Enzymic Assimilation of Nitrate in Tomato Plants. I. Reduction of Nitrate to Nitrite

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Abstract
A study is presented of the properties of nitrate reductase extracted from tomato leaves and roots: cysteine is required for its extraction in an active state. At pH 7.5 (the pH optimum) the enzyme was specific for diphosphopyridine nucleotide. Presence of nitrate reductase in tomato roots was confirmed by its extraction from excised roots grown in sterile culture. The level of activity in excised roots was comparable to that in roots of intact plants. Nitrate reductase was also detected in 6 other species. It is suggested that failure to account for the sensitivity of this enzyme to environmental factors such as light and nitrate supply and to extraction procedures is probably responsible for any inability to detect the enzyme. A cell-free reductase system obtained from tomato plants is described which brings about the rapid (within 60 seconds) reduction of nitrite in the dark, but illuminated grana are required for sustained activity. Cysteine was required for the extraction of this enzyme system in an active state. The Michaelis constant for nitrite was 2.7 x 10-6 [image] and ammonia was the product of nitrite reduction. The enzyme would reduce nitrite in the absence of grana when reduced benzyl viologen was used as the electron donor. Reduced pyridine nucleotides were ineffective in bringing about nitrite reduction. The natural electron donor is unidentified but it was shown to be present on all organs of the tomato plant. The demonstration of the presence of an enzyme system in plant organs reducing nitrite to ammonia supports the supposition that nitrate reductase is involved in nitrate assimilation in higher plants.