Protein phosphatase from rat liver nuclei

Area of Science:

• Biochemistry
• Molecular Biology
• Cell Biology

Background:

• Rat liver nuclei possess diverse protein phosphatases.
• Understanding these enzymes is crucial for regulating nuclear functions.

Purpose of the Study:

• To isolate and characterize a specific protein phosphatase from rat liver nuclei.
• To investigate its substrate specificity and biochemical properties.

Main Methods:

• Partial purification using salt extraction, ammonium sulfate precipitation, DEAE cellulose chromatography, gel filtration, and preparative isoelectric focusing.
• Characterization of enzyme kinetics, pH optimum, and cofactor dependencies.

Main Results:

• A heterogeneous population of low molecular weight (12-31 kDa) protein phosphatases was identified.
• A specific fraction (pH 5.5 peak) showed a pH optimum of 6.8 for nuclear phosphoprotein dephosphorylation.
• The enzyme was activated by Mg2+ and Mn2+ but inhibited by Na+ and high Mn2+ concentrations.
• While active on histones and casein, its activity was significantly lower on histones, indicating substrate specificity.

Conclusions:

• Rat liver nuclei contain a distinct protein phosphatase with a preference for non-histone phosphoprotein substrates.
• This enzyme plays a specific role in nuclear protein dephosphorylation, separate from histone or casein phosphatases.