D-mannoheptulose phosphorylation by hexokinase isoenzymes

D-mannoheptulose is a specific inhibitor of D-glucose phosphorylation by he xokinase isoenzymes. In the present study, the phosphorylation of this hept ose was investigated by either a spectrophotometric or radioisotopic proced ure. Using yeast hexokinase, the phosphorylation of 25 mM D-mannoheptulose only represented 0.02% of that of 5 mM D-glucose. Such a percentage was inc reased to 3.93% in the case of bovine heart hexokinase. In the latter case, the Km for D-mannoheptulose was close to 0.2 mM and both D-glucose (0.1-1. 0 mM) and D-glucose 6-phosphate (also 0.1-1.0 mM) inhibited the phosphoryla tion of the heptose (0.03-0.60 mM). Human B-cell glucokinase also catalyzed the phosphorylation of D-mannoheptulose (0.1 mM), which was now increased in a bell-shaped manner by D-glucose (1.0-20 mM). Likewise, rat parotid gla nd, liver and pancreatic islet homogenates catalyzed the phosphorylation of D-[H-3] mannoheptulose. The results obtained in these three tissues differ ed from one another by their absolute values (per mg wet wt.), relative val ues (by reference to the phosphorylation rate of 10 mM D-glucose), and sens itivity to inhibition by D-glucose(10 mM).