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NADH-nitrate reductase has been highly purified from leaves of 8-day-old wheat (Triticum aestivum L. cv. Olympic) seedlings by affinity chromatography, using blue dextran-Sepharose 4B. Purification was assessed by polyacrylamide gel electrophoresis. The enzyme was isolated with a specific activity of 23 micromoles nitrite produced per minute per milligram protein at 25 C. At pH 7.5, the optimum pH for stability of NADH-nitrate reductase, this enzyme, and a component enzyme reduced flavin adenine mononucleotide (FMNH(2))-nitrate reductase has a similar stability at both 10 and 25 C. Two other component enzymes-methylviologen-nitrate reductase and NADH-ferricyanide reductase-also have a similar but higher stability. At this pH the Arrhenius plot for decay of NADH-nitrate reductase and methylviologen-nitrate reductase indicates a transition temperature at approximately 30 C above which the energy of activation for denaturation increases. FMNH(2)-nitrate reductase and NADH-ferricyanide reductase do now show this transition. The energy of activation for denaturation (approximately 9 kcal per mole) of each enzyme is similar between 15 and 30 C. The optimum pH for stability of the component enzymes was: NADH-ferricyanide reductase, 6.6; FMNH(2)-nitrate reductase and methylviologen-nitrate reductase, 8.9. All of our studies indicate that the NADH-ferricyanide reductase was the most stable component of the purified nitrate reductase (at pH 6.6, t((1/2)) [25 C] = 704 minutes). Data are presented which suggest that methylviologen and FMNH(2) do not donate electrons to the same site of the nitrate reductase protein.

In Vitro Stability of Nitrate Reductase from Wheat Leaves