Purification and Properties of Nitrite Reductase from Spinach Leaves

Nitrite reductase (EC 1.6.6.4) has been purified 730-fold from spinach leaves. The enzyme catalyzes the reduction of nitrite to ammonia, with the use of reduced form of methyl viologen and ferredoxin. A stoichiometry of one molecule of nitrite reduced per molecule of ammonia formed has been found. KCN at 2.5×10^-4 m inhibited nitrite reductase activity almost completely. Purified enzyme was almost homogeneous by disk electrophoresis with polyacrylamide gel. The molecular weight of the enzyme was estimated to be 61,000 from gel filtration. Nitrite reductase, in the oxidized form, has absorption maxima at 276, 388 and 573 mμ. Both methyl viologen and ferredoxin linked nitrite reductase activities of the enzyme were inactivated on exposure to low ionic strength.