Change of nuclear protein phosphatase activity in the liver of old rats

Nuclear protein phosphatase (NPP-ase) activity was measured in non-histone protein (NHP) preparations obtained from liver nuclei of young and old rats with exogenous substrates labelled with ³²P. NPP-ase activity can be measured in a rather broad range of pH between 5.5 and 7.5 and it shows several characteristic differences between young (4–6 months) and old (24–28 months) animals: (a) under the effect of NPP-ase a limited dephosphorylation of nuclear phosphoprotein (NPP) substrate ensues reaching a 25 per cent liberation maximum of incorporated ³²P in the case of NHP preparations from young animals; the same liberation maximum is over 45 per cent in the case of old animals, (b) an age-related change of NPP-ase activity is assumed to occur in old rats in comparison with young ones. The NPP-ase activity measurable for its homologous—NPP—substrate increases in old animals while there is a concomitant decrease of casein phosphatase activity. The possible implication of the phosphorylation of chromosomal non-histone proteins in the cellular aging is discussed.