[The mechanism of carbon isotope fractionation in the metabolic process]

The mechanism of carbon isotope fractionation in metabolic paths of autotrophic organisms is considered. The principal features of the mechanism proposed are: 1) the emergence of a one-stage kinetic isotope effect of pyruvate decarboxylation during respiration resulting in the formation of two flows of the carbon substrate with different isotope compositions; 2) the multiplication of the one-stage isotope effect by means of the repeated circulation of a light isotope fraction (C2-fragments) in lipid-carbohydrate metabolism and by the simultaneous removal of a heavy isotope carbon dioxide in the Krebs cycle. On the basis of the above mechanism carbon isotope effects are explained of CO2 assimilation and respiration as well as sequential decrease of 13C content in the series of lipids, carbohydrates and proteins. The cuase of the enrichment of the whole organisms in the light isotope in respect to the carbon dioxide of the environment is discussed.