Comparative characterization of membrane‐associated and cytosolic Tyr‐protein kinases in human erythrocytes

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Abstract
In recent years, two protein-tyrosine kinase activities, phosphorylating tyrosine residues on the transmembrane band-3 protein, have been isolated from human erythrocyte membranes and partially characterized by different laboratories, i.e. one extracted by non-ionic detergent (Triton X-100 or Nonidet P-40), the other solubilized by 0.25 M NaCl from the detergent-insoluble residue. The present paper shows that these two membrane-associated Tyr-protein kinases purified, in the presence of bovine serum albumin, by phosphocellulose chromatography followed by heparin-Sepharose chromatography, have the same apparent molecular mass (36 kDa) determined by Ultrogel Ac44 filtration. Moreover, both Tyr-protein kinases exhibit several identical properties, including Km values for band 3, the random acidic copolymer poly(Glu,Tyr)4:1 and angiotensin II, pH dependence, response to Mn2+ and Mg2+, response to NaCl and 2,3-bisphosphoglycerate. All these properties are identical or very similar to those exhibited by the Tyr-protein kinase previously isolated by us from human erythrocyte cytosol. These results suggest that the two membrane-associated and the cytosolic Tyr-protein kinase activities are mediated by the same enzyme, distributed between the cytosol and the membrane structures.