Intramolecular 13C pattern in hexoses from autotrophic and heterotrophic C3 plant tissues

The stable carbon isotope C-13 is used as a universal tracer in plant eco-physiology and studies of carbon exchange between vegetation and atmosphere. Photosynthesis fractionates against (CO2)-C-13 so that source sugars (photosynthates) are on average C-13 depleted by 20 parts per thousand compared with atmospheric CO2. The carbon isotope distribution within sugars has been shown to be heterogeneous, with relatively C-13-enriched and C-13-depleted C-atom positions. The C-13 pattern within sugars is the cornerstone of C-13 distribution in plants, because all metabolites inherit the C-13 abundance in their specific precursor C-atom positions. However, the intramolecular isotope pattern in source leaf glucose and the isotope fractionation associated with key enzymes involved in sugar interconversions are currently unknown. To gain insight into these, we have analyzed the intramolecular isotope composition in source leaf transient starch, grain storage starch, and root storage sucrose and measured the site-specific isotope fractionation associated with the invertase (EC 3.2.1.26) and glucose isomerase (EC 5.3.1.5) reactions. When these data are integrated into a simple steady-state model of plant isotopic fluxes, the enzyme-dependent fractionations satisfactorily predict the observed intramolecular patterns. These results demonstrate that glucose and sucrose metabolism is the primary determinant of the C-13 abundance in source and sink tissue and is, therefore, of fundamental importance to the interpretation of plant isotopic signals.