QUANTITATIVE ISOLATION AND PROPERTIES OF NEARLY HOMOGENEOUS POPULATIONS OF UNDEGRADED FREE AND BOUND POLYSOMES FROM RAT BRAIN1

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Abstract
Abstract— A procedure is described for the preparation of free and bound polysomes from whole homogenate of rat brain tissue. Brain is homogenized in a sucrose‐polysome buffer medium high in KCl (250 mm). After a 12‐min centrifugation at 135,000 g, the free polysomes in the supernatant are decanted and saved, while the membrane bound polysomes in the pellet are resuspended in homogenizing medium, homogenized in the presence of detergent (Triton X‐100), centrifuged for 5min at 1470 g to remove nuclei, decanted, treated with deoxycholate and centrifuged for 10 min at 24,000 g to remove deoxycholate‐insoluble material. Polysomes in the two supernatants are harvested by centrifugation through sucrose gradients prepared in high KCl polysome buffer, and with or without cell sap. Free and bound polysomes prepared in this manner are undegraded, equally active in cell‐free protein synthesis, and largely free of the usual contaminants. Cross‐contamination is minimal (>10%). The recovery of polysomes is at least 95%. The distribution of ribosomes and polysomes in rat brain is 58% free and 42% membrane‐bound. The distribution of rat brain RNA is 65% ribosomal and 35% non‐ribosomal. Conditions are described for the visualization and analysis of the entire complement of free and bound ribosomes. The size fractionation procedure is rapid and reproducible, requires much less ultracentrifugation than the density‐gradient technique, and provides a nearly quantitative means of isolating undegraded free and bound polysomes of rat brain tissue.