Tryptophan hydroxylase exists in two isoforms: Isoform 1 catalyses the first and rate-limiting step in the synthesis of serotonin in the peripheral parts of the body while isoform 2 catalyses this step in the brain. The catalytic domains of human tryptophan hydroxylase 1 and 2 have been expressed, purified and the kinetic properties have been studied and are compared. Substrate inhibition by tryptophan is observed for isoform 1 but not for isoform 2. Large differences are observed in the K (m,tetrahydrobiopterin) values for the two isoforms, being >10 times larger for isoform 1 compared to isoform 2.
Tryptophan hydroxylase 2 (
TPH
2) 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
TPH
2 is poorly characterized due to low purification quantities caused by its inherent instability. Three truncated variants of human
TPH
2 (rc
h
TPH
2; regulatory and catalytic domain, NΔ47‐rc
h
TPH
2; truncation of 47 residues in the N terminus of rc
h
TPH
2, and c
h
TPH
2; catalytic domain) were expressed, purified, and examined for changes in transition temperature, inactivation rate, and oligomeric state. c
h
TPH
2 displayed 14‐ and 11‐fold higher half‐lives compared to rc
h
TPH
2 and NΔ47‐rc
h
TPH
2, 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
h
TPH
2 and NΔ47‐rc
h
TPH
2 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
h
TPH
2 and NΔ47‐rc
h
TPH
2 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
h
TPH
2 and NΔ47‐rc
h
TPH
2. Addition of
l
‐Phe to the purification buffer significantly increases the purification yields, which will facilitate characterization of
h
TPH
2.
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