Inhibition of photosynthetic carbon metabolism in isolated chloroplasts by iodoacetol phosphate

Methylglyoxal functions as Hill oxidant and stimulates the photoreduction of O(2) at photosystem I: a symptom of plant diabetes

We elucidated the metabolism of methylglyoxal (MG) in chloroplasts of higher plants. Spinach chloroplasts showed MG-dependent NADPH oxidation because of aldo-keto reductase (AKR) activity. K(m) for MG and V(max) of AKR activity were 6.5 mm and 3.3 µmol NADPH (mg Chl)(-1) h(-1) , respectively. Addition of MG to illuminated chloroplasts induced photochemical quenching (Qp) of Chl fluorescence, indicating that MG stimulated photosynthetic electron transport (PET). Furthermore, MG enhanced the light-dependent uptake of O(2) into chloroplasts. After illumination of chloroplasts, accumulation of H(2) O(2) was observed. K(m) for MG and V(max) of O(2) uptake were about 100 µm and 200 µmol O(2) (mg Chl)(-1) h(-1) , respectively. MG-dependent O(2) uptake was inhibited by 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) and 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone (DBMIB). Under anaerobic conditions, the Qp of Chl fluorescence was suppressed. These results indicate that MG was reduced as a Hill oxidant by the photosystem I (PSI), and that O(2) was reduced to O(2) (-) by the reduced MG. In other words, MG produced in chloroplasts is preferentially reduced by PSI rather than through AKR. This triggers a type of oxidative stress that may be referred to as 'plant diabetes', because it ultimately originates from a common metabolite of the primary pathways of sugar anabolism and catabolism.

Inhibition of the photosynthetic activities of isolated spinach chloroplasts by phosphonate compounds

The effects of three closely related phosphonate compounds on several photosynthetic activities of isolated chloroplasts were investigated. Phosphonoformic and phosphonopropionic acid were found to inhibit both CO(2) fixation and the reduction of 3-phosphoglyceric acid, with CO(2) fixation being more sensitive. In contrast, phosphonoacetic acid was only slightly inhibitory. The lack of inhibition appeared to be due to its inability to enter the stroma via the phosphate translocator. Measurements of changes in stromal metabolite levels following the inhibition of CO(2) fixation by either phosphonoformic or phosphonopropionic acid indicated that the activity of ribulose bisphosphate carboxylase/oxygenase was reduced. Studies with the isolated enzyme confirmed that both of these compounds were effective competitive inhibitors of the carboxylase activity of the enzyme.