Leucine-induced HH is a feature of the HI/HA syndrome and is due to gain of function mutations in GLUD1. In the pancreatic β-cells, α-ketoglutarate enters the tricarboxylic acid cycle and leads to an increase in the concentration of cellular ATP. This rise in the cellular ATP causes closure of the ATP sensitive potassium channel (KATP channel); resulting in cell membrane depolarization, Ca2+ influx via voltage gated calcium channels and insulin exocytosis.
GDH is allosterically activated by leucine and inhibited by GTP . Activating mutations in the GLUD1 gene reduce the sensitivity of the enzyme to allosteric inhibition by GTP and ATP  or less frequently cause an increase in the basal GDH activity . The loss of inhibition by GTP leads to increased leucine induced glutamate oxidation to α-ketoglutarate. Hence leucine sensitivity is manifested by hypoglycaemia following protein-rich meals which is a classical feature of this condition [5, 16]. The mechanism of persistent hyperammonaemia is not completely understood, although a recent paper by Treberg et al  suggests that this is due to renal ammoniagenesis.
Protein sensitivity (but no leucine sensitivity) is also a feature observed in patients with mutations in the KATP channel genes . We recently described severe protein sensitivity in patients with loss of function mutations in the HADH gene . We now show for the first time, in a larger number of patients, that loss of function mutations in the HADH gene causes severe leucine hypersensitivity. These clinical observations suggest that HADH acts in some way to limit leucine induced insulin secretion. Interestingly, in contrast to patients with mutations in the GLUD1 gene the serum ammonia level is not elevated in patients with HADH mutations (data not shown). Our results show that despite the leucine hypersensitivity, mutations in HADH do not cause an increase either in the basal activity of GDH, or a change in the IC50 for GTP. This suggests that HADH mutations cause leucine sensitivity and dysregulated insulin secretion via a novel pathway not involving GTP regulation of GDH.
Our results show that a protein-protein interaction exists between GDH and HADH thus suggesting that HADH in some way regulates the activity of GDH. This regulatory mechanism would not seem to involve GTP but must occur by another as yet unidentified mechanism. It is possible that an interaction between HADH and GDH has allosteric effects that affect how leucine stimulates GDH activity. Reduced expression of HADH protein, which can be seen in patient lymphoblasts, could allow leucine to over stimulate GDH and hence lead to excessive insulin secretion. In support of this hypothesis Li et al have shown an interaction between HADH and GDH both in liver mitochondria and islets from hadh +/+ but not hadh −/− mice. In hadh −/− liver His-tagged HADH was used to immunoprecipitate GDH as identified by mass spectrometry but this was not the only protein pulled down, it may be speculated that this could be true of islet cells also. Interestingly, basal activity of GDH in islets were similar in +/+ and −/− islets and also the GTP IC50 which agrees with our data on human lymphoblasts. Li et al also showed a reduced K
for α-ketoglutarate in hadh −/− islets that was not seen in −/− liver or kidney which resulted in a 50 % increase in enzyme efficiency. This increase in enzyme efficiency shown in pancreas may be due to the fact that HADH is so highly expressed  in this organ compared to others, that a deficiency of it is detrimental with regard to regulation of GDH. In other organs the expression of HADH is much lower and so does not have the same effect on GDH. This highlights the difference between patients with mutations in GDH where activity of this enzyme is affected in all tissues in the body.
HH on administration of leucine was demonstrated in the hadh −/− mouse by Li et al . This HH was exacerbated by the addition of glutamine and alanine and it is possible that the HH we see in our patients would also be responsive to these amino acids.
Martens et al showed that expression of HADH in pancreatic β-cells is higher than that of other enzymes of β–oxidation pathway suggesting an important role for this protein in β–cell physiology. Li et al show that the ratio of HADH to GDH mRNA was 5 fold higher in mouse islets than in liver. Martens et al showed however that suppression of the HADH protein resulted in increased basal and glucose stimulated insulin secretion which was not due to increased rates of glucose metabolism or an inhibition of fatty acid oxidation. The data obtained by Li et al suggested that deletion of HADH protein caused an increase in basal islet metabolism but that this did not have a major effect on glucose stimulated insulin secretion. These data appear to contradict each other but may be due to an incomplete KO of HADH in the study by Martens et al .