Some observations on mitochondrial-bound hexokinase and creatine kinase of the heart

Hexokinase-mitochondrial interaction in cardiac tissue: implications for cardiac glucose uptake, the 18FDG lumped constant and cardiac protection

The hexokinases are fundamental regulators of cardiac glucose uptake; by phosphorylating free intracellular glucose, they maintain the concentration gradient driving myocardial extraction of glucose from the bloodstream. Hexokinases are highly regulated proteins, subject to activation by insulin, hypoxia or ischaemia, and inhibition by their enzymatic product glucose-6-phosphate. In vitro and in many non-cardiac cell types, hexokinases have been shown to bind to the mitochondria, both increasing their phosphorylative capacity, and having a putative role in the anti-apoptotic function of protein kinase B (PKB)/Akt. Whether hexokinase-mitochondrial interaction is a dynamic and responsive process in the heart has been difficult to prove, but there is growing evidence that this association does indeed increase in response to insulin stimulation or ischaemia. In this review I discuss the relevance of hexokinase-mitochondrial interaction to cardiac glycolytic control, our interpretation of (18)FDG cardiac PET scans, and its possible role in protecting the myocardium from ischaemic injury.

Insulin does not change the intracellular distribution of hexokinase in rat heart

Preliminary evidence has suggested that hexokinase in rat heart changes its kinetic properties in response to insulin through translocation to the outer mitochondrial membrane. We reexamined this hypothesis in light of tracer kinetic evidence to the contrary. Our methods were as follows. Working rat hearts were perfused with Krebs-Henseleit buffer containing glucose (5 mmol/l) and sodium oleate (0.4 mmol/l). Dynamic glucose uptake was measured with [2-3H]glucose and with 2-deoxy-2-[18F]fluoroglucose (2-[18F]DG). Hexokinase activity was determined in the cytosolic and mitochondrial fractions. Our results are as follows. Uptake of glucose and uptake of 2-[18F]DG were parallel. Insulin (1 mU/ml) increased glucose uptake threefold but had no effect on 2-[18F]DG uptake. The tracer-to-tracee ratio decreased significantly. The Michaelis-Menten constant of hexokinase for 2-deoxyglucose was up to 10 times higher than for glucose. There was no difference in maximal reaction velocity. Two-thirds of hexokinase was bound to mitochondria. Insulin neither caused translocation nor changed Michaelis-Menten constant or maximal reaction velocity. In conclusion, the insulin-induced changes in the tracer-to-tracee ratio are due to a shift of the rate-limiting step for glucose uptake from transport to phosphorylation by hexokinase. Insulin does not affect the intracellular distribution or the kinetics of hexokinase.