Intramolecular¹³C pattern in hexoses from autotrophic and heterotrophic C₃plant tissues

Abstract

The stable carbon isotope 13 C is used as a universal tracer in plant eco-physiology and studies of carbon exchange between vegetation and atmosphere. Photosynthesis fractionates against 13 CO 2 so that source sugars (photosynthates) are on average 13 C depleted by 20‰ compared with atmospheric CO 2 . The carbon isotope distribution within sugars has been shown to be heterogeneous, with relatively C 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 13 C abundance in source and sink tissue and is, therefore, of fundamental importance to the interpretation of plant isotopic signals.

Nonstatistical 13C Distribution during Carbon Transfer from Glucose to Ethanol during Fermentation Is Determined by the Catabolic Pathway Exploited

“…Of these three pathways, only the EMP has been closely examined for position-specific 13 C fractionation in ethanol biosynthesis (13,14,18), whereas some data from the RPP are also available (12). These studies clearly demonstrated that fermentation by the EMP route leads to 13 C/ 12 C isotopic ratios for the methyl (CH 3 ) and methylene (CH 2 ) carbon positions that differ depending on the source glucose.…”

“…However, at that time (1961) isotope measurement by mass spectrometry gave only a value for ␦ 13 C g (‰), the global or mean isotope content, and the technique is poorly adapted to the determination of the intramolecular distribution of isotopes. Subsequently, measurement of ␦ 13 C i (‰), the position specific isotope distribution, has proved the first part of the above conclusion to be incorrect (12,18): as yet, evidence is still to be presented that glycolysis proceeds without position-specific isotopic discrimination. To probe this, we have carried out an analysis of ␦ 13 C i (‰) in ethanol from the EMP pathway.…”

A retro-biosynthetic approach to the prediction of biosynthetic pathways from position-specific isotope analysis as shown for tramadol

“…In addition, the oxidative deamination of cadaverine should imply an intramolecular kinetic carbon isotope effect (40). As a result, competition between equivalent positions of the symmetrical precursors should select in favor of isotopomers containing 12 C at this position, leading to a 13 C depletion of the C atom bearing the oxygen function (C5 T ) relative to that bound to N (C1″ T ). Both these predictions are not met: on the contrary, C1″ T is depleted relative to C5 T (ΔC5 T − C1″ T = 10.2‰).…”

“…The information that can be obtained from examining the global isotope deviations (δ 13 C g ) in whole molecules (14,15) by isotope ratio monitoring by mass spectrometry (irm-MS) is inadequate because this technique, in which the target is combusted to CO 2 in an O 2 -rich atmosphere, has the disadvantage of only determining the average distribution of 13 C among the isotopomers (δ 13 C g ‰). This leads to considerable loss of information about isotope fractionation, because not only are heavy isotopes unevenly distributed within a compound, but position-specific isotopic fractionation at natural abundance is defined by the specific reactions taking place during biosynthesis (12,16,17).…”

“…The phenomenon of isotopic fractionation during reactions is well established as a method to characterize reaction mechanisms and help distinguish different (bio)synthetic origins (11)(12)(13). The information that can be obtained from examining the global isotope deviations (δ 13 C g ) in whole molecules (14,15) by isotope ratio monitoring by mass spectrometry (irm-MS) is inadequate because this technique, in which the target is combusted to CO 2 in an O 2 -rich atmosphere, has the disadvantage of only determining the average distribution of 13 C among the isotopomers (δ 13 C g ‰).…”