ELSEVIER Amino acid composition of dietary proteins affects plasma cholesterol concentration through alteration of hepatic phospholipid metabolism in rats fed a cholesterol-free diet Kimio Sugiyama, Hironori Kanamori, Toshiyuki Akachi, and Akihiro Yamakawa Department of Applied Biological Chemistry, Faculty of Agriculture, Shizuoka University Shizuoka, Japan This study was designed to investigate the relationship between the amino acid composition of dietary proteins and the plasma cholesterol concentration and to examine whether the alteration of hepatic phospholipid me- tabolism participates in the effect of dietary proteins in rats fed a cholesterol-free diet. There was a significant positive correlation between the plasma total cholesterol concentration and the plasma concentration of methi- onine and valine and the hepatic concentration of valine and alanine in rats fed seven types of proteins. In contrast, the plasma cholesterol concentration exhibited a significant negative correlation with the hepatic-free ethanolamine concentration. As far as methionine and ethanolamine, theirpositive and negative correlation with the plasma cholesterol concentration was consistent with their known hyper- and hypocholesterolemic effects, respectively. The phospholipid profile as represented by the ratio of phosphatidylcholine (PC) to phosphatidyl- ethanolamine fPE) in liver microsomes, but not in other tissues tested, was largely influenced by the type of dietary proteins; the PWPE ratio exhibited a significant positive correlation with the plasma total cholesterol concentration. There was a significant correlation between the methionine content of dietary proteins and several biochemical variables including plasma cholesterol and liver microsomal PC/PE ratio. A significantly lower concentration of hepatic S-adenosylmethionine was observed with soybean protein diet than with casein diet suggesting a decrease in the PC biosynthesis via the PE N-methylation pathway in rats fed soybean protein. From these results, it is suggested that the plasma cholesterol concentration might be influenced by the mefhionine content of dietary proteins at least in part through an alteration of hepatic phospholipid metabolism. (J. Nutr. Biochem. 7:4@48, 1996.) Keywords: dietary proteins; plasma cholesterol; phospholipid metabolism; methionine; etbanolamine; phosphatidylethanol- amine N-methylation Introduction It is confirmed that the plasma cholesterol concentration can be affected by the type of dietary proteins.lm3 The ingestion of plant proteins, as compared with animal proteins, gener- ally results in lower plasmaconcentrations in various spe- Address reprint requests to Dr. Kimio Sugiyama at the Department of Applied Biological Chemistry, Faculty of Agriculture, Shizuoka Univer- sity, 836 Ohya, Shizuoka 422, Japan. Received March 20, 1995; accepted August 25, 1995. ties of experimental animals. The hypocholesterolemic ac- tion of soybeanprotein was also demonstrated in humans.4 A number of explanations have so far beenprovided for the differential effects of plant and animal proteins. These ex- planations can be divided into two categories: one is based on a difference in the amino acid composition of dietary proteins and the other is based on a difference in the phys- icochemical properties of dietary proteins or luminal diges- tion products. The former concernswhether specific amino acid(s) of dietary proteins affect, after being absorbed,the metabolismof cholesterol either directly or indirectly. The latter concerns whether the extent of intestinal absorptionor Nutritional Biochemistry 7:40-48, 1996 0 Elsevier Science Inc. 1996 655 Avenue of the Americas, New York, NY 10010 0955-2863/96/$15&l SSDI 0955-2863(95)00165-4