Abstract
Abstract Postmitochondrial supernatant fractions derived from livers of hypophysectomized rats were found to contain twice the level of total (active + latent) alkaline RNase found in similar preparations from normal animals. Polysomes isolated from both sources often had similar optical density profiles, and both supported amino acid incorporation in vitro to about the same extent. Occasionally, however, polysomes obtained from hypophysectomized rats were of a smaller size class distribution than preparations from normal animals, and these preparations exhibited a correspondingly decreased capacity to support cell-free protein synthesis. This breakdown of polysomes was associated with an increase in the active RNase content of the corresponding postmitochondrial supernatant fraction. It is suggested that the apparent translational defect, reported by others, of polysomes isolated from livers of hypophysectomized rats may result from the effect of RNase upon polysomes during isolation and subsequent incubation. The elevated RNase levels of postmitochondrial supernatant fractions obtained from hypophysectomized animals were reduced to normal levels by intraperitoneal injection of bovine growth hormone, and optical density profiles and protein-synthesizing capacity of polysomes obtained from growth hormone-treated hypophysectomized rats were comparable to those of normal preparations.
Highlights
Our results indicate that the hypophysectomized rat liver postmitochondrial supernatant fraction, from which the polysomes are isolated, contains twice the normal level of alkaline RNase, suggesting that the apparent defect, reported by others, in the ribosomes from these animals was caused by polysome breakdown during isolation and subsequent incubation
Amino Acid-incorporating Capacity of Polysomes from Normal and Hypophysectomized Rats-Noll, Staehelin, and Wettstein [15] have shown that the ability of normal rat liver polysome preparations to support amino acid incorporation into protein in a cell-free system is related to the size distribution of polysomes in the preparation
To determine whether the ribosomes of hypophysectomized rats were defective in translation of endogenous mRNA, the relationship between polysome profiles and amino acid-incorporating ability of hypophysectomized rat preparations was determined, and this relationship was compared to that for normal polysome preparations (Fig. 1)
Summary
PolysomePreparation-Rats were fasted for 18 hours before decapitation. Livers were quickly removed, transferred to 2 volumes of cold Littlefield’s medium [7], minced with scissors, and homogenized in Potter-Elvehjem glass homogenizers equipped with a motor-driven Teflon pestle. Polysomes were isolated as described previously [4, 8], except that the deoxycholate concentration was adjusted to 1% of the. Ribonudease and Protein Synthesis in Hypophysectomized Rats Vol 244, No 6 “‘1 j&-N, O I- o.2 1’0 “:E OL’+.
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