Dietary soybean protein moderates the deleterious disturbance of lipid metabolism caused by exogenous oxidized cholesterol in rats

Effects of dietary protein on oxidized cholesterol-induced disturbance of l ipid metabolism were examined in 4 week old male Sprague-Dawley rats, using casein and soybean protein as dietary protein source. The rats were given one of the two proteins in 0.078% cholesterol (control), 0.25% cholesterol or 0.25% oxidized cholesterol mixture (containing 0.078% cholesterol) diets . Dietary oxidized cholesterol, compared to cholesterol, tended to inhibit hepatic sterol biosynthesis in casein-fed rats, whereas this inhibitory act ion was slightly moderated by intake of soybean protein. As a result, the h epatic 3-hydroxy-3-methylglutaryl-CoA reductase activity was rather higher in the rats fed oxidized cholesterol than in those fed cholesterol in the s oybean protein-fed group. The hepatic cholesterol 7 alpha-hydroxylase activ ity tended to be higher in the rats fed oxidized cholesterol than in those fed control diet in the soybean protein-fed group, despite the fact that ox idized cholesterol lowered the hydroxylase activity in the casein-fed group . On the other hand, dietary oxidized cholesterol tended to slightly enhanc e the hepatic Delta 6 desaturase activity in the casein-fed group I however , this observation was not shown in the soybean protein-fed group. Moreover , dietary soybean protein facilitated fecal oxidized cholesterol excretion and simultaneously inhibited the accumulation of oxidized cholesterol in se rum and liver. In conclusion, dietary soybean protein alleviated the delete rious actions of exogenous oxidized cholesterol on hepatic cholesterol and linoleic acid metabolism, although these efficacies were not necessarily si gnificant. A great part of these moderations may be exerted by the specific hypocholesterolemic function of soybean protein, such as the stimulation o f fecal oxidized cholesterol excretion, the change of hormonal release and modulation of lipoprotein catabolism. (C) 1999 Elsevier Science B.V. All ri ghts reserved.