A DISSIMILATORY NITRATE REDUCTASE FROM NEUROSPORA CRASSA.

Abstract

A dissimilatory nitrate reductase (cytochrome: nitrate oxidoreductase, EC 1.9.6.1) has been purified 35-fold from submerged felts of Neurospora crassa . It requires iron as well as molybdenum for its activity and is thus similar to the dissimilatory enzyme found in denitrifying bacteria. Reduced methyl or benzyl viologen was the most effective hydrogen donor and under these conditions there is no evidence for a flavin requirement. NADPH 2 , an effective hydrogen donor for the assimilatory nitrate reductase (NADH 2 : nitrate oxidoreductase, EC 1.6.6.1 and NAD(P)H 2 : nitrate oxidoreductase, EC 1.6.6.2), did not function for the purified dissimilatory enzyme. Although iron concentrated in purified fractions of the enzyme the most purified preparation conatined less iron than did less pure ones. It is shown that the purified enzyme contains a molybdenum-protein component which may also be common to the assimilatory enzyme. Assimilatory and dissimilatory nitrate reduction systems differ only in the penultimate electron transfer sequence to the terminal nitrate reductase which is a molybdenum-containing protein. There is no close parallel between the activity of the dissimilatory BVH: nitrate oxidoreductase and that of the NADPH 2 : cytochrome c oxidoreductase, (EC 1.6.2.3) in the most purified fractions as is known to occur with the assimilatory NADPH 2 : nitrate reductase and cytochrome c reductase. The purified BVH: nitrate reductase does not utilize NADPH 2 or FADH 2 as hydrogen donors and has negligible NADPH 2 : cytochrome c oxidoreductase activity.