Effect of Casein on the Extractability and Stability of Nitrate Reductase From Wheat Leaves

The effect of casein on the extraction and stability of nitratereductase (EC 1.6.6.1 EC] ) from the primary leaf of the wheat (Triticumaestivum L.) cultivar Olympic was examined. The use of 2 percent casein in the extraction medium resulted in a small butsignificant (P > 0·05) increase in nitrate reductaseactivity at the time of assay. This increase in activity wasnot attributable to improved extraction but was a result ofincreased stability of the enzyme between time of extractionand assay. The presence of casein actually resulted in lessactivity being extracted during the grinding procedure. Othercultivars of wheat examined showed a small increase in activityat the time of assay when extracted in the presence of casein.However, the ranking of these cultivars with respect to nitratereductase activity did not alter markedly when casein was used.A number of other species examined showed a wide variation inresponse to casein, while buckwheat (Fagopyrum esculenta) andoats (Avena sativa) had an absolute requirement, maize (Zeamays cv. XL45) was relatively unaffected. Nitrate reductasein crude extracts decayed exponentially with time at both 0°and 10 °C with a half-life of 216 and 109 min respectively.Casein added before or after extraction completely stabilizedthe enzyme over a period of several hours at 0 °C whileat 10 °C the enzyme had a half-life of approximately 16h. The effect of casein at both 0° and 10 °C was independentof the concentration used within the range 0·5 to 4·0per ant. casein, genotype, nitrate reductase, stability, Triticum aestivum L.