Multiple Forms of Human Red Blood Cell Hexokinase

Human red blood cell hexokinase (EC 2.7.1.1) has been shown to exists in multiple molecular forms which are separable by ion exchange chromatography. Of the major forms, designated hexokinase Ia, Ib, and IC, only hexokinase La corresponds to hexokinase type I from human liver, while the others differ from every other previously reported hexokinase isozyme. Hexokinase Ib is the predominant form in the fetal erythrocytes, while it is present at lower levels in the red blood cells of adults. Analysis of the hexokinase isozymic pattern in red cells of different mean age shows that the level of hexokinase Ib is also dependent on the age of the cell. The three major forms of hexokinase have the same molecular weight of 100,000, by sedimentation velocity on sucrose density gradients, the same Michaelis constants, substrate and coenzyme specificity, pHdependent activity, and the same thermal stability. The only significant differences were found in the isoelectric points which were 5.7 pH units for hexokinase Ia, 5.5 pH units for hexokinase Ib, and 5.35 pH units for hexokinase IC. These data, together with that previously reported for rabbit erythrocytes (Stocchi, V., Magnani, M., Canestrari, F., Dacha, M., and Fornaini, G. (1981) J. BioL Chem. 256,7856-7861) suggest that the presence of multiple forms of hexokinase is a common phenomenon in mammalian red blood cells.