The Inducible Formation and Stability of Nitrate Reductase in Higher Plants: I. EFFECTS OF NITRATE AND MOLYBDENUM ON ENZYME ACTIVITY IN CAULIFLOWER (BRASSICA OLERACEAVAR. BOTRYTIS)
The enzyme nitrate reductase could not be detected in leaf tissues of cauliflower plants grown in sterile cultures with glutamic acid or ammonium sulphate if nitrate was absent. Excised leaf tissues from these plants formed the enzyme for several hours at a steady rate when infiltrated with nitrate. Plants starved of nitrate for short periods lost enzyme activity which was restored in excised tissues upon infiltration with nitrate but not with ammonium sulphate or nitrite. Molybdenum-deficient plants grown with nitrate also lacked enzyme activity which was restored in excised tissues after infiltration with molybdenum. Both nitrate and molybdenum were required to produce maximal rates of enzyme formation in excised tissues of plants grown with ammonium sulphate and no molybdenum. Apparent Michaelis constants for nitrate and molybdenum were found to be about 10-5 and 10-7 respectively. The capacity of excised tissues to respond to the inducer varied with their age and leaf position on the plant and was exercised under conditions where growth was unlikely. Increases in specific activities were similar. There was no evidence of a lag in response to nitrate or molybdenum with tissues of plants grown with ammonium sulphate or glutamic acid in sterile cultures but lag periods were observed with tissues from plants deprived of nitrate. Cell-free preparations were unable to respond to either factor. The results are interpreted as evidence for induced enzyme formation in vivo in response to the substrate or the constituent metal.