A single dose of [1–¹⁴C] or [U-¹⁴C]glycine or L-[1–¹⁴C] or [U-¹⁴C]valine was injected intraperitoneally into 1- and 3-week-old chicks. Continuous automatic determination of respiratory ¹⁴CO₂ and determination of ¹⁴C in excreta and uric acid were carried out for 8 h. Computer-aided analysis of the kinetics of respiratory ¹⁴CO₂ demonstrated two exponentially decreasing components, ‘fast ¹⁴CO₂’ and ‘slow ¹⁴CO₂’. Fast ¹⁴CO₂production represents the immediate oxidation and/or decarboxylation of the ¹⁴C-labeled amino acid. The half-lives differed according to the nature of the 14C-labeled amino acid but not to the age of the chick. The size of the acetyl-CoA pool labeled by [¹⁴C]glycine was between 10 and 15 mmol/l00 g of chick. Metabolic partition of a circulating plasma free amino acid between oxidation and retention could be quantified by an isotopic index.

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Analysis of the respiratory ¹⁴CO₂ of growing chicks injected intraperitoneally with [1–¹⁴C] or [U-¹⁴C]glycine or valine has shown the presence of a delayed ‘slow ¹⁴CO₂’ exponential component. The half-lives, about 3 h, are independent of the nature of the amino acid and of the age (1–3 weeks) of the animal. As supported by investigations in cycloheximide-treated chicks of the oxidative catabolism of glycine, valine and acetate, slow ¹⁴CO₂ is significantly associated with the degradation of short-lived proteins.