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

Abstract

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 δ 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 δ 13 C value, while Chenopodium (closely related to Spinacia ), Oxalis (more distantly related to Pelargonium ) and two members of the Polygonaceae had oxalate δ 13 C values close to the whole-leaf δ 13 C value, which suggests derivation of both oxalate C atoms from carboxylase activity of the enzyme ribulose biphosphate carboxylase–oxygenase.

The influence of N metabolism and organic acid synthesis on the natural abundance of isotopes of carbon in plants

“…phases the two cell types is maintained (F. A. Smith, cited II and IV (Griffiths, 1988), so that non-Rubisco by Raven, 1977 a The only data available on the ^^^C of an endmalic acid synthesis using stored carbohydrate and product of non-Rubisco carboxylation in CAM exogenous and/or respired CO2 with PEPC as the plants seem to be those of Rivera & Smith (1979) on carboxylase in the dark, and CO2 regeneration oxalate in cacti. The synthetic pathways for oxalate followed by CO2 fixation by Rubisco during malic are still a matter of dispute (see Raven et al, 19826). acid decarboxylation in the light.…”

“…The data activity, a pattern consistent with leakage of CO2 of Rivera & Smith (1979) Leary & Osmond, 1980;Holtum et al, 1982Holtum et al, , 1983Holtum et al, , 1984. Howeverŝ uch a source for both C atoms of oxalate is not in accord with any of the current schemes for oxalate biosynthesis (see Raven et a/., 19826). A final point which involves both C^ and CAM plants relates to the much-wanted, but less frequently demonstrated, increase in N use efficiency predicted to result from the operation of the CO2 concentrating mechanism in these plants (Griffiths, 1988).…”

“…The action of the appropriate phosphatases on the first products of the carboxylase or the oxygenase activity of Rubisco yield organic acids (glyceric, glycolic) with relatively low pK^ values (respectively 3-52 and 3-38 at 25°C: Dawson et al, 1986). Further oxidations of glycolic acid yields a stronger dibasic acid, oxalic (Raven et al, 19826;Raven, 19856). Even if the phosphoglyceric acid produced by Rubisco is reduced to a compound without carboxyl functions, further metabolism can produce strong acids without the need for further COg fixation reactions.…”

“…Even if the phosphoglyceric acid produced by Rubisco is reduced to a compound without carboxyl functions, further metabolism can produce strong acids without the need for further COg fixation reactions. Examples of such oxidative production of carboxyl groups include synthesis of uronic acids, ascorbic acid (and hence, by further oxidation, oxalic and threonic or tartaric acids: Raven et al, 19826), lactic acid and even (via the glyoxylate cycle) malic acid (see Fig. 1).…”

Oscillatory Nature of Metabolism and Carbon Isotope Distribution in Photosynthesizing Cells

“…Table 1 illustrates 13 C enrichment of leaf oxalates of different C 3 and CAM-plants (Rivera & Smith, 1979;Raven et al, 1982 Table 1. Carbon isotope ratio of leaf biomass and oxalates of some oxalate accumulating plants.…”