In vivo 14C isotopic evaluation of amino acid oxidative catabolism in the growing chick. I. Fast respiratory 14CO2 and acetyl-CoA renewal.

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 14CO2 production 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/100 g of chick. Metabolic partition of a circulating plasma free amino acid between oxidation and retention could be quantified by an isotopic index.

In vivo 14C isotopic evaluation of amino acid oxidative catabolism in the growing chick. II. Slow respiratory 14CO2 and protein renewal.

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.