HIGHLY PURIFIED SOYBEAN PROTEIN IS NOT HYPOCHOLESTEROLEMIC IN RATS BUT STIMULATES CHOLESTEROL-SYNTHESIS AND EXCRETION AND REDUCES POLYUNSATURATED FATTY-ACID BIOSYNTHESIS

The specific effects of soybean protein on lipid metabolism were deter mined with highly purified soybean protein. At 5 wk of age, growing ra ts were fed diets containing 20% highly purified soybean protein or ca sein supplemented or not with 0.1% cholesterol for 2 mo. Plasma and li ver lipid composition, fecal steroid excretion and several hepatic enz yme activities were measured. There were no significant dietary protei n-related differences in plasma and liver cholesterol concentrations. When diets were cholesterol free, highly purified soybean protein stim ulated fecal neutral and acidic steroid excretion associated with conc omitantly higher hydroxy methylglutaryl CoA (HMG-CoA) reductase activi ty, but lower cholesterol 7 alpha-hydroxylase activity. Soybean protei n lowered the linoleate desaturation index [20:4(n-6)/18:2(n-6)] in li ver microsomal lipids and phospholipids. This may have been due to the reduced microsomal Delta 6(n-6) desaturase activity in rats fed soybe an protein, whereas Delta 5(n-6) desaturase activity did not differ be tween groups fed the two proteins. Cholesterol supplementation (0.1 %) did not affect plasma cholesterol but increased liver cholesterol and triacylglycerol concentrations and reduced HMG-CoA reductase activity ; this latter effect was greatest in rats fed soybean protein. Cholest erol 7 alpha-hydroxylase activity, however, was diminished only in rat s fed casein. Desaturase activities, and particularly Delta 5(n-6) act ivity, were lowered by cholesterol supplementation in rats fed both pr otein diets, including a significantly lower 20:4(n-6)/18:2(n-6) ratio in liver microsomal lipids and liver phospholipids. Thus although die tary proteins have no effect on serum cholesterol in rats, they affect enzyme activities involved in cholesterol metabolism and fatty acid d esaturation.