Of three casein phosphatases isolated from the cytosol of human cord blood erythrocytes two were cobalt-dependent, E(2) and E(3). In the presence of C0CI(2), E(2) activity was the most prominent. In addition to casein, E(2) dephosphorylated phosvitin and p-nitrophenyl phosphate (p-NPP) with pH optima at Ó.8-7.2 for proteins and 9.0 for p-NPP. The native enzyme had a molecular weight of 104,000 daltons after AcA-44 Ultrogel filtration. According to SDS/PAGE it consisted of two subunits, 78,000 and 15,000 daltons. The 104,000-dalton form exhibited Michaelis-Menten kinetics and had the greatest affinity for casein between protein substrates tested. Ethanol denaturated the enzyme by 80%. Optimal activation of E(2) phosphatase was achieved with 5 mmol/1 CoCl(2) which did not affect the catalytic properties of the enzyme but did affect the rate of ‘E-S’ complex formation. Inorganic pyrophosphate was not inhibitory for the 104,000-dalton enzyme. Judging by all these properties the natural substrate for E(2) casein phosphatase could be P-pyruvate kinase.

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