Purification and characterization of the membrane-bound ferrochelatase from Spirillum itersonii

The membrane-bound enzyme ferrochelatase (protoheme ferro-lyase, EC 4.99.1.1) was purified from isolated membrane fragments of Spirillum itersonii approximately 490-fold. Purification was achieved by solubilization with chaotropic salts followed by ammonium sulfate fractionation, diethylaminoethyl-cellulose chromatography, and gel filtration on Sephadex G-200. The purified enzyme has an apparent minimum molecular weight of approximately 50,000, as determined by gel filtration in the presence of 0.1% Brij 35 and 1 mM dithiothreitol but forms high-molecular-weight aggregates in the absence of detergent. Purified ferrochelatase is strongly stimulated in the presence of copper. The apparent Km for Fe2+ is 20 micrometer in the absence of copper and 9.5 micrometer in the presence of 20 micrometer CuCl2. The apparent Km for protoporphyrin is 50 micrometer, and it is unaltered by copper. Ferrochelatase has a single pH optimum of 7.50, and it is inhibited 50% by 20 micrometer heme. Certain divalent cations and sulfhydryl reagents also inhibit the enzyme.