Soybean protein-dependent changes in triacylglycerol synthesis and concentration of diacylglycerol in the liver microsomes of fasted-refed rats.

Effects of soybean protein, casein and whole egg protein on various indices for lipid biosynthesis in the liver were compared in fasted-refed rats. Soybean protein compared to casein and whole egg protein significantly reduced activities of glucose 6-phosphate dehydrogenase and fatty acid synthetase. The protein source also slightly reduced the activity of the malic enzyme. Soybean protein compared to other proteins not only reduced the microsomal triacylglycerol but also phosphatidylcholine syntheses when the activities were measured with endogenous diacylglycerol substrate. The protein-dependent changes disappeared, if artificial dispersion of dioleoylglycerol was employed as a substrate. The concentrations of microsomal diacylglycerol and triacylglycerol in whole liver in rats fed soybean protein were lower than those fed other proteins. When the diets containing soybean protein and casein were supplemented with DL-methionine (0.5 and 0.3%, respectively) to meet the nutritional requirement of the animals, soybean protein-dependent reductions in these indices for lipid biosynthesis were still detectable but considerably attenuated. Thus, it is plausible that a soybean protein-dependent decrease in fatty acid synthesis reduced the availability of microsomal diacylglycerol substrate for triacylglycerol synthesis and in turn modified hepatic triacylglycerol concentration. The dietary availability of sulfur amino acids may, at least in part, be responsible for the consequence observed in the present study.