Iron dependence of tryptophan hydroxylase activity in RBL2H3 cells and itsmanipulation by chelators

Tryptophan hydroxylase requires Fe2+ for in vitro enzyme activity. In this study, the intracellular activity of tryptophan hydroxylase was assessed by applying 3-hydroxybenzylhydrazine (NSD-1015), an inhibitor of aromatic L-a mino acid decarboxylase, to monolayer cultures of RBL2H3 cells, a serotonin producing mast cell line. The effect of manipulating intracellular 'free' iron levels on enzyme activity was analyzed by administration of iron chela tors. Desferrioxamine (DFO suppressed the intracellular enzyme activity. Sa licyldehyde isonicotinoyl hydrazone (SIH) also suppressed enzyme activity, but stimulated it when administered in the Fe-bound form. Hemin also stimul ated enzyme activity, which progressively increased over several hours to m ore than sixfold the initial level DFO and SUI inhibited the hemin stimulat ory effect when administered simultaneously with hemin. Both suppression an d stimulation with these chelators took place without a significant decreas e or increase ill the amount of enzyme. These results indicate that there w as an inadequate supply of Fe2+ in the cells to support full activity of tr yptophan hydroxylase.