Leucine flux: total flux, oxidative catabolism, protein synthesis and breakdown during development in the rat

Total leucine flux (flux into or out of the free leucine pool) and its different components --oxidative catabolism, protein synthesis and protein degradation--were studied in male rats weighing 160 and 300 g and infused intravenously for 6 h with L-(14C) leucine. Plasma L-(14C) leucine specific activity was measured after 4 and 6 h of infusion. The excretion rate of 14CO2 was estimated during infusion. Total leucine flux and oxidative catabolism flux were calculated from the specific radioactivity in the plasma at plateau value. Rates of protein synthesis were calculated from the total flux after subtracting the proportion oxidized. Protein breakdown flux was estimated by the difference between total flux and dietary intake flux. Total leucine flux (mumoles/h/100 g body weight) was not significantly lower at 300 than at 160 g. Oxidation accounted for about 10 p. 100 of the leucine flux; this proportion was significantly larger at 300 than at 160 g. A comparison of protein synthesis flux between the two groups of tats, for the same value of total flux, showed a decrease of leucine utilization for protein synthesis in the older animals. The quantity of leucine provided by protein breakdown was comparatively more important at 300 than at 160 g. Protein deposition fell by about 50 p. 100 between these two weights.