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

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 X 10[6] 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 b5 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 b5 reductase, mostly associated with intracellular membranes and in part with the plasma membrane.