeared when methionine (Met) was supplemented at a level equivalent to casein. The effects of casein and SPI were reproduced by their equivalent amino acid mixtures. The ratio of Met/glycine (Gly) in postpr13EA\ : andial portal blood was significantly higher when casein or casein-simulated amino acids mixture was fed than when SPI or SPI-simulated amino acid mixture was fed. The casein-induced increase in the level of plasma cholesterol concentration was attr13EA\ : ibuted to an increase in high density lipoprotein (HDL) cholesterol. HDL-cholesterol concentration showed a positive correlation with the lecithin : cholesterol acyltransferase activity. HDL-apolipoprotein level in aortic blood and synthetic rate of13EA\ : HDL-apolipoproteins in vivo was higher in casein-fed rats than in SPI-fed rats. From all of these observations, we conclude that the dietary protein-induced alterations of the plasma cholesterol concentration would be caused through the change in a13EA\ : polipoproteins composing HDL.