Intracellular Tryptophan Hydroxylase Activity Assessed by Metabolic Assay: An Interpretation of in vitro Activation of the Enzyme

Abstract

The intracellular actlvlty of tryptophan hydroxylase was assessed by applying NSD-1055, an inhibitor of aromatic L-amino acid decarboxylase, to a monolayer culture of RBL2H3 cells from a serotonin producing rat basophilic leukemia cell line. Tryptophan hydroxylase activity was also measured in a cell-free system. In this system, enzyme activity was enhanced 14 fold by incubation with Fe2+ and DIT. Enzyme activity measured by the metabolic assay was similar to the non-enhanced activity measured in the cellfree system. When 30 MM hemin were added to the cell culture, intracellular tryptophan hydroxylase activity was enhanced 1.8 fold. Administration of desferrioxamine, a cell permeable chelator, suppressed enzyme activity to about 25% the normal activity even in the presence of hemin. The results are consistent with the idea that a large part tryptophan hydroxylase exists in a latent form which can be activated by iron either in a cell-free system or in living cells.

Keywords

• tryptophan hydroxylase
• dtt-activation
• fe2+ -activation