Hyperinsulinism/hyperammonemia syndrome in children with regulatory mutations in the inhibitory guanosine triphosphate-binding domain of glutamate dehydrogenase

The hyperinsulinism/hyperammonemia (HI/HA) syndrome is a form of congenital hyperinsulinism in which affected children have recurrent symptomatic hypo glycemia together with asymptomatic, persistent elevations of plasma ammoni um levels. We have shown that the disorder is caused by dominant mutations of the mitochondrial enzyme, glutamate dehydrogenase (GDH), that impair sen sitivity to the allosteric inhibitor, GTP. In 65 HI/HA probands screened fo r GDH mutations, we identified 19 (29%) who had mutations in a new domain, encoded by exons 6 and 7. Six new mutations were found: Ser(217)Cys, Arg(22 1)Cys, Arg(265)Thr, Tyr(266)Cys, Arg(269)Cys and Arg(269)His. In all five m utations tested, lymphoblast GDH showed reduced sensitivity to allosteric i nhibition by GTP (IC50, 60-250 vs. 20-50 nmol/L in normal subjects), consis tent with a gain of enzyme function. Studies of ATP allosteric effects on G DH showed a triphasic response with a decrease in high affinity inhibition of enzyme activity in HI/HA lymphoblasts. All of the residues altered by ex ons 6 and 7 HI/HA mutations lie in the GTP-binding domain of the enzyme. Th ese data confirm the importance of allosteric regulation of GDH as a contro l site for amino acid-stimulated insulin secretion and indicate that the GT P-binding site is essential for regulation of GDH activity by both GTP and ATP.