Age-dependent change of the protein-bound phosphorus in rat liver cell nuclei

Some aspects of the metabolism of the protein-bound phosphorus were examined on isolated rat liver cell nuclei in function of age. The protein complement of the nuclei was separated into 5 non-histone and one histone fractions by means of salt and acid extraction. The phosphorus content of the fractions was determined and the rate of phosphorylation and dephosphorylation was measured after 32P labelling. The supernatant of the in vitro phosphorylation system contained a lower amount phosphorus but showed high phosphorylation rate. For several parameters such as the relative amounts of the non-histone proteins, the net phosphorus contents, the newly incorporated phosphorus/unit protein and the phosphatase activity in the nuclei a common pattern was found in function of age; intermediate, maximum and low or intermediate values for the age-groups of 4--6 months, 16--18 months and 24--28 months respectively. The possible implication of the nuclear non-histone proteins in the process of ageing is discussed.