Ferredoxin-sulfite reductase and ferredoxin-nitrite reductase activities in leaves of Pisum sativum: Changes during ontogeny and in vitro regulation by sulfide

Since ferredoxin‐dependent sulfite reductase (EC 1.8.7.1) and nitrite reductase (EC 1.7.7.1) can both catalyze the reduction of SO2‐3 and NO−2, physiological and biochemical evidence is needed for properly classifying the two enzyme activities. They were therefore compared during ontogeny of pea leaves and in the effect of their products, sulfide and ammonium, on their catalytic activity. In the crude extract of the young second leaf of pea plants, Pisum sativum L. cv. Vatters Frühbusch, no ferredoxin‐nitrite reductase activity could be detected, but ferredoxin‐sulfite reductase and ATP‐sulfurylase (EC 2.7.7.4), measured for comparison, were at 24 and 14%, respectively, of their maximal activity per leaf. After 11 and 12 days, respectively, ATP‐sulfurylase and ferredoxin‐sulfite reductase were no longer detectable, whereas ferredoxin‐nitrite reductase was still at more than 30% of its maximal activity per leaf. Ferredoxin‐sulfite reductase was inhibited by 50% with 18 μM and 100% with 30 μM sulfide produced by this enzyme during its assay. Sulfide at 100 μM added to the assay mixture completely inhibited ferredoxin‐sulfite reductase activity in the crude extract, the 30000 g pellet and its supernatant. The same addition reduced ferredoxinnitrite reductase activity by 20% in the crude extract and by 100% in the 30000 g pellet. NH+4 at 100 μM did not affect ferredoxin‐sulfite reductase or ferredoxin‐nitrite reductase activity. The inhibition by sulfide and the changes in activity during ontogeny similar to ATP‐sulfurylase (which catalyzes the first step of assimilatory sulfate reduction) represent biochemical and physiological evidence for the correct classification of ferredoxin‐sulfite reductase. The complete inhibition of ferredoxin‐nitrite reductase activity in the 30000 g pellet by S2‐ indicates that this activity was due to a ferredoxin‐sulfite reductase