Mannose Phosphorylation by Glucokinase from Liver and Transplantable Insulinoma: Cooperativity and Discrimination of Anomers

Glucokinase from rat liver or transplantable, radiation-induced insulinomas was partially purified by ion exchange chromatography using DEAE-Cibacron Blue F3GA agarose. Phosphorylation of α,β-D-mannose by glucokinase occurred with cooperative rate dependence on mannose concentration (n_H: 1.50). Half-maximal phosphorylation rate occurred at 14 mM α,β-D-mannose. The α- and β-anomers of mannose were phosphorylated with sigmoidal kinetics (n_H: 1.57 and 1.42, respectively). The affinity of glucokinase for α-D-mannose is higher than for β-D-mannose (S₀₅: 12 mM versus 19 mM). The maximum phosphorylation rate is slightly higher, about 10%, with β-D-mannose than with α-D-mannose. Islet glucokinase has previously been shown to be chromatographically and kinetically identical to glucokinase from insulinoma and liver; therefore, evidence that glucokinase from these two tissues phosphorylates mannose with cooperative rate dependence and differentiates mannose anomers supports the glucokinase-glucose sensor hypothesis.