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Metabolism and Enzymology

Effect of Inorganic Orthophosphate on in Vitro Activity of NADH-Nitrate Reductase Isolated from 2-Row Barley Leaves

Department of Agricultural Chemistry, Faculty of Agriculture, Kobe University, Rokkodai-cho, Nada-ku, Kobe 657, Japan

Inorganic orthophosphate (25 millimolar in assay media; Pi) was found to increase in vitro activity of NADH-nitrate reductase (NR) isolated from 2-row barley (Hordeum vulgare L.) leaves with a saturating concentration of nitrate (2 millimolar) but to decrease it with low nitrate levels (<0.1 millimolar). The response to nitrate concentrations was Pi specific. The Lineweaver-Burk plot showed that Pi increases the apparent K_m for nitrate as well as V_max, whereas it does not alter the K_m for NADH significantly. These results suggest that the interaction between a molybdenum site of the enzyme and Pi results in alteration of the properties of NR molecule.