Stabilized, NAD(P)H-dependent, soluble nitrate reductase, a process for the preparation thereof and a reagent containing it

A stabilized NAD(P)H-dependent, soluble nitrate reductase enzyme of the assimilartory type, having a molecular weight of about 90,000 daltons as determined by SDS electrophoresis wherein said nitrate reductase enzyme retains greater than 60% residual activity after 3 weeks at 35.degree. C. and is obtainable by a method which comprises:

• (a) preparing a suspension of a comminuted biological starting material having nitrate reductase activity in a tris/tartaric acid buffer at a pH of from 7 to 8.5;
• (b) adding a soluble polyethyleneimine thereto in an amount sufficient to form a precipitate containing from 10 to 20% of nitrate reductase activity of said suspension and a supernatant containing from 80 to 90% of nitrate reductase activity of said suspension;
• (c) separating precipitate from supernatant;
• (d) fractionating said supernatant in a tris/tartaric acid buffer to form a solution containing said nitrate reductase enzyme;
• (e) dialyzing said solution to separate said nitrate reductase enzyme therefrom;
• and
• (f) lyophilizing said nitrate reductase enzyme in a zwitterionic buffer.