On the decarboxylation of α-keto acids by isolated chloroplasts

The mechanism of the decarboxylation of α-keto acids by isolated chloroplasts has been studied with the aid of superoxide dismutase and catalase. Using photosynthetic and enzymatic systems, which are known to catalyze peroxidic oxidations, we have been able to demonstrate that both the superoxide free radical ion and H₂O₂ are necessary for maximal rates of decarboxylation. In isolated chloroplasts, an auto-oxidizable electron acceptor as well as an electron donor for Photosystem I are absolute requirements for the decarboxylation. H₂O₂ seems to be the primary oxidant in the decarboxylation of pyruvate or glyoxylate by isolated chloroplasts. A secondary rate of decarboxylation is superimposed on the primary one, mediated by superoxide free radical ion. Mn²⁺ stimulates the decarboxylation probably via intermediarily-formed Mn³⁺ in a reaction, which is neither inhibited by catalase nor by superoxide dismutase. A decarboxylation of pyruvate or glyoxylate by isolated chloroplasts in the presence of NADP⁺ is initiated, as soon as the available NADP⁺ is fully reduced. In this case, the open-chain electron transport seems to switch from NADP⁺ to oxygen as the terminal electron acceptor.