Cerebral protein synthesis. I. Physical properties of cerebral ribosomes and polyribosomes.

Ribonucleoprotein preparations from rat cerebral cortex, isolated in 0·25 M -sucrose, 4 m M -MgCl2, 25 m M -KCl and 50 m M -tris-HCl buffer (pH 7·4), exhibited 3 major sedimenting species in the analytical ultracentrifuge. The S 20,w values and percentage distribution of these cerebral mixed ribosomes averaged 56% of 78 s (monomer), 33% of 114 s (dimer), and 11% of 147 s (trimer). Occasionally, small amounts of tetrameric ribosomes appeared. Little dependence of S 20,w values on ribosomal concentration was noted. Ultraviolet absorption analyses and RNA : protein ratios revealed that this preparation was relatively free of contamination with non-ribosomal protein. When cerebral mixed ribosomes were suspended in media low in Mg 2+ (1 m M or less) and high in K + (100 m M ), a 57 s RNP† component appeared. An additional, smaller RNP particle was noted after dialysis against buffer of low ionic strength (35 s) and after treatment with EDTA (26 s). Inasmuch as the larger and smaller RNP components could be caused to reassociate to RNP particles with normal S 20,w values (79 sand 121 s), they probably represented normal subunits of cerebral ribosomes. When cerebral ribosomes were prepared by alternate methods which produced relatively large amounts of polyribosomes with rat liver, the proportions of trimers and larger aggregates increased. However, unlike hepatic preparations, large polyribosomes predominated in cerebral ribosomal pellets only when the isolation media contained relatively high concentrations of Mg 2+ (10 to 12 m M ) and K + (100 m M ). Resuspension of polyribosomal pellets in buffer containing 1 m M -Mg 2+ and 25 m M -K + markedly increased the proportion of mono ribosomes in cerebral preparations, but had little effect on the physical properties of hepatic polyribosomes. The maximum yield of ribosomal aggregates from cerebral tissue was very low (about 0·1 mg RNA/g tissue). This value was about one-sixth the recovery value for cerebral mixed ribosomes and only about 10% of the maximum recovery value for liver polyribosomes. The results indicate that the ribosomal population of cerebral cortical tissue is very low compared to that of liver, and suggest that cerebral polyribosomes are relatively unstable.