Characterization of a novel NADH-specific, FAD-containing, soluble reductase with ferric citrate reductase activity from maize seedlings

A novel NADH-dependent, soluble flavoreductase of 60 kDa, active toward fer ric chelates and quinones, has been purified from maize seedlings. Two clos ely related isoforms were separated. The two isoforms are similar in severa l biochemical features, with the exception of the apparent molecular mass o f their subunits (29 and 31 kDa, respectively). They are homodimers in the native state, they bind FAD as the prosthetic group and show strong prefere nce for NADH over NADPH as the electron donor, Ferric chelates (chiefly fer ric citrate, K-m 3-5 x 10(-5) M; k(cat)/K-m 3.4-3.7 x 10(5) M-1 s(-1)), and some quinones (benzoquinone, coenzyme Q-0, and juglone) are used as electr on accepters. Enzymatic reduction of benzoquinone occurs with formation of radical semiquinones. Both soluble ferric chelate reductase isoforms are st rongly inhibited by p-hydroxymercuribenzoic acid (I-50, 5 nM) and by cibach ron blue, the latter giving nonlinear inhibition. It is suggested that solu ble ferric chelate reductase might be involved in the symplastic reduction of iron chelates which is required for the assembly of iron-containing macr omolecules such as cytochromes and ferritin. (C) 1999 Academic Press.