Studies on a soluble human erythrocyte protein kinase

A cyclic AMP-independent protein kinase has been isolated from human erythrocyte cytosol and purified about 28,000-fold. The enzyme uses ATP as the phosphoryl donor and exhibits a high activity towards casein and phosvitin. The kinase has a molecular weight of about 30,000 to 32,000 as estimated by sucrose density gradient centrifugation and Sephacryl S-200 gel filtration. Upon isoelectrofocusing, two kinase activities are resolved. The major component, which comprises about 85% of the activities, focuses at a pH of about 9.6 and the minor component at about 4.6. The relationship between these two kinase activities is not clear. NaF, ADP and, to a lesser extent, AMP, all inhibit the kinase activity. 2,3-Diphosphoglyceric acid, on the other hand, has no effect. The kinase also catalyzes the phosphorylation of a number of erythrocyte membrane proteins, including spectrin, band 3, and the sialoglycoproteins.