Characterization of glucokinase regulatory protein-deficient mice

The glucokinase regulatory protein (GKRP) inhibits glucokinase competitivel y with respect to glucose by forming a protein-protein complex with this en zyme. The physiological role of GKRP in controlling hepatic glucokinase act ivity was addressed using gene targeting to disrupt GKRP gene expression. H eterozygote and homozygote knockout mice have a substantial decrease in hep atic glucokinase expression and enzymatic activity as measured at saturatin g glucose concentrations when compared with wild-type mice, with no change in basal blood glucose levels. interestingly, when assayed under conditions to promote the association between glucokinase and GKRP,liver glucokinase activity in wild-type and nub mice displayed comparable glucose phosphoryla tion capacities at physiological glucose concentrations (5 mM). Thus, despi te reduced hepatic glucokinase expression levels in the null mice, glucokin ase activity in the liver homogenates was maintained at nearly normal level s due to the absence of the inhibitory effects of GKRP. However, following a glucose tolerance test, the homozygote knockout mice show impaired glucos e clearance, indicating that they cannot recruit sufficient glucokinase due to the absence of a nuclear reserve. These data suggest both a regulatory and a stabilizing role for GKRP in maintaining adequate glucokinase in the liver. Furthermore, this study provides evidence for the important role GKR P plays in acutely regulating of hepatic glucose metabolism.