Molecular forms of red blood cell hexokinase

Mammalian red blood cell hexokinase has been shown to exist in two or more distinct molecular forms, which are separable by ion-exchange chromatography. Of these forms just one corresponds to hexokinase type I from other tissues, while the others differ from any previously reported hexokinase isozyme. Analysis of several molecular properties of the three major forms (Ia, Ib and Ic in the order of their elution from DE-52 columns) of hexokinase prepared from human red cells and of the two forms purified from rabbit reticulocytes, shows significant differences in the isoelectric point. The kinetic and regulatory characteristics, the molecular weight, the temperature and pH-dependence of the various isozymes were similar. The hexokinase isozymic pattern is largely dependent upon red blood cell age. Among all, hexokinase Ib is the predominant form in rabbit reticulocytes and becomes the minor component in the older cells; a similar situation has also been found in the human erythrocyte. At present the molecular basis of hexokinase heterogeneity remains unknown, however preliminary experimental findings indicate a post-translational modification as a possible mechanism.