Equilibration of metabolic CO2 with preformed CO2 and bicarbonate

Abstract

Entry of metabolic ¹⁴CO₂ into urea is shown to occur more readily than it equilibrates with the general pool of cellular plus extracellular bicarbonate plus CO₂. Since the sites of CO₂ production (pyruvate dehydrogenase and oxoglutarate dehydrogenase) and of fixation (carbamoylphosphate synthetase) are intramitochondrial, it is likely that the fixation of CO₂ is also more rapid than its equilibration with the cytoplasmic pool of bicarbonate plus CO₂. This observation may point to a more general problem concerning the interpretation of isotope data, when compartmentation or proximity of sites of production and utilisation of metabolites may result in the isotope following a preferred pathway.