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

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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.