RNA Polymerase Activities and other Aspects of Hepatic Protein Synthesis during Early Protein Depletion in the Rat

Abstract

Certain aspects of the hepatic protein-synthesizing system were studied in vivo or in vitro in young male rats fed diets with various levels (0 to 40%) of casein for periods up to 42 days. First, the incorporation of L-leucine-1-¹⁴C or L-methionine-³⁵S into microsomal protein was established to be related inversely to the protein level of the diet; data in vitro indicated these differences were more closely related to the capacity of the microsomal fraction. A similar inverse relationship was shown for RNA synthesis in vivo; the specific activity of ¹⁴C-incorporation (using orotic acid) into the total RNA of various subcellular fractions was found to be greatest in the nuclear isolate. To determine the nature of this phenomenon, the activity of DNA-dependent RNA polymerase was examined; two proposed polymerase assay methods were used, a conventional Mg²⁺-dependent or a Mn²⁺-(NH₄)₂SO₄ system. The activity of the former system was found to be significantly higher in nuclei from rats fed the low protein diets and lower in the high protein group, whereas the latter system was unresponsive to the dietary protein level. Finally, liver polysomal profiles from rats fed a low protein for 14 days showed a relatively larger proportion of polysomal aggregates with a decrease in monosomes. Over a period of prolonged depletion, however, the relatively higher levels of protein and RNA synthesis in the low protein groups diminished, indicating an inevitable breakdown in this adaptative phenomenon.