Assuming you already have the enzyme solution, reactants, and products of nitrate reductase, this may be helpful. The first half is just the stuff that you should already know, but the second half outlines a classic enzyme kinetics experiment.

thanks,perhaps I have questions again.

Hi Ramin,

this is pretty old school but should be exactly what you are looking for. I used it a long time ago for exactly that purpose and it worked well.

http://www.biochemj.org/bj/164/0199/1640199.pdf

thanks Jochen

Hi Rahim

5mL of supearnatant + 5 mL of 30mM potassium nitrate + 2.5mL of 1% sulphanilamide in 25N HCL + 2.5 mL of 0.2% NED solution in water == Read at 540 nm to measure the amount of nitrite formed by nitrate reductase activity and calculate by nitrite standard curve

Can i use silver nitrate?

I think you can as your interest here is only nitrate. But as you know that SPR of Silver nanoparticles is also about 420-450 nm that may also contribute to the abs which is due to nitrite formation. Just check it out and bye the way Silver nitrate is costlier than potassium nitrate so avoid it.

Redox sensor Green is a reliable marker of bacterial reductase activity