Male rats aged 45, 85, 145 and 270 days (daily body mass increments on ar, optimal diet containing casein were 6.73, 2.88, 0.53 and 0.31 g respectively) were fed 15 days ad libitum on a diet with a nutrient content physiological for their age, in which the protein source was milk casein (ratio of essential to nonessential amino acids E/N = 0.79, compensation coefficient K = 14) or wheat gluten (E/N = 0.30, K= -8). In the case of gluten, net protein utilization (NPU) fell markedly in rapidly growing animals aged 45 and 85 days (33 and 30 % more than with casein), indicating that without essential amino acid compensation, gluten is inadequate for animals of this age, whose organism requires fully ensured proteosynthesis for growth and development. In adolescence and adulthood (145 and 270 days), the utilization of proteins is not dependent on their quality (the decrease in NPU 13 and 12 % - is nonsignificant). That means that a smaller amount of essential amino acids, including the limiting amino acid in uncompensated protein, is sufficient for the maintenance and renewal of organs and tissues, i.e. for proteosynthesis. The activation of gluconeogenesis (phosphoenolpyruvate carboxvkinase activity in the liver) after the intake of plant protein confirms the effect of proteins on catabolic processes.
Male weaning rats were put on a diet with a physiological nutrient combination adjusted for age, milk casein (E7N = 0.79) and wheat gluten (E/M = 0.30) being the sources of protein. The net protein ratio (NPR) was evaluated weekly until 140 days of age. On days 70 and 140, L-((J-14C)-tyrosine was administered intraperitoneally and 12 h later specific tyrosine activity was determined in the protein fraction of liver and muscle by measuring the incorporation of the labeled amino acid in order to assess protein synthesis over the corresponding time period. Regression lines describing the relationship between animals' weight, age and protein source suggested that the daily weight increase was 6.99 g between days 30-77, 2.97 g between days 77-105 and 0.64 g between days 105-140. Muscle tyrosine levels in rapidly growing animals aged 70 days were 91.0 /¿g/g/12 h for casein and 65.6/ig/g/12 h for gluten. Liver tyrosine levels were 336.4 and 189.6 /rg/g/12 h, respectively. The differences observed at this age were highly significant. In adult animals (140 days old) there were non-significant differences between tyrosine levels in the casein- and gluten-fed groups. The isotope study clearly showed that protein synthesis was reduced in growing and developing animals on vegetable nutrition, which is deficient in essential amino acids, (especially the limiting amino acid lysine, crucial for the utilization of all other amino acids in peptide chain synthesis). The low rate of amino acid utilization found in animals younger than 105 days is consistent with the findings obtained by the isotope method.