J. Neurochem.

Tryptophan hydroxylase (TPH) catalyses the first and rate limiting step in the biosynthesis of the neurotransmitter serotonin. There are two TPH isoenzymes in humans, encoded by two different genes: TPH1 and the recently described TPH2. We have expressed both human enzymes and various deletion mutants of TPH2 (ΔN44, ΔC17, ΔC19, ΔC51) in COS7 cells. TPH1 and 2 displayed different kinetic properties with a lower Km value of TPH1. Removal of 44 amino acids from the N-terminus of TPH2 resulted in a 3–4-fold increased Vmax, which indicates a strong inhibitory function of this part on the enzymes activity. TPH1 and 2 were able to form homooligomers and also heterooligomers with each other. The different deletion mutants (ΔC17, ΔC19 and ΔC51), which lack the putative C-terminal leucine zipper tetramerization domain, existed as monomeric enzymes. While short deletions (ΔC17 and ΔC19) hardly changed Vmax values, the ΔC51 mutant lost 99% of TPH activity. These data identify a region between the C-terminal oligomerization domain and the catalytic domain, which is indispensable for TPH2 activity