Different renal oxidation rates of intramolecular carbon atoms in lactate and pyruvate

In order to get more detailed information on the different renal modes of utilization of individual intramolecular carbon atoms, formation rates of 14CO2 and metabolic products from single-carbon labelled exogenous substrates have been studied in isolated tubular fragments from rat kidney cortex. The results showed that carboxyl groups from acetate, lactate, and pyruvate were converted to CO2 at a much higher rate than substrate-2-C or 3-C. This effect was not due to a physico-chemical splitting. The corresponding C-2 or C-3-atoms were incorporated into newly formed glucose to a higher extent than carboxyl groups. Apparently, the maximum chance to be converted to CO2 for an intramolecular carbon of an exogenous substrate lies at the stereospecific site of action of decarboxylating enzymes. With rising distance from this point the probability to be built into glucose (i.e. to escape the oxidative pathway) increases. Due to the different share of the intramolecular substrate carbon (in the total CO2 formation) without the use of single-carbon labelled substrates one cannot calculate the oxygen requirements for an observed 14CO2 formation.