Determination of 14C-labelled ketone bodies by liquid-scintillation counting.

1. A method of assaying (14)C in ketone bodies present in blood by using liquid-scintillation counting is described. 2. d(-)-beta-Hydroxy[(14)C]butyrate is converted quantitatively into [(14)C]acetoacetate by means of a coupled oxidoreduction reaction involving NAD(+), d(-)-beta-hydroxybutyrate dehydrogenase and malic dehydrogenase in the presence of a high concentration of oxaloacetate. 3. [(14)C]Acetoacetate is decarboxylated to acetone and carbon dioxide which are trapped separately in a double-well flask and counted subsequently. 4. The method permits the determination of (14)C activity in the individual ketone bodies and allows the activity in the carboxyl carbon atoms of acetoacetate or of d(-)-beta-hydroxybutyrate to be assayed separately from the activity in the remainder of the molecule. 5. Recoveries of (14)C-labelled ketone bodies added to blood approach 100% with good reproducibility in replicate analyses.