The absolute configuration of β-aminoisobutyric acid formed by degradation of thymine in man

1. 1. Serum and urine samples from a healthy person have been examined for enantiomorphs of β-aminoisobutyric acid (BAIBA) after loading with thymine and dihydrothymine. The identity of BAIBA and its enantiomorphs was verified by combined gas-liquid chromatography and mass spectrometry. 2. 2. After loading with the pyrimidines a considerable increase of BAIBA in serum and urine was observed. This increase was higher after loading with thymine. 3. 3. In both cases the increase of BAIBA in serum and urine was found to be caused by the- R-enantiomorph only. 4. 4. The results show that thymine degradation gives rise to the R-form of BAIBA and that no steric inversion takes place before excretion. The S-isomer which apparently predominates in serum has a metabolic origin other than the catabolism of thymine. Furthermore it seems likely that only one isomer of the racemic dihydrothymine is further degradated to BAIBA. 5. 5. The determination of R-BAIBA in serum may represent a sensitive parameter for the metabolism of thymine and of DNA.