Role of the superoxide free radical ion in photosynthetic ascorbate oxidation and ascorbate-mediated photophosphorylation

The mechanism of ascorbate photooxidation in isolated chloroplasts has been studied. The enzyme superoxide dismutase has been used as a tool to show that ascorbate is oxidized by the superoxide free radical ion, which is formed during the autooxidation of a low-potential electron acceptor.

In the absence of an artificial, low-potential electron acceptor, addition of ascorbate stimulates photophosphorylation in isolated chloroplasts. This effect of ascorbate is abolished by superoxide dismutase, indicating that both the superoxide free radical ion and ascorbate are responsible for the stimulation of photophosphorylation. In this case, the superoxide free radical ion seems to be formed during the autooxidation of an endogenous electron acceptor.

In the presence of ferredoxin and NADP⁺, photophosphorylation in isolated chloroplasts stops as soon as the available NADP⁺ is fully reduced. If ascorbate is present in this system, however, a linear rate of photophosphorylation is maintained in spite of the fact, that NADP⁺ is fully reduced. This ascorbate-mediated photophosphorylation again is abolished by superoxide dismutase.

During the catalysis of this oxygen-dependent photophosphorylation, ascorbate consumption is not observed. These findings support the idea, that in chloroplasts ascorbate together with the superoxide free radical ion may function in providing additional ATP by an oxygen-dependent photophosphorylation.