Characterization of low-molecular-weight acid phosphoprotein phosphatase associated with rat splenic cell nucleoli

Evidence is presented on the existence of an acid phosphoprotein phosphatase (APPase) associated with rat splenic cell nucleoli. The enzyme is purified 1250-fold from 0.3 M NaCl nucleolar extract by means of chromatography on P cellulose and Sephacryl S-200. The nucleolar acid phosphoprotein phosphatase is a very basic protein (pI 8.3) and shows maximal activity at pH 5.8. It dephosphorylates acidic phosphoproteins (casein and phosvitin), ATP, and p-nitrophenyl phosphate, but not basic phosphoproteins (histones and protamine phosphate). The enzyme activity is very dependent on reducing agents, especially on ascorbic acid. Divalent and monovalent cations did not affect phosphatase activity, but heavier divalent metals, Co2+ and Zn2+, strongly inhibit the enzyme activity. The activity was also inhibited by N-ethylmaleimide, indicating a requirement for free sulfhydryl groups. The estimated molecular weight of the purified enzyme is approximately 38,000 by gel filtration and sedimentation in sucrose gradient concentration.