Anomeric specificity of human liver and B-cell glucokinase: Modulation by the glucokinase regulatory protein

The anomeric specificity of the wild-type recombinant forms of human liver and B-cell glucokinase was investigated using radioactive anomers of D-gluc ose as tracers. With D-glucose at anomeric equilibrium and at 30 degrees C, the maximal velocity, Hill number, and K-s amounted, respectively, to 16 m u mol min(-1) mg(-1), 1.8 and 6.9 mM in the case of liver glucokinase, and 7.3 mu mol min-l mg(-1), 2.0 and 7.1 mM in the case of B-cell glucokinase. Whether at 20-22 or 30 degrees C, the maximal velocity, Hill number, and K- m were significantly lower with alpha-D-glucose than with P-D-glucose in bo th liver and B-cell glucokinase. As a result of these differences, the reac tion velocity was higher with alpha-D-glucose at low hexose concentrations, while the opposite situation prevailed at high hexose concentrations. In t he presence of 0.2 mM D-fructose 6-phosphate, the glucokinase regulatory pr otein caused a concentration-related inhibition of D-glucose phosphorylatio n, such an effect fading out at high concentrations of either D-glucose or glucokinase relative to that of its regulatory protein. The phosphorylation of alpha-D-glucose by liver glucokinase appeared more resistant than that of beta-D-glucose to the inhibitory action of D-fructose 6-phosphate, as me diated by the glucokinase regulatory protein, Such a phenomenon failed to a chieve statistical significance in the case of the B-cell glucokinase, It i s proposed that this information, especially the novel findings concerning the anomeric difference in both Hill number and sensitivity to the glucokin ase regulatory protein, should be taken into account when considering the r espective contributions of alpha- and beta-D-glucose to the overall phospho rylation of equilibrated D-glucose by glucokinase. (C) 2000 Academic Press.