The Relation of Glycolic Acid Synthesis to the Primary Photosynthetic Carboxylation Reaction in Leaves.

The studies initiated by Calvin and his colleagues led to the elucidation of a well supported sequence of reactions known as the photosynthetic carbon reduction cycle (1). According to this scheme, which was largely deduced from kinetic experiments, ribulose-1,5-di-P is first carboxylated to give 2 molecules of 3-P-glyceric acid as the first stable carboxylation product (2, 3). The experiments from which the reactions of the photosynthetic cycle were inferred were always conducted at high concentrations of COz in air, 1.0% or more. Wilson and Calvin (4), however, found that on lowering the COZ concentration glycolic acid was pr.oduced in large amounts, and they suggested that it was derived from an intermediate of the photosynthetic cycle, presumably ribulose-5-P. The recent finding of a specific P-glycolic acid phosphatase in leaves (5) was taken as further evidence that glycolic acid is a secondary product of the cycle. On the other hand, I have shown that when the oxidation of glycolic acid in leaves is blocked with a suitable ol-hydroxysulfonate (6), as much as half of the 14C02 fixed during photosynthesis in air may accumulate as glycolic acid (7). In addition, Warburg and Krippahl (8) have reported a 92% conversion into glycolic acid of the COz taken up during photosynthesis by Chlmella when the cells were maintained in an atmosphere containing not more than 0.1 to 0.2% COZ. This observation implies that glycolic acid may be synthesized by a carboxylation reaction different from that catalyzed by ribulose diphosphate carboxylase. Experiments were therefore carried out to determine the optimal conditions for glycolic acid synthesis in leaves, and to attempt to discover whether glycolic acid is derived by cleavage of a metabolite that is part of the photosynthetic cycle or by an additional carboxylation. The results obtained from measurements of specific activity provide evidence that glycolic acid is synthesized in the light from COZ by reactions that are largely separate from those of the photosynthetic cycle, especially at concentrations of CO2 close to those normally found in air.