Plasma membrane-bound NADH: Fe3+-EDTA reductase and iron deficiency in tomato (Lycopersicon esculentum). Is there a Turbo reductase?

The properties of NADH-dependent Fe3+-EDTA reductase in plasma membranes (PM) from roots of iron-deficient and -sufficient tomato plants [Lycopersicon esculentum L. (Mill.) cv. Abunda] were examined. Iron deficiency resulted in a 3-fold increase of in vivo root iron-chelate reductase activity with a Km (Fe3+-EDTA) of 230 μM. In purified root PM, average specific activities of ferric chelate reductase of 410 and 254 nmol Fe (mg protein)−1 min−1 were obtained for iron-deficient and -sufficient plants, respectively. In both cases, the PM-bound activity showed a pH optimum at pH 6.8. Activity depended on NADH and not on NADPH and on the presence of detergent. The activity was inhibited 40-50% by superoxide dismutase (EC 1.15.1.1) and ca 30% by oxygen. Kinetic analysis of the membrane-bound enzyme revealed a Km (Fe3+-EDTA) of ca 200 μM for both iron-stressed and -sufficient plants. For NADH, Km values around 230 μM were obtained. The ferric chelate reductase could be solubilised from salt-washed PM with Triton X-100 at a protein:detergent ratio of 1:2.8 (w/w). The Triton-soluble fraction revealed one enzyme-stained band in native polyacrylamide electrophoresis. Although the membranes showed no nitrate reductase (NR; EC 1.6.6.1) activity, anti-spinach NR immunoglobulin G (IgG) recognized a 54 kDa band both in the PM and the Triton-soluble fraction, but not in the enzymatically active material obtained from the native gel. No evidence could be found for the synthesis of a new, biochemically distinct PM-bound ferric chelate reductase under iron deficiency, which might be identified as the so-called Turbo reductase. It is concluded that iron deficiency in tomato induces increased expression of a ferric chelate reductase in root PM, which is already present in iron-sufficient plants and probably also in plants, which do not contain the Turbo reductase, like the grasses. The iron reductase is not identical with the recently reported PM-associated nitrate reductase.