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The Mechanism of Oxalate Biosynthesis in Higher Plants: Investigations with the Stable Isotopes $^{18}$O and $^{13}$C

J. A. Raven, H. Griffiths, Sheila M. Glidewell, T. Preston


Substantial incorporation of $^{18}$O$_{2}$ into photorespiratory carbon oxidation cycle intermediates in illuminated Spinacia oleracea leaves confirms that oxygenase activity of the enzyme ribulose biphosphate carboxylase--oxygenase is a major source of glycollate in illuminated leaves. No $^{18}$O$_{2}$ incorporation into oxalate was detected in these experiments, although $^{13}$C incorporation from $^{13}$CO$_{2}$ shows that oxalate synthesis is occurring under the experimental conditions. This result tends to minimize the role of a direct oxidation of glyoxylate derived (via phosphoglycollate and glycollate) from ribulose biphosphate oxygenase activity in oxalate synthesis in Spinacia. Measurements of $\delta ^{13}$C show (in confirmation of earlier reports) that oxalate from Spinacia is less depleted in $^{13}$C than is bulk organic C in the plant; it is possible the phosphoenolpyruvate carboxylase is involved in the production of the oxalate precursor. Of the plants tested, Mercurialis and Pelargonium shared with Spinacia the high $\delta ^{13}$C value, while Chenopodium (closely related to Spinacia), Oxalis (more distantly related to Pelargonium) and two members of the Polygonaceae had oxalate $\delta ^{13}$C values close to the whole-leaf $\delta ^{13}$C value, which suggests derivation of both oxalate C atoms from carboxylase activity of the enzyme ribulose biphosphate carboxylase-oxygenase.

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