Mitochondrial hexokinase in brain: coexistence of forms differing in sensitivity to solubilization by...

Hexokinase (ATP:D-hexose 6-phosphotransferase, EC 2.7.1.1) is associated with mitochondria from brain of various species. The fraction of the bound activity that can be released in soluble form after incubation of the mitochondria with glucose-6-phosphate (Glc-6-P) has been shown to vary markedly among species (F. Kabir and J.E. Wilson, Arch. Biochem. Biophys. 300, 641-650, 1993). A method has been developed by which mitochondria bearing significant amounts of bound hexokinase can be selectively immunoprecipitated by Staphylococcus aureus cells coated with anti-Type I hexokinase antibodies. Treatment of mitochondria from guinea pig, bovine, and human brain with Glc-6-P solubilized approximately 60, 40, and 20% of the bound hexokinase activity, respectively, but had no effect on the extent to which the mitochondria could be immunoprecipitated. This is consistent with the view that the residual hexokinase, resistant to solubilization with Glc-6-P, coexists on the same mitochondria bearing the Glc-6-P-sensitive form, i.e., removal of the latter does not prevent immunoprecipitation mediated by the Glc-6-P-resistant form. Mitochondrial porin has previously been shown to be involved in binding of hexokinase to mitochondria. Four porin species, having a common molecular weight but differing in isoelectric point, were detected, in the same relative amounts, in mitochondria from bovine and rat brain. The extent to which Glc-6-P solubilizes hexokinase from rat and bovine brain mitochondria is approximately 90 and approximately 40%, respectively. Thus the marked difference in sensitivity to solubilization with Glc-6-P cannot be attributed to differences in the relative amounts of different porin species present in rat and bovine mitochondria.