Stabilization of tryptophan hydroxylase 2 by l-phenylalanine-induced dimerization.

Tryptophan hydroxylase 2 (TPH2) catalyses the initial and rate-limiting step in the biosynthesis of serotonin， which is associated with a variety of disorders such as depression， obsessive compulsive disorder， and schizophrenia. Full-length TPH2 is poorly characterized due to low purification quantities caused by its inherent instability. Three truncated variants of human TPH2 (rc TPH2; regulatory and catalytic domain， NΔ47-rc TPH2; truncation of 47 residues in the N terminus of rc TPH2， and c TPH2; catalytic domain) were expressed， purified， and examined for changes in transition temperature， inactivation rate， and oligomeric state. c TPH2 displayed 14- and 11-fold higher half-lives compared to rc TPH2 and NΔ47-rc TPH2， respectively. Differential scanning calorimetry experiments demonstrated that this is caused by premature unfolding of the less stable regulatory domain. By differential scanning fluorimetry， the unfolding transitions of rc TPH2 and NΔ47-rc TPH2 are found to shift from polyphasic to apparent two-state by the addition of l-Trp or l-Phe. Analytical gel filtration revealed that rc TPH2 and NΔ47-rc TPH2 reside in a monomer-dimer equilibrium which is significantly shifted toward dimer in the presence of l-Phe. The dimerizing effect induced by l-Phe is accompanied by a stabilizing effect， which resulted in a threefold increase in half-lives of rc TPH2 and NΔ47-rc TPH2. Addition of l-Phe to the purification buffer significantly increases the purification yields， which will facilitate characterization of TPH2.