Phosphoprotein phosphatases in human erythrocyte cytosol.

Abstract

More than 97% of spectrin phosphatase activity in human erythrocyte hemolysate was recovered in cytosol. The cytosolic phosphatase activity was resolved into four peaks, namely phosphatases I (22%, Mr = 180,000), II (3%, Mr = 42,000), III (8%, Mr = 177,000), and IV (62%, Mr = 104,000), by aminohexyl-Sepharose column chromatography. Although these phosphoprotein phosphatases also catalyzed the dephosphorylation of phosphorylase a, glycogen synthase b, and phosphorylated H1 and H2B histones, the phosphatases differed from each other in preferences for substrates and the Mg2+ or Mn2+ requirements for their activities. The treatment with 80% ethanol converted phosphatases I, III, and IV to Mr = 31,000 forms which had essentially the same physical and catalytic properties. By contrast, the molecular weight and catalytic properties of phosphatase II, which was Mg2+- or Mn2+-dependent, were not changed by the same ethanol treatment. The major spectrin phosphatase, phosphatase IV, was purified to near homogeneity, as judged by polyacrylamide gel electrophoresis. Sodium dodecyl sulfate-gel electrophoresis revealed that the enzyme was composed of one 32,000-Da polypeptide (alpha) and one 69,000-Da polypeptide (beta). Km values of the enzyme for phosphorylated spectrin and H2B histone were 1.63 +/- 0.45 and 48.2 +/- 7.6 microM, respectively. The spectrin phosphatase activity was stimulated about 2-fold by 5-25 mM Mg2+, but was completely inhibited by the same concentration of Mn2+. Physiological concentrations of adenine nucleotides, 2,3-diphosphoglyceric acid, cyclic nucleotides, or Ca2+ and/or calmodulin had no significant effect on the reaction, but 20 mg/ml of hemoglobin inhibited the reaction by 60%. -SH-blocking agents but not iodoacetate inhibited the reaction.