METHIONINE DEFICIENCY IN RATS FED SOY PROTEIN INDUCES HYPERCHOLESTEROLEMIA AND POTENTIATES LIPOPROTEIN SUSCEPTIBILITY TO PEROXIDATION

A number of studies have provided evidence that plant proteins, especi ally soy protein, have a cholesterol-lowering effect as compared with casein. However, dietary supply of sulfur amino acids may be deficient when soy protein is present in the diet at a suboptimal level, which could affect lipid metabolism. Accordingly, in rats fed 13% protein di ets, soy protein feeding resulted in a cholesterol-increasing effect ( +18%), which could be counteracted by methionine supplementation (0.4% ). In contrast, soy protein was effective in decreasing plasma triglyc eride, as compared with levels in rats fed casein; this triglyceride-l owering effect was entirely abolished by methionine supplementation. T he hypercholesterolemic effect of soy protein was characterized by a h igher cholesterol content in low-density lipoprotein (LDL) and high-de nsity lipoprotein 1 (HDL(1)) fractions, together with a marked inducti on of hepatic hydroxymethyl glutaryl coenzyme A (HMG CoA) reductase ac tivity and to a lesser extent cholesterol 7 alpha-hydroxylase. There w as practically no induction of these enzymes, as compared with levels in rats fed casein diets, when the soy protein diet was supplemented w ith methionine. Very-low-density lipoprotein (VLDL) plus LDL susceptib ility to peroxidation was higher in rats fed soy protein than in casei n-fed rats, which could reflect in part the lack of sulfur amino acid availability, since methionine supplementation led to a partial recove ry of lipoprotein resistance to peroxidation. These findings suggest t hat amino acid imbalance could be atherogenic by increasing circulatin g cholesterol and leading to a higher lipoprotein susceptibility to pe roxidation. Copyright (C) 1995 by W.B. Saunders Company