NADH:Fe(III)‐chelate reductase of maize roots is an active cytochrome b5 reductase

Abstract
Microsomal NADH:Fe(III)‐chelate reductase (NFR) of maize roots has been purified as a monomeric flavoprotein of 32 kDa with non‐covalently bound FAD. In the presence of NADH, NFR efficiently reduced the physiological iron‐chelate Fe(III)‐citrate (K cat/K m(Fe(III)‐citrate)=6.0×106 M−1 s−1) with a sequential reaction mechanism. Purified NFR was totally inhibited by the sulfhydryl reagent PHMB at 10−9 M, and it could use cyt b 5 as alternative electron acceptor with a maximal reduction rate as high as with Fe(III)‐citrate. We conclude that in maize roots the reduction of Fe(III)‐citrate is chiefly performed by a cytochrome b 5 reductase, mostly associated with intracellular membranes and in part with the plasma membrane.