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

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 Tyrprotein kinases exhibit several identical properties, including K m values for band 3, the random acidic copolymer poly(Glu, Tyr) 4:1 and angiotensin II, pH dependence, response to Mn 2+ and Mg 2+ , 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.