Effect of nitrite upon leghemoglobin and interaction with nitrogen fixation.

Nitrite (0.4 mM) added to soybean bacteroid preparations strongly inhibited C2H2 reduction. In the presence of leghemoglobin (0.1mM), a 3-fold enhancement of nitrogen fixation occurred but the inhibitory effect of nitrite was delayed. Spectra of leghemoglobin showed a rapid disappearance of the 574 nm and 541 nm peaks of oxyleghemoglobin the presence of nitrite. Concomitant oxidation of this hemoprotein gave ferric leghemoglobin as the single final product. High nitrite levels could depress nitrogen fixation both by inactivation of nitrogenase and by conversion of leghemoglobin into an inactive form. Nitrite present at low concentrations reacts with this hemoprotein and is then no longer able to penetrate into bacteroids.