Ferredoxin-sulfite reductase from spinach.

Abstract

Ferredoxin-sulfite reductase (Fd-SiR) [hydrogen-sulfide: ferredoxin oxidoreductase, EC 1.8.7.1] from spinach leaves has been purified to homogeneity by a new procedure. Subunit analysis by sodium dodecyl sulfate gel electrophoresis yielded a single protein band with a molecular weight of 71, 000. Gel electrophoresis in non-denaturing media at different acrylamide concentrations gave a molecular weight of 270, 000, suggesting that the native enzyme was composed of four identical subunits. In the presence of 0, 2M sodium chloride, however, gel filtration produced a value of 136, 000, indicating the presence of dimer in this ionic environment. A plot of substrate (sulfite) concentration versus enzymatic (Fd-SiR) activity yielded a sigmoidal curve, giving a Hill coefficient (ñ) of 2.1. Purified Fd-SiR, in the oxidized form, had absorption maxima at 279, 385, 588 and 714 nm. Thus the enzyme has the property of a siroheme-containing protein.