Isolation and characterization of a subchloroplast particle enriched in iron-sulfur protein and P700

Treatment of isolated spinach thylakoid fragments with Triton X-100 followed by differential centrifugation and Sephadex G-200 and DEAE Bio-Gel A chromatography results in isolation of two distinct particles containing iron-sulfur protein. The first is a P700-containing particle that contains 8-10 g-atoms each of nonheme iron and labile sulfide and 23 chlorophylls per mole of P700 but no detectable levels of chlorophyll b or cytochromes f, b₆, or b₅₅₉. The second particle exhibits no P700 activity but does contain cytochromes f and b₆ in equimolar amounts in addition to 2-4 g-atoms each of nonheme iron and labile sulfide per mole of cytochrome f. Virtually all the nonheme iron and labile sulfide present in spinach thylakoids is accounted for in these two particles. Further treatment of the P700-enriched particle with urea and potassium ferricyanide causes a time-dependent loss of labile sulfide in concert with the loss of photoactive P700. In contrast, the environmental integrity of P700 is unaffected by this treatment since there is no corresponding absorbance change in the chemical oxidized-minus-reduced difference spectrum. Control levels of labile sulfide are reestablished in the depleted particles by overnight treatment with dithiothreitol. Pretreatment of the depleted particles with cyanide prevented the recovery of labile sulfide by preincubation with dithiothreitol. In accord with these data, a mechanism is invoked for the oxidation of labile sulfide to zero-valence sulfur, S⁰, in the bound iron-sulfur proteins, which results in destruction of the iron-sulfur core.