I. Carbon Isotope Distribution in Food Chains. II. Mechanism of Carbon Isotope Fractionation Associated with Lipid Synthesis

The distribution of carbon isotopes in food chains was investigated by analyzing animals grown in the laboratory on diets of constant isotopic composition. The isotopic composition of an animal reflects the isotopic composition of its diet, but the animal is generally enriched by about 1 ‰ in δ13C relative to the diet. An isotopic mass balance exists for an animal-diet system. In three of four cases analyzed, the 13C enrichment of the body relative to the diet is balanced by a 13C depletion of the respired CO2. The carbon isotope distribution among different suborganismic components of animals was also analyzed. The relationships among the 13C/12C ratios of the major biochemical fractions, such as lipid, carbohydrate, and protein, appear to be preserved in going from one trophic level to the next, but the actual isotopic composition of a fraction in an animal and its diet can differ considerably. The relationship between the carbon isotopic composition of a tissue in a mouse and the carbon isotopic composition of the diet is affected both by the type of tissue analyzed and by the diet. The 13C/12C ratios of the biochemical components collagen, chitin, and the insoluble organic fraction of shells, which are commonly preserved in fossil material, also show a correlation with the isotopic composition of the diet. However, the magnitude of the difference between the 13C/12C ratios of the component and the diet depends on the animal, the diet, and the biochemical nature of the component. These results suggest that it will be possible to perform dietary analysis based on the relationship between the 13C/12C ratios of animals and their diets. The limits of accuracy of this method, as established by the experiments reported here, will restrict its application to situations in which the diet is derived from sources with large differences in their δ13C values, such as terrestrial vs. aquatic organisms and C3 vs. C4 plants. This method should be applicable in fossil as well as modern situations. The second part of this study involved the elucidation of the basis of the low 13C/12C ratio of the lipid fraction relative to the carbon isotopic composition of the whole organism and the other major biochemical fractions. Experiments in which E. coli was grown on carbon sources which enter at different steps of the metabolic sequence leading to lipid formation indicate that the carbon isotope fractionation resulting in the 13C depletion of the lipid fraction occurs during the oxidative decarboxylation of pyruvate to acetyl CoA by the pyruvate dehydrogenase complex. The isotopic fractionations occurring during this step were analyzed indirectly in a series of in vitro experiments with pyruvate decarboxylase. These experiments indicate that: 1) An isotopic fractionation of the expected direction and magnitude occurs during the pyruvate dehydrogenase step. 2) The 13C depletion of the acetyl CoA formed in the reaction is concentrated primarily in the carbonyl carbon atom, with the methyl carbon atom retaining the 13C/12C ratio of pyruvate. 3) The difference in the 13C/12C ratios of the methyl and carbonyl carbon atoms of acetyl CoA is twice as large as the 13C depletion of the lipid fraction. 4) The difference in the 13C/12C ratios of the methyl and carbonyl carbon atoms of acetyl CoA is temperature-dependent. 5) There will be a large, temperature-dependent difference in the carbon isotopic composition of those carbon atoms of lipid components which derive from the methyl and carbonyl carbon atoms of acetyl CoA.