Effect of oxygen on the synthesis and assembly of mitochondrial encoded subunits of cytochrome oxidase and cytochrome b.c1 in mouse embryo fibroblasts.

Mouse embryo fibroblasts were grown in low and control O2 for 24 h (average medium oxygen tensions, 7 torr and 143 torr, respectively). Relative to controls, there was a reduction in radiolabeled subunits in immunoprecipitates of cytochrome oxidase and cytochrome b.c1 prepared from low O2 cells. Incorporation of radiolabeled amino acids into subunit I of cytochrome oxidase and the apocytochrome b protein of the b.c1 complex ranged from 51-100% of control, whereas the appearance of these pulse-labeled subunits into holoenzymes immunoprecipitated from low O2 cells was in the range of 6-39% of control. The synthesis of subunit II of cytochrome oxidase by low O2 cells ranged from 63-100% of control, and assembly of this protein into the low O2 immunoprecipitated enzyme ranged from 15-61% of control. Thus, the data suggest that O2 had an effect on the assembly of these mitochondrially translated proteins that was independent of any effect on their synthesis.