Protein catabolism in rat liver nuclei
Review articleOpen access

AbstractA rat liver nuclear fraction has been purified by differential sedimentation. With increasing purification this fraction exhibited a relatively constant protein catabolic activity at neutral pH and a diminished ability to degrade protein at acid pH. Certain metabolic inhibitors previously observed to inhibit protein breakdown in liver homogenates inhibited protein degradation in isolated nuclei as well. However, when the nuclear fraction was vigorously disrupted these inhibitions were lost, although catabolic activity was maintained. Certain other inhibitions were not affected by this treatment. These results imply that nuclear protein catabolism involves both structural and proteolytic components.Native and denatured rat serum proteins and liver cytoplasmic proteins were not substrates for the liver nuclei, while denatured bovine hemoglobin, active bovine spleen ribonuclease and calf thymus deoxyribonuclease were degraded to some extent, and rat liver nucleoproteins were hydrolyzed approx. 0.5–1% per h.There appears to be both a cytoplasmic inhibitor and a stimulator for nuclear protein catabolism. The inhibition is not due to a competition for substrate since the cytoplasmic proteins are not degraded by the nuclei. The pH 5 fraction stimulated protein catabolism, and this stimulation was lost by pre-treating the pH 5 fraction with ribonuclease or by adding the ribonuclease to the nuclei with the pH 5 fraction. Ribonuclease added to the nuclei inhibited protein degradation. These data suggest an RNA involvement in protein catabolism, but our initial attempts to demonstrate a stimulation of protein catabolism by adding various RNA preparations were not successful.

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