Molybdenum and iron as functional consitituents of the enzymes of the nitrate-reducing system of Azotobacter chroococcum.

The roles of molybdenum and iron in the enzymes of the assimilatory nitrate-reducing system from Azotobacter chroococcum have been investigated. 1. By adding 99 Mo-molybdate to a cell culture of A. chrocococcum with nitrate as the nitrogen source, it has been possible to incroporate the radioactive metal into a purified preparation of the enzyme nitrate reductase. 2. When 185 W-tungstate was supplied to a culture medium lacking added molybdate, a 185 W-labelled nitrate reductase preparation with negligible activity could be obtained. This in vivo incorporation of tungsten was competitively hindered by molybdenum. 3. The cellular level of nitrite reductase activity gradually increased in response to the addition of increasing amounts of iron to the culture medium. Under the same conditions, of the level of nitrate reductase activity was not affected.