The Identity of Hexokinase Activities from Mitochondrial and Cytoplasmic Fractions of Rat Brain Homogenates

Cytoplasmic hexokinase (ATP: D-hexose 6-phosphotransferase, EC 2.7.1.1) was purified from the soluble fraction of a rat brain homogenate by a procedure that included a unique affinity elution of the enzyme from Blue Dextran-Sepharose. The purified enzyme was examined with respect to properties in which the impure cytoplasmic enzyme has been reported to differ from the solubilized mitochondrial enzyme. These included the ability to bind to mitochondria, inhibition by quercetin, effect of pH on activity, and kinetics. In all regards the purified mitochondrial and cytoplasmic enzymes appeared identical. In addition, comparative peptide maps after partial proteolysis showed no detectable differences. These results do not support the view that there exist distinct mitochondrial and cytoplasmic forms of hexokinase, the latter being permanently relegated to a cytoplasmic location and unable to participate in a dynamic equilibrium with the mitochondrially-bound enzyme. Alternatives are proposed to explain previous results that had been interpreted as indirect evidence for the existence of a distinct cytoplasmic hexokinase.