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Effects of Dietary Proteins on the Regulation of Liver Lipoprotein Receptors in Rats¹

Cesare R. Sirtori^*, Giovanni Galli, Maria Rosa Lovati, Patrizia Carrara, Enrica Bosisio and Marzia Galli Kienle

^* Chair of Chemotherapy Institute of Pharmacology and Pharmacognosy Department of Medical Chemistry and Biochemistry, University of Milano, Via A. Del Sarto 21, 20129 Milano, Italy

Female rats fed a 1.2% cholesterol diet with animal proteins (casein) develop a significant hypercholesterolemia, with a marked increase of very low density lipoprotein (VLDL)-associated cholesterol. Substitution of soy proteins for casein in the diet counteracts the increase of both total and VLDL cholesterol. Studies of liver receptor activity were carried out with both casein and soybean-cholesterol diets, to define the site of action of soy proteins. Binding of a cholesterol-rich lipoprotein fraction (ß-VLDL) to hepatic membranes is normal when a soybean-cholesterol diet is administered, and markedly reduced with casein-cholesterol. The activity of receptor-linked enzymes, HMG-CoA reductase, cholesterol 7-hydroxylase and acyl-CoA:cholesterol O-acyltransferase (ACATase), is differently affected by the two diets. HMG-CoA reductase activity is reduced by both diets with, however, significantly higher enzyme activities in the soybean-cholesterol-fed group. Both 7-hydroxylase and ACATase activity levels are significantly raised by casein-cholesterol but are in a normal range with soybean-cholesterol. These findings suggest that the hepatic receptor regulation of cholesterol metabolism is differently affected by animal and vegetable proteins in the diet.

KEY WORDS: • soybean protein • casein • cholesterol • liver lipoprotein receptors

¹ Supported in part by the Consiglio Nazionale delle Ricerche of Italy (PF Basi Molecolari delle Malattie Ereditarie, SP₃) and by a Grant-in-aid from Gipharmex, Milano, Italy.

Manuscript received 21 February 1984.