Chloramphenicol‐ and Cycloheximide‐sensitive Protein Synthetic Systems in Brain Mitochondrial and Nerve‐ending Preparations

Mitochondria isolated from rat brain contained ribosomes, with an apparent sedimentation constant (j) of 60, which were dissociable into 45 s and 32 s subunits. The RNA associated with the 45 s subunit had an s value of 16; the RNA of the 32 s subunit had an s value of 13. These values were essentially like those found for the mitochondria of rat liver. Nascent protein associated with the 60 s monosome was reduced by chloramphenicol but not by cycloheximide. All cycloheximide‐sensitive activity found in brain mitochondria prepared from sucrose or Ficoll‐sucrose gradients and most of that associated with nerve‐ending preparations that were similarly prepared could be attributed to a contaminant that was probably a membrane‐enclosed, cytoplasmic‐like system which contained, in addition to 18 s and 28 s RNA, one or more mitochondria. Nerve‐ending preparations from which most or all of this contamination had been removed exhibited very little cycloheximide‐sensitive protein synthetic activity.