Intramolecular deuterium distributions reveal disequilibrium of chloroplast phosphoglucose isomerase

Intramolecular deuterium distributions of the carbon-bound hydrogens of glu cose were measured using deuterium nuclear magnetic resonance. Glucose isol ated from leaf starch of common bean (Phaseolus vulgaris cv, Linden) or spi nach (Spinacia oleracea cv, Giant nobel) was depleted in deuterium in the C (2) position, compared with glucose isolated from leaf sucrose or bean endo sperm starch, In beans, the depletion of C(2) was independent of the light intensity during growth (150 or 700 mu mol photons s(-1) m(-2)). The ratio of glucose-6-phosphate to fructose-6-phosphate ([G6P]/[F6P]) in bean chloro plasts was 0.9 in high light, indicating that the phosphoglucose isomerase reaction was not in equilibrium ([G6P]/[F6P]) approximate to 3), This impli es that the kinetic isotope effect of phosphoglucose isomerase depleted deu terium in the C(2) position of G6P, Because the depletion was the same, the chloroplastic ([G6P]/[F6P]) ratio was in disequilibrium irrespective of th e light intensity. If the ([G6P]/[F6P]) ratio was in equilibrium, a large c hloroplastic pool of G6P would be unavailable for regeneration of ribulose- 1,5-bisphospate. We argue that chloroplast phosphoglucose isomerase activit y is regulated to avoid this. The deuterium depletion of C(2) explains the known low overall deuterium abundance of leaf starch. This example shows th at measurements of intramolecular deuterium distributions can be essential to understand overall deuterium abundances of plant material.