The fate of isotopic carbon in kidney cortex synthesizing glucose from lactate.

1. When slices of kidney cortex are incubated with lactate and acetoacetate, lactate is almost quantitatively converted into glucose whereas acetoacetate provides a major part of the fuel of respiration. 2. In apparent contrast with these findings a large fraction of (14)C-labelled acetoacetate appears in glucose and a large fraction of (14)C-labelled lactate appears in the carbon dioxide. 3. The findings can be explained by: (a) the participation of oxaloacetate as an intermediate of both gluconeogenesis and respiration; (b) the fact that the carbon dioxide formed in the course of one turn of the tricarboxylic acid cycle is exclusively derived from oxaloacetate. 4. As a result there is a ;crossing over' of the carbon of the substrate of respiration to the pathway of gluconeogenesis and of the carbon of the glucogenic precursors to the pathway of respiration. 5. In the given situation the fate of the label does not allow predictions to be made about the net fate of the labelled metabolites. 6. The implications of the findings on the interpretation of isotopic data are discussed.