X-band EPR spectrum of F417L recombinant human ferrochelatase. Spectra are from whole cells of E coli DW35 (A) with the plasmid encoding wild-type human ferrochelatase, (B) with the plasmid encoding F417L human ferrochelatase, and (C) alone, containing no plasmid. Cell pellets were washed and suspended anerobically in 0.1 mol/L Tris MOPS, pH 8.1, containing an excess of sodium dithionite. Immediately before freezing, approximately 10% (vol/vol) chloroform was added. EPR conditions: temperature, 35°K; microwave power, 10 mW; microwave frequency, 9.60 Ghz; modulation amplitude, 0.64 mT.