Iron Ions Increase the Thermal Stability In Vitro and Activity In Vivo of the 447R Mutant Form of Mouse Tryptophan Hydroxylase 2

Tryptophan hydroxylase 2 (TPH2) hydroxylates L-tryptophan to L-5-hydroxytryptophan (5-HTP) the first and rate-limiting step of serotonin (5-HT) synthesis in the mammalian brain. Some mutations in the Tph2 gene reducing TPH2 activity are associated with hereditary depressive disorders. The P447R substitution in the mouse TPH2 molecule reduces its thermal stability in vitro and its activity in the brain. The effects of iron ions on thermal stability in vitro and the activity in the brain of the mutant TPH2 were investigated. In the in vitro experiment effects of 0.01, 0.05, and 0.2 mM of FeSO4 and FeCl3 on the enthalpy (ΔH) and Gibbs free energy (ΔG) of thermal denaturation of the mutant TPH2 extracted from the midbrain of Balb/c mice were assayed. All FeSO4 concentrations and 0.05 and 0.2 mM concentrations of FeCl3 increased these thermodynamic characteristics of the mutant TPH2. Repeated (for 7 days) intramuscular administration of Fe(III) hydroxide dextran complex (15 and 30 mg/kg/day) increased TPH2 activity in the hippocampus, but not in the midbrain in Balb/c mice. Repeated (for 7 days) intramuscular administration of Fe(III) hydroxide dextran complex (15 and 30 mg/kg/day) together with thiamine (8 mg/kg/day) and cyanocobalamin (0.8 mg/kg/day) increased TPH2 activity in the hippocampus, while 30 mg/kg of Fe(III) hydroxide dextran also increased the enzyme activity in the midbrain in Balb/c mice. These results are the first evidence for chaperone-like effects of iron ions on thermal stability in vitro and activity in the brain of the mutant TPH2.