Isolation and characterization of nitrate reductase from the halophilic sulfur-oxidizing bacterium Thioalkalivibrio nitratireducens

A novel nitrate reductase (NR) was isolated from cell extract of the haloalkaliphilic bacterium Thioalkalivibrio nitratireducens strain ALEN 2 and characterized. This enzyme is a classical nitrate reductase containing molybdopterin cofactor in the active site and at least one iron-sulfur cluster per subunit. Mass spectrometric analysis showed high homology of NR with the catalytic subunit NarG of the membrane nitrate reductase from the moderately halophilic bacterium Halomonas halodenitrificans. In solution, NR exists as a monomer with a molecular weight of 130-140 kDa and as a homotetramer of about 600 kDa. The specific nitrate reductase activity of NR is 12 mu mol/min per mg protein, the maximal values being observed within the neutral range of pH. Like other membrane nitrate reductases, NR reduces chlorate and is inhibited by azide and cyanide. It exhibits a higher thermal stability than most mesophilic enzymes.