Molecular mechanisms of liver regeneration: VI. Incorporation of [ 14C]orotic acid into cytoplasmic RNA in normal and regenerating liver

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The incorporation of [ 14]orotic acid into cytoplasmic RNA was measured in non-hepatectomized and partially hepatectomized rats 6, 12, 18 and 24 h following the operation. At each of these times after partial hepatectomy groups of 3 rats were killed 20, 40 and 60 min after intraperitoneal injection of labeled orotic acid. Cytoplasmic RNA was extracted from the post-mitochondrial supernatant with cold phenol in the presence of bentonite. The specific activity of total cytoplasmic RNA increases 6 h after partial hepatectomy, reaches a maximum at 12 h and decreases thereafter. Sucrose gradient analysis of the cytoplasmic RNA shows three radioactive peaks corresponding to 4-S, 18-S and 28-S RNA. After orotic acid injection, 4-S cytoplasmic RNA is labeled first followed by the labeling of 18-S and 28-S RNA. The labeling of 18-S and 28-S RNA is accelerated after partial hepatectomy. A distinct peak of radioactivity is already found in 18-S RNA of 12-, 18- and 24-h regenerating liver 20 min after injection of the precursor. The highest specific activity of 18-S RNA was found 12 h after partial hepatectomy while the specific activity of 28-S RNA reaches a peak at 24 h. In normal and regenerating liver 18-S RNA has a higher specific activity than that of 28-S. During regeneration the ratio between the specific activities of 28-S and 18-S RNA does not remain constant in the first hour after orotic acid injection. The results suggest that the turnover rates of 18-S and 28-S RNA are increased after partial hepatectomy but the increase is greater for 18-S RNA. It is possible that the 18-S RNA is a mixture of ribosomal and messenger RNA.