Photosynthetic Nitrite Reduction by Dithioerythritol and the Effect of Nitrite on Electron Transport in Isolated Chloroplasts

Photosynthetic reduction of nitrite to ammonia with type C chloroplasts from the heterocont alga Bumilleriopsis filiformis was investigated using 3,6‐diaminodurene/ascorbate and 3,6‐diaminodurene/dithioerythritol (DAD/DTE) as electron donor couple. Rates approach 6–10 μmol NO ‐ 2 reduced/mg chlorophyll/h and are steady for up to 30 min. The presence of oxygen or NADP + only slightly diminished the rates of nitrite reduction obtained with DAD/DTE. Illuminated chloroplasts reduce oxygen in the presence of DAD/DTE at 135 μmol/mg chlorophyll/h without acceptor supplied. Photosynthetic oxygen uptake by this system in the presence of ferredoxin and NO ‐ 2 , however, is inhibited to 42% by nitrite reductase with concurrent nitrite reduction. NO ‐ 3 and NO ‐ 2 have no effect on photosystem I‐mediated NADP + reduction, NO ‐ 2 (10 m M ) inhibits ferricyanide‐mediated oxygen evolution to 72%. Also photosystem II reactions assayed e.g. with silicomolybdate are inhibited significantly by NO ‐ 2 (1 m M ), but only slightly by NO ‐ 3 . Nitrite reductase is inhibited by p ‐chloromercuribenzoate ( p CMB), and this inhibition is prevented by DTE. Results suggest that photosynthetic nitrite reduction can cope with low concentrations of either compound, provided relevant thiol groups are protected.