Protein Synthesis by Highly Aggregated and Purified Polysomes From Young and Adult Rat Brain

The state of aggregation and the activity of polyribosomes as well as the activity of the pH 5 enzyme fraction were studied at two stages of postnatal brain development, 9 and 50 days after birth. When the polyribosomes were prepared at 0°C in the presence of 5 mm‐Mg2+, more than 85 per cent of the polyribosome material exhibited a sedimentation coefficient higher than 110 S. High Mg2+ concentrations are, therefore, unnecessary to obtain highly aggregated brain polyribosomes. The basal amino acid incorporating activity of both 9‐ and 50‐day‐old rat brain preparations is at least equal to that of rat liver. When prepared by the same procedure as above, 9‐day‐old rat brain polyribosomes seem to be more active (20 per cent) than those of adult brain. However, this difference in activity depends on the presence of a non‐ribosomal inactive contaminant which is always present in higher amounts in adult brain preparations. When purified from this contaminant, the preparations do not differ in activity. High Mg2+ concentrations are also not necessary for optimal protein synthetic activity and, in fact, are inhibitory. When assayed with both types of highly aggregated polyribosomes, the pH 5 enzyme fraction from adult brain is clearly less active than that of 9‐day‐old rats. These results suggest that the loss of brain protein synthesis during development does not depend on the stability of the messenger RNA‐ribosome complex but only on the soluble pH 5 enzyme fraction.