Cerebral Protein Synthesis

Sedimentation analyses of postmitochondrial supernatant fluids and preparations of purified polyribosomes from rat cerebral cortex revealed that the proportion of large ribosomal aggregates diminished during development. This phenomenon was not associated with variations in ribonuclease activity in the different preparations. Polyribosomes isolated from cerebral tissue of animals 14 days of age or older exhibited a loss of heavy components in vitro when the Mg2+ concentration of the medium in which they were suspended for analysis was reduced from 10 to 1 mm. Similar preparations from infant rats (0 to 2 days old) disaggregated only slightly in the presence of 1 mm Mg2+. Developmental alterations in the stability of cerebral messenger RNA-ribosome complexes did not result in differences in the over-all capacity of the polyribosomal preparations for amino acid incorporation in vitro. The largest polyribosomes were initially most active in vivo in incorporation of [3H]leucine into ribosomal bound protein of cerebral cortices of rats varying in age from 0 to 42 days. However, the fraction of ribosomal bound radioactive protein which was associated with ribosomes lighter than the tetramer as early as 5 min after intracisternal administration of the isotopic amino acid was considerably greater in immature rats (14 days old) and adult rats (42 days old) than in newborn animals. Moreover, this fraction was further enhanced in the succeeding 10-min interval in the older animals, but not in newborn rats. The proportion of nascent, nonribosomal protein found in cerebral postmitochondrial supernatant fluids 5 min after administration of labeled leucine was also greater in the immature and adult rats than in newborn animals. These results suggest that the proportion of unstable messenger RNA-ribosome complexes in cerebral cortex of the rat increases during development. The data also indicate that polyribosomal instability may be associated with specialized function in the mature brain.