Effect of dietary proteins on time-dependent changes in plasma amino acid levels and on liver protein synthesis in rats.

The effects of four dietary proteins on plasma amino acid (PAA) levels in portal vein and aorta and on hepatic ribosome aggregation were investigated in rats fed 10% protein diets ad libitum. Proteins studied were beef, casein, rapeseed and soybean. Measurements were made at 2000, 2300, 0200, 0500 and 0800 h. Portoaortic differences in essential amino acids (EAA) in plasma were calculated and compared to dietary EAA to verify the time-dependent changes in their intestinal absorption. Each protein generated different variations in portal and aortic PAA levels. Rapeseed produced the lowest plasma concentrations of EAA, but its portoaortic differences were as large as those of the other proteins. With each protein, proportions of EAA in plasma (in contrast to concentrations) tended to remain constant in portal vein and aorta over the 12-h period. Significant correlations between portoaortic differences and dietary EAA were found in all groups, but at varying times, indicating that the kinetics of intestinal absorption were dependent on the protein fed. So were variations in liver polysome distribution. For rapeseed and soybean, times of most active protein synthesis were 0200 and 0500 h; with beef and casein, variations between sampling times were not significant. No direct relationship was found between magnitude and composition of PAA portoaortic differences and ribosome aggregation. Thus these two parameters were both affected by dietary proteins, but no synchrony was evidenced between them.