Comparison of Protein Synthesis in Mitochondria, Synaptosomes, and Intact Brain Cells

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Abstract
Qualitative aspects of protein synthesis in organelles and intact cultured cells of [rat] brain origin were compared to clarify the distinction between synaptosomal and mitochondrial protein synthesis. Brain mitochondria and synaptosomes were isolated either on a traditional Ficoll-sucrose gradient or by a new Percoll gradient procedure; they were incubated in an amino acid incorporation system containing [35S]methionine, and then were electrophoresed on gradient slab gels. Autoradiography of the gels revealed that, in the presence of cycloheximide, both mitochondria and synaptosomes synthesized at least 17 proteins in the 6000-50,000 MW range, and that incubation with chloramphenicol reduced or eliminated these bands. With minor variation, these patterns in the low MW region also resembled patterns obtained from cycloheximide-inhibited rat liver mitochondria and intact brain cells (cultured glia, glioma and neuroblastoma). In the higher MW region of the gels (> 50,000), banding patterns were more complex, and they tended to differ between organelles and intact cells. These polypeptides probably reflect nonmitochondrial protein synthesis; their variable response to inhibitors may account for confusion in the literature with regard to the effects of inhibitors of protein synthesis in brain mitochondria and synaptosomes.