Protein synthesis by brain-cortex mitochondria. Characterization of a 55S mitochondrial ribosome as the functional unit in protein synthesis by cortex mitochondria and its distinction from a contaminant cytoplasmic protein-synthesizing system

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Abstract
Homogenates of rat brain cortex were fractionated by conventional methods of velocity sedimentation and separated into a microsomal and a washed mitochondrial fraction. By electron microscopy the mitochondrial fraction was shown to be rich in synaptosomes. The mitochondria–synaptosome fraction synthesized protein in vitro by a route that was partially inhibited by cycloheximide and partly by chloramphenicol. The relative effectiveness of the two inhibitors varied greatly with the medium used. In the mitochondria–synaptosome fraction active 80S cytoplasmic ribosomes and active 55S mitochondrial ribosomes were detected; these were also seen in the electron microscope. Mild osmotic shock of the mitochondria–synaptosome fraction followed by velocity sedimentation in sucrose–EDTA allowed isolation of a mitochondrial fraction free of synaptosomes. Protein synthesis in this fraction was entirely inhibited by chloramphenicol, but was completely resistant to cycloheximide both in a medium promoting oxidative phosphorylation and in ATP-generating medium. Ouabain had no inhibitory effect on protein synthesis in a purified mitochondrial preparation. It is concluded that brain-cortex mitochondria synthesize protein entirely on 55S mitochondrial ribosomes.