Effects of a GTP-insensitive mutation of glutamate dehydrogenase on insulin secretion in transgenic mice.

Glutamate dehydrogenase (GDH) plays an important role in insulin secretion as evidenced in children by gain of function mutations of this enzyme that cause a hyperinsulinism-hyperammonemia syndrome (GDH-HI) and sensitize beta-cells to leucine stimulation. GDH transgenic mice were generated to express the human GDH-HI H454Y mutation and human wild-type GDH in islets driven by the rat insulin promoter. H454Y transgene expression was confirmed by increased GDH enzyme activity in islets and decreased sensitivity to GTP inhibition. The H454Y GDH transgenic mice had hypoglycemia with normal growth rates. H454Y GDH transgenic islets were more sensitive to leucine- and glutamine-stimulated insulin secretion but had decreased response to glucose stimulation. The fluxes via GDH and glutaminase were measured by tracing 15N flux from [2-15N]glutamine. The H454Y transgene in islets had higher insulin secretion in response to glutamine alone and had 2-fold greater GDH flux. High glucose inhibited both glutaminase and GDH flux, and leucine could not override this inhibition. 15NH4Cl tracing studies showed 15N was not incorporated into glutamate in either H454Y transgenic or normal islets. In conclusion, we generated a GDH-HI disease mouse model that has a hypoglycemia phenotype and confirmed that the mutation of H454Y is disease causing. Stimulation of insulin release by the H454Y GDH mutation or by leucine activation is associated with increased oxidative deamination of glutamate via GDH. This study suggests that GDH functions predominantly in the direction of glutamate oxidation rather than glutamate synthesis in mouse islets and that this flux is tightly controlled by glucose.

Carnitine membrane transporter deficiency: a long-term follow up and OCTN2 mutation in the first documented case of primary carnitine deficiency.

Three older patients were diagnosed with systemic carnitine deficiency in childhood nearly a generation ago and have together been treated for more than 50 patient years. Treatment improved tissue carnitine stores (proven in two) and eliminated most of the signs and symptoms of carnitine deficiency. All three have continued to respond to carnitine therapy and remain well except for the irreversible sequelae of the pretreatment illnesses. We demonstrate here that transformed lymphocytes from the first documented case of plasma membrane carnitine transporter deficiency fail to take up carnitine from the medium. The analysis of the cDNA of this patient and his parents revealed a homozygous frameshift mutation, 1027delT in exon 4. The resulting polypeptide terminates after amino acid 295. His parents are heterozygous for this mutation. The deletion resulted in predominately abnormal mRNA splicing with either a 13 or 19bp insertion between the junction of exons 3 and 4. The 13/19bp insertions were found in both parents, predominantly in cis with the deletion, and rarely seen with normal alleles from either parents or controls.

Expression, purification and characterization of human glutamate dehydrogenase (GDH) allosteric regulatory mutations.

Glutamate dehydrogenase (GDH) catalyses the reversible oxidative deamination of l-glutamate to 2-oxoglutarate in the mitochondrial matrix. In mammals, this enzyme is highly regulated by allosteric effectors. The major allosteric activator and inhibitor are ADP and GTP, respectively; allosteric activation by leucine may play an important role in amino acid-stimulated insulin secretion. The physiological significance of this regulation has been highlighted by the identification of children with an unusual hyperinsulinism/hyperammonaemia syndrome associated with dominant mutations in GDH that cause a loss in GTP inhibition. In order to determine the effects of these mutations on the function of the human GDH homohexamer, we studied the expression, purification and characterization of two of these regulatory mutations (H454Y, which affects the putative GTP-binding site, and S448P, which affects the antenna region) and a mutation designed to alter the putative binding site for ADP (R463A). The sensitivity to GTP inhibition was impaired markedly in the purified H454Y (ED(50), 210 microM) and S448P (ED(50), 3.1 microM) human GDH mutants compared with the wild-type human GDH (ED(50), 42 nM) or GDH isolated from heterozygous patient cells (ED(50), 290 and 280 nM, respectively). Sensitivity to ADP or leucine stimulation was unaffected by these mutations, confirming that they interfere specifically with the inhibitory GTP-binding site. Conversely, the R463A mutation completely eliminated ADP activation of human GDH, but had little effect on either GTP inhibition or leucine activation. The effects of these three mutations on ATP regulation indicated that this nucleotide inhibits human GDH through binding of its triphosphate tail to the GTP site and, at higher concentrations, activates the enzyme through binding of the nucleotide to the ADP site. These data confirm the assignment of the GTP and ADP allosteric regulatory sites on GDH based on X-ray crystallography and provide insight into the structural mechanisms involved in positive and negative allosteric control and in inter-subunit co-operativity of human GDH.