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

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.