The hexokinase inhibitor glucosamine exerts a concentration dependent dual effect on protein kinase activity in vitro

Hexokinase inhibitors are used as tools to address the function of hexokinase in sugar dependent signal transduction pathways of plants. We have characterised glucosamine, N-acetyl-glucosamine and manno-heptulose for their ability to inhibit hexokinase and found pronounced differences in the inhibition of glucose-phosphorylating activity. As it is known that phosphorylation reactions are involved in sugar signaling, we have also evaluated the effect of the three hexokinase inhibitors on the activity of a glucose activated MAP kinase purified from tomato suspension culture cells. Glucosamine was shown not only to be a weak hexokinase inhibitor but specifically affected protein kinase activity. Depending on the glucosamine concentration, the in vitro activity of the protein kinase was stimulated or inhibited. In contrast, the hexokinase inhibitors mannoheptulose and N-acetylglucosamine did not show this effect.