Interpretation of Experiments on Metabolic processes, using Isotopic Tracer Elements

IN two instances, the distribution of isotopic carbon in the product of a metabolic process has been used to infer that a symmetrical intermediate compound is not involved. Wood et al.1 showed that isotopic carbon, introduced as carbon dioxide together with pyruvate, led to the formation of ketoglutarate which contained isotopic carbon only in the carboxy group next to the keto group; on these grounds, they excluded citrate as an intermediate. Sbemin2 found that when glycine is formed from serine, containing isotopic nitrogen and isotopic carbon in its carboxy group, the relative abundance of nitrogen-15 and carbon-13 in the glycine was the same as in the serine; he argued that amino-malonic acid is therefore not an intermediate in this process.