Abstract
Polyribosomes from ethionine-induced transplanted hepatomas are compared with liver polyribosomes from rats fed ad lib. , fasted overnight, or fed on ethionine for two weeks. The polyribosomes were prepared from post-mitochondrial supernatant in all experiments. Hepatoma polyribosomes were significantly smaller than those from normal liver; this was not due to excess free ribonuclease in the tumour homogenate. Polyribosomes from all four sources remained intact after incubation at 37°; however, p-chloromercuribenzoate treatment resulted in breakdown to monomers (hepatoma was not tested), suggesting that a complex of ribonuclease and ribonuclease-inhibitor was present. Very little rapidly labelled RNA was released in a non-sedimentable form from normal liver polyribosomes after incubation, and by the same criterion hepatoma polyribosomes were relatively resistant both to incubation and to pancreatic ribonuclease. Rapidly labelled RNA from unincubated normal liver polyribosomes was found after phenol extraction to be polydisperse with respect to s value. The ratio of free to endoplasmic reticulum-bound polyribosomes from hepatoma was extremely high, compared with that from normal liver. Bound polyribosomes were slightly larger than free in liver from both normal and ethionine-fed rats. Free polyribosomes from fed rats were markedly delayed in sedimenting through 2M-sucrose, compared with those from fasted rats. It is suggested that the larger polyribosome found in fed rats "tangle" with the membranes at the sucrose interface. Recommendations are given for optimum separation of free from bound polyribosomes. Free polyribosomes from ethionine-fed rats, did not show this delay in sedimentation, regardless of whether the animals were fed or fasted. Amino acid incorporation in vivo into free and bound polyribosome fractions followed a similar time-course for both normal liver and hepatoma. For both tissues, the bound polyribosomes were more active per ribosome than the free. Since the majority of hepatoma post-mitochondrial supernatant polyribosomes are free, the bulk of hepatoma protein must be made by free polyribosomes, while the reverse is true for normal liver.