Flooding-dose of various amino acids for measurement of whole-body protein synthesis in the rat.

These studies were undertaken to investigate the influence of the precursor amino acid on the measurement of whole-body protein turnover by the flooding-dose method. Whole-body protein synthesis rates were estimated in 70 g rats using an intravenous injection of L-(U(14)C) Threonine, L-(U(14)C) Lysine (200µmoles/100g; 0.15µCi/µmol or 1000µmoles/100 g; 0.15µCi/µmol), L-(U(14)C) Phenylalanine, L-(1(14)C) Leucine or L-(U(14)C) Histidine (200µmoles/100 g; 0.15µCi/µmol). Forty two rats were divided into seven groups. Each group received a large dose of one of the labelled amino acids. In each group, one animal was killed every 2 min between 5 and 15 min after the injection. Whole-body protein fractional rate was determined from the slope of the linear regression of Sb(t) (protein specific radioactivity) against Si'(t) ×t, where Si'(t) is the average specific radioactivity of free tissue amino acids between 0 andt. Whole-body protein fractional synthesis rates were 41.4, 25.6, 31.1, 31.4 and 22.8%/day with threonine, lysine, phenylalanine, leucine and histidine respectively. These data suggest that the estimation of whole-body protein synthesis rate varies according to the amino acid used because of the heterogeneity of the protein pool.