Covalent modification of nuclear proteins during aging.

An in vitro assay system has been established to study acetylation and phosphorylation of nuclear proteins from isolated nuclei. Phosphorylation of neclear proteins reached a peak within 5 min while maximum acetylation occurred about 10 min later. The rate of acetylation of liver nuclear proteins in 15 min incubation was significantly higher in "old" mice (29 mo) than in "young" mice (2 mo), while there was no difference in phosphorylation. When nuclear histones were fractionated by polyacrylamide-urea electrophoresis the acetylation of histone F3 was increased in "old" mice to 129% and F2al to 112% of the values in "young" mice. Acetylation of phenol-soluble nuclear acidic proteins was increased to 250% and phosphorylation to 138% in "old" mice as compared to "young" mice. This increase in covalent modification of acidic proteins was found in tow specific fractions when separated by SDS-polyacrylamide gel electrophoresis. By contrast, the labeling of nucleoplasmic proteins, soluble in 0.14 M NaCl, showed no significant difference between the two ages.