In vivo and in vitro activation of soluble tryptophan hydroxylase from rat brainstem

Abstract

TRYPTOPHAN hydroxylation is the rate-limiting step in the biosynthesis of 5-hydroxytryptamine (5-HT) in central serotonergic neurones. In physiological states, tryptophan hydroxylase (EC 1.14.16.4), the key enzyme, is not saturated by its substrate¹. Thus, three factors which modulate the concentration of tryptophan in brain—the levels of free tryptophan in plasma^2,3, the ratio of total tryptophan levels to those of other neutral amino acids⁴, and the uptake process for tryptophan in brain tissues^5,6—are important in the control of 5-HT synthesis. On the basis of indirect evidence^7,8, it was suggested that tryptophan hydroxylase could also play a regulatory role. Among the mechanisms possibly involved, rapid changes in the enzyme affinity for its substrate and/or its cofactor may take place. Indeed, such changes are responsible for the activation of tyrosine hydroxylase seen in dopaminergic terminals after blockade of dopaminergic receptors by neuroleptics⁹. We used methiothepin(1-[10,11-dihydro-8-(methylthio)-dibenzo (b,f) thiepin-10-yl]-4-methylpiperazine maleate), a potent blocker of central serotonergic receptors¹⁰, to look for a similar activation of tryptophan hydroxylase. Our results indicate that the affinity of this enzyme for its substrate is markedly enhanced in methiothepin-treated tissue.