Subunit Interactions of Glucose‐6‐Phosphate Dehydrogenase from Human Erythrocytes

Glucose‐6‐phosphate dehydrogenase from human erythrocytes shows successive aggregation‐dissociation equilibria. The effects of salt concentration and pH show that a molecule of 210,000 molecular weight and s_20,w of 9.0 S dissociated to a half‐molecule of 105,000 molecular weight and s_20,w of 5.6 S. Evidence for this being a discrete dissociation was provided by molecular weight determinations, relation between sedimentation coefficients and molecular weights, and the occurrence of skew patterns at a particular degree of dissociation. Some aggregation of the molecule to give apparent molecular weights above 210,000 occurred at low salt concentrations in the pH range 6–7. Reaction with maleic anhydride gave a species of molecular weight 53,000 and s_20,w 3.4 S, indicating that in the native protein the predominant molecular species (210,000) was a tetramer. The nature of the dissociations indicate the dissimilarities between the subunit contacts in the tetrameric molecule. Optimum conditions for the enzyme assay suggest that the dimer contains the essential requirements for the catalytic function of the protein, and the implications of these findings are discussed.