On the molecular sieving property of the human erythrocyte membrane. Localization of some proteins within the cell.

The membrane of swollen erythrocytes was found to be more resistant to hypotonicity at 37 degrees C than at 0 degrees C either in the presence or absence of added glucose and even more resistant if ATP formation was stimulated by preincubation of the sedimented cells with adenosine. The membrane, irrespective of temperature and metabolism behaves as a molecular sieve for proteins, i.e. the smaller the molecular radius of a protein, the higher is the amount released by molecular sieving. This phenomenon allows one to discriminate between uniform distribution of proteins within the cell and complex formation of enzymes. Some glycolytic enzymes catalyzing reactions between triosephosphate and lactate production were found to be clustered near the surface of the cell but not bound to the membrane. The data indicate that some of these enzymes may form complexes under in vivo conditions.