Early Effects of Protein Depletion on Hepatic Glycoprotein Synthesis in the Rat

Abstract

The purpose of this study was to determine how dietary protein depletion affects glycoprotein metabolism. Groups of young adult male rats were fed diets containing 0, 5 or 20% casein for 2 weeks. Glycoproteins were then analyzed in several liver subcellular fractions, and in blood plasma. Also, the kinetics of incorporation of 14C-glucosamine in vivo into the protein-bound hexosamines and sialic acids were carried out in three different liver fractions; the highest specific activities were seen in rats fed the casein-free diets. The “acid insoluble” (0.6 M perchloric acid) glycoproteins showed significant decreases in the hexosamine to sialic acid ratio with an actual increase in total protein-bound sialic acid in liver fractions as well as in blood plasma from rats fed the low protein diets. Among the “acid soluble” glycoproteins there was a significant increase in total protein-bound hexoses in the liver fractions, but such changes were not noted in the blood plasma. The significance of these adaptations is discussed in terms of the importance of the bound sugars in the regulation of the secretion of glycoproteins.