Abstract
Abstract The cytochrome “ a 1 ” or P-428, previously proposed to be a high affinity terminal oxidase in nitrogen-fixing bacteroids of Bradyrhizobium japonicum has been purified. The water-soluble native hemoprotein has an M r of 136 000, lacks heme a and is a high-spin ferric protohemoprotein: It is slowly reduced with dithionite to give a species with an optical spectrum like that of hemoprotein b -590 ( Escherichia coli ; peak at 555 nm, shoulder at 590 nm), and which reacts slowly with CO. It has catalase and peroxidase activities, again resembling the E. coli b -590. Neither hemoprotein forms a stable oxy complex under conditions in which dithionite-reduced horseradish per-oxidase reacts with oxygen to form such a complex. The hemoprotein, which we name hemoprotein, b -590 ( Bradyrhizobium japonicum , may play a role in removal of peroxides generated during respiration in the bacteroids of several Rhizobium and Bradyrhizobium species. The high-affinity terminal oxidase under nitrogen-fixing conditions remains to be identified.
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