Mutants of glucokinase cause hypoglycaemia- and hyperglycaemia syndromes and their analysis illuminates fundamental quantitative concepts of glucose homeostasis

Aims/hypothesis. Mutations of the glucokinase gene cause hyperglycaemia or hypoglycaemia. A quantitative understanding of these defects of glucose hom eostasis linked to the glucokinase gene was lacking. Therefore a database o f kinetic variables of wild-type and 20 missense mutants of glucokinase was developed and used in mathematical modelling to predict the thresholds for glucose-stimulated insulin release. Methods. Recombinant human glucokinase was generated in E. coli. The k(cat) , glucose S-0.5, ATP K-m, and Hill number of glucokinase were determined. I nhibition by Stearoyl CoA and glucokinase regulatory protein and thermal st ability were assayed for all mutants kinetically similar to wild-type gluco kinase. A mathematical model predicting the threshold for glucose-stimulate d insulin release was constructed. This model is based on the two substrate kinetics of glucokinase and the kinetic variables of the database. It is a ssumed that both glucokinase gene alleles are equally expressed in beta-cel ls and that induction of glucokinase occurs as a function of basal blood gl ucose. Results. Large changes, varying greatly between mutants were found in nearl y all variables. Glucokinase flux at threshold for glucose-stimulated insul in release was about 25% of total phosphorylating potential in the normal b eta-cell and this was used to predict thresholds for the mutant heterozygot es. Clinical data for maturity onset diabetes of the young type linked to t he glucokinase gene and familial hyperinsulinaemic hypoglycaemia linked to the glucokinase gene and the glucokinase kinetic data of this study were us ed to test the model. The model predicts fasting blood glucose between 3 an d 7 mmol/l in these cases. Conclusion/interpretation. A kinetics database of wild-type and 20 mutants of glucokinase was developed. Many kinetic differences were found for the m utants. The mathematical model to calculate the threshold for glucose-stimu lated insulin release predicts fasting blood glucose between 3 and 7 mmol/l in subjects with glucokinase gene mutations.