A low-spin iron complex in human melanoma and rat hepatoma cells and a high-spin iron(II) complex in rat hepatoma cells.

Human melanoma and rat hepatoma cells cultured in the presence of low concentrations (2.5 microM) of low-molecular-weight iron (Fe) chelates and Fe-transferrin complexes have been studied with 57Fe M?ssbauer spectroscopy. The spectra show that holoferritin is only a minor fraction of the total iron present in the cells. The major form of Fe was in a low-spin state unlike the high-spin Fe(III) found in ferritin. Only about 10% of the Fe could be attributed to ferritin. In addition, the hepatoma cells had a high-spin Fe(II) spectral component which made up about 20% of the Fe present.     

Role of Bradyrhizobium japonicum cytochrome c550 in nitrite and nitrate respiration.

Bradyrhizobium japonicum cytochrome c(550), encoded by cycA, has been previously suggested to play a role in denitrification, the respiratory reduction of nitrate to dinitrogen. However, the exact role of this cytochrome in the denitrification process is unknown. This study shows that cytochrome c(550) is involved in electron transfer to the copper-containing nitrite reductase of B. japonicum, as revealed by the inability of a cycA mutant strain to consume nitrite and, consequently, to grow under denitrifying conditions with nitrite as the electron acceptor. Mutation of cycA had no apparent effect on methylviologen-dependent nitrite reductase activity. However, succinate-dependent nitrite reduction was largely inhibited, suggesting that c(550) is the in vivo electron donor to copper-containing nitrite reductase. In addition, this study demonstrates that a cytochrome c(550) mutation has a negative effect on expression of the periplasmic nitrate reductase. This phenotype can be rescued by extending the growth period of the cells. A model is proposed whereby a mutation in cycA reduces expression of the cbb(3)-type oxidase, affecting oxygen consumption rate by the cells and consequently preventing maximal expression of the periplasmic nitrate reductase during the first days of the growth period.