Mitochondrial hexokinase in brain of various species: differences in sensitivity to solubilization by glucose...

Approximately 90% of the hexokinase (ATP:D-hexose 6-phosphotransferase, EC 2.7.1.1) activity was solubilized by treatment of rat brain mitochondria with glucose 6-phosphate (Glc-6-P), while only about 20% of the hexokinase could be solubilized from human brain mitochondria. Intermediate amounts of solubilized activity were obtained with brain mitochondria from other species. In contrast, > or = 80% of the activity could be released by 0.5 M potassium thiocyanate (KSCN), regardless of the species from which the mitochondria were obtained. Hexokinase activities solubilized by treatment of bovine brain mitochondria with Glc-6-P (HKG6P) and by a subsequent treatment with KSCN (HKKSCN) were indistinguishable in their isoelectric focusing pattern and molecular weight, and both were inhibited by Glc-6-P with Ki approximately 20 microM. Both HKG6P and HKKSCN could bind to mitochondria from rat liver or brain, and both were again solubilized by a subsequent treatment with Glc-6-P. These results do not suggest any intrinsic molecular difference between HKG6P and HKKSCN. Rather, the difference in susceptibility to release by Glc-6-P is reasonably attributed to discrete types of binding sites for hexokinase on brain mitochondria, with the relative proportion of these varying with species. Bovine brain mitochondria bearing HKG6P and HKKSCN were not resolved by sucrose density gradient fractionation, suggesting that both forms may coexist on the same mitochondrion. Given the probable importance of mitochondrially bound hexokinase in regulating aerobic glycolysis in brain, these differences in hexokinase-mitochondrial interactions may be related to previously documented differences in cerebral energy metabolism of these various species.