Investigation of the form of selenium in the hydrogenase from chemolithotrophically cultured Bradyrhizobium japonicum

It has been established that the hydrogenase from autotrophically cultured Bradyrhizobium japonicum contains selenium as a bound constituent. About 80% of the enzyme selenium remains bound during precipitation with 5% trichloroacetic acid (TCA). However, 85% of the selenium bound to the enzyme is released by a combined treatment of urea, heat and TCA. Neither selenomethionine nor selenocysteine could be detected on analysis of anaerobically hydrolyzed enzyme. These results are consistent with the report showing that the structural genes for this enzyme do not contain a TGA codon (Sayavedra-Soto et al. 1988) which has been reported to code for selenocysteine incorporation into several proteins (Chambers et al. 1986; Zinoni et al. 1986; Stadtman 1987). We have demonstrated that ⁷⁵Se from the labeled hydrolyzed enzyme forms the derivative' selenodicysteine. The form of selenium resulting in the synthesis of this derivative apparently is SeO_inf3^sup=or a compound such as Se⁼ which is easily oxidized to SeO_inf3^sup=. In a separate approach it was established that 12–16% of the total ⁷⁵Se in the native enzyme reacted with 2,3-diaminonaphthalene indicating that this fraction was present as SeO_inf3^sup=. The remaining ⁷⁵Se was bound to the enzyme protein. From this research, we concluded that Se in Bradyrhizobium japonicum hydrogenase is present in a labile bound form. In this respect, this enzyme is similar to xanthine dehydrogenase and nicotinic acid hydroxylase, both of which contain labile Se constituents that have not been defined.Technical paper no. 8980 from the Oregon Agricultural Experiment Station