The mechanism of oxalate biosynthesis in higher plants: investigations with the stable isotopes 18O and 13C

Substantial incorporation of 18O2 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 18O2 incorporation into oxalate was detected in these experiments, although 13C incorporation from 13CO2 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 δ13C show (in confirmation of earlier reports) that oxalate from Spinacia is less depleted in 13C 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 δ13C value, while Chenopodium (closely related to Spinacia), Oxalis (more distantly related to Pelargonium) and two members of the Polygonaceae had oxalate δ13C values close to the whole-leaf δ13C value, which suggests derivation of both oxalate C atoms from carboxylase activity of the enzyme ribulose biphosphate carboxylase–oxygenase.