Kinetic 12C/13C isotope fractionation by invertase: evidence for a small in vitro isotope effect and comparison of two techniques for the isotopic analysis of carbohydrates

The natural C-13/C-12 isotope composition (delta C-13) of plants and organic compounds within plant organs is a powerful tool to understand carbon allocation patterns and the regulation of photosynthetic or respiratory metabolism. However, many enzymatic fractionations are currently unknown, thus impeding our understanding of carbon trafficking pathways within plant cells. One of them is the C-12/C-13 isotope effect associated with invertases (EC 3.2.1.26) that are cornerstone enzymes for Suc metabolism and translocation in plants. Another conundrum of isotopic plant biology is the need to measure accurately the specific delta C-13 of individual carbohydrates. Here, we examined two complementary methods for measuring the delta C-13 value of sucrose, glucose and fructose, that is, off-line high-performance liquid chromatography (HPLC) purification followed by elemental analysis and isotope ratio mass spectrometry (EA-IRMS) analysis, and gas chromatography-combustion (GC-C)-IRMS. We also used these methods to determine the in vitro C-12/C-13 isotope effect associated with the yeast invertase. Our results show that, although providing more variable values than HPLC similar to EA-IRMS, and being sensitive to derivatization conditions, the GC-C-IRMS method gives reliable results. When applied to the invertase reaction, both methods indicate that the C-12/C-13 isotope effect is rather small and it is not affected by the use of heavy water (D2O). Copyright (C) 2009 John Wiley & Sons, Ltd.