| Abstract: | Two human subjects were given separate oral doses of sodium 2H6]isobutyrate and methyl-2H3]thymine and the labelling patterns of urinary metabolites were determined. Ingestion of deuterated isobutyrate resulted in the excretion of 2H5-labelled S-3-hydroxyisobutyric acid, formed on the direct catabolic pathway, and of S- and R-2H4]-3-hydroxyisobutyric acids, formed by the reduction of S- and R-methylmalonic semialdehydes respectively. Only the R-enantiomer of urinary 3-hydroxyisobutyric acid was labelled by thymine. This labelling pattern indicates a flow from S- to R-methylmalonic semialdehyde, suggesting that the R-enantiomer is the substrate of methylmalonic semialdehyde dehydrogenase. |
