Purification and characterization of esterases D1 and D2 from human erythrocytes

Esterase D1 and esterase D2, two common esterase D (EC 3.1.1.1) isozymes, were isolated and purified from human erythrocytes. Their substrate specificity, pH profile and Km values were essentially identical. Their molecular mass was the same at 34 kDa on sodium dodecyl sulfate/polyacrylamide electrophoresis and at 27 kDa on Sephadex G-100 gel filtration. Antisera to each of the esterase D1 and esterase D2 isozymes were successfully raised in chickens; each antiserum reacted identically with both isozymes. These findings indicate that the isozymes are close to each other in structure. The fact that the molecular mass of the esterase D1 and esterase D2 isozymes computed on sodium dodecyl sulfate/polyacrylamide electrophoresis was close to that obtained on Sephadex G-100 gel filtration in non-dissociating buffer indicates that the isozymes are not dimers bound by disulfide bonds or a noncovalent force. These facts together indicate that the esterase D isozymes are monomers, contrary to the prevailing view that they are dimers.