Further characterization of the assimilatory nitrate reductase from the yeast Candida nitratophila

Nitrate reductase from the yeast Candida nitratophila was found to contain one molecule of cytochrome b557 and one atom of molybdenum per subunit. FAD/haem-dependent diaphorase activity (haem domain) was associated with a 40 kDa tryptic fragment of the subunit. The 50 amino-terminal residues of this fragment were determined, and the sequence did not show significant similarity to deduced sequences of other nitrate reductases previously published. Increasing ionic strength in vitro had a stimulatory effect on enzymic activity via stimulation of the molybdenum-dependent terminal nitrate-reducing activity. Stimulation of activity by exogenous protein (bovine serum albumin or casein) also appeared to be an ionic effect. Stimulation of catalytic activity by phosphate was a separate effect.