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Dietary Protein Modifies Oxidized Cholesterol-Induced Alterations of Linoleic Acid and Cholesterol Metabolism in Rats^1,2,

Laboratory of Food Science, Department of Food Science and Technology, Faculty of Agriculture, Kyushu University, Fukuoka 812, Japan

Effects of dietary protein on oxidized cholesterol-induced alterations in linoleic acid and cholesterol metabolism were studied in 4-wk-old male Sprague-Dawley rats, using casein and soybean protein as dietary protein sources. The rats were fed one of the two proteins in cholesterol-free, 0.3% cholesterol or 0.3% oxidized cholesterol mixture diets using a pair-feeding protocol for 3 wk. In the soybean protein-fed group, rats fed oxidized cholesterol did not have lower activity of liver microsomal 6 desaturase, the rate-limiting enzyme in the metabolism of linoleic acid to arachidonic acid, compared with rats fed cholesterol-free diet, whereas in the casein-fed group the desaturase activity was significantly greater in rats fed oxidized cholesterol than in those fed cholesterol-free diet. This was in contrast to a significant reduction in liver microsomal 6 desaturase activity by cholesterol, irrespective of protein source. In general, these changes were reflected in the desaturation indices of liver phospholipids. Furthermore, soybean protein significantly increased the fecal excretion of neutral and acidic steroids and tended to reduce (P = 0.082) the accumulation of oxidized cholesterols in the liver. Thus, soybean protein partly modified some of the undesirable effects of oxidized cholesterol through its hypocholesterolemic effect and possibly through the modulation of hepatic 6 desaturase activity.

KEY WORDS: • oxidized cholesterol • soybean protein • casein • 6 desaturase • rats

¹ Supported by the Research Projects on Aging and Health (90A 1102) from the Ministry of Public Welfare, Japan.

² The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 USC section 1734 solely to indicate this fact.

³ To whom correspondence should be addressed. Current address: Laboratory of Science of Bioproducts, Faculty of Agriculture, Hirosaki University.

Manuscript received 1 August 1995. Revision accepted 14 February 1996.