Characteristics of the activation by dithiothreitol and Fe(2+) of tryptophan hydroxylase from the rat brain

Distinctive iron requirement of tryptophan 5-monooxygenase: TPH1 requires dissociable ferrous iron

A peripheral type of tryptophan 5-monooxygenase (EC 1.14.16.4), TPH1, is very unstable in vitro, but the inactivation was reversible and full reactivation occurs upon anaerobic incubation with a high concentration of dithiothreitol (DTT, 15 mM). In this study, distinctive iron requirement of TPH1 was revealed through analysis of the enzyme's inactivation and activation by DTT. For this purpose, all the glasswares, plastics, Sephadex G-25 gels, and reagents including protein solutions had been treated with metal chelators, and apo-TPH was prepared by treatment with EDTA. Apo-TPH thus prepared exclusively required free Fe2+ for its catalytic activity; 10(-8) M was enough under the strict absence of Fe3+ but 10(-12) M was too low. No other metal ions including Fe3+ were effective. It appeared that Fe3+ bound to the enzyme with a higher affinity than Fe2+, resulting in the inactivation. Ascorbate, a non-thiol reducing agent, did not substitute DTT in the activation of TPH1, but enhanced the Fe2+-dependent activity of apo-TPH as effectively as DTT. Thus, the DTT-activation was essentially substituted by preparation of apo-TPH by the EDTA treatment and the assay of apo-TPH in the presence of Fe2+ and ascorbate. The activation of TPH1 by incubation with DTT was accompanied by exposure of 9 sulfhydryls out of the total 10 cysteine residues, but the cleavage of disulfide bonds seemed not to be crucial, even if it occurred. The effect of DTT was substituted by some other sulfhydryls whose structure was analogous to that of commonly used metal chelators. Based on these observations, the following dual roles of DTT are proposed: (1) in the activation of TPH, DTT removes inappropriate bound iron (Fe3+) as a chelator, keeping Fe3+ away from the enzyme's binding site which needs to bind Fe2+ for the catalytic activity, and (2) in both the activation and reaction processes, DTT prevents oxidation of Fe2+ to Fe3+ as a reducing agent.

Iron dependence of tryptophan hydroxylase activity in RBL2H3 cells and its manipulation 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-amino 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 chelators. Desferrioxamine (DFO) suppressed the intracellular enzyme activity. Salicylaldehyde isonicotinoyl hydrazone (SIH) also suppressed enzyme activity, but stimulated it when administered in the Fe-bound form. Hemin also stimulated enzyme activity, which progressively increased over several hours to more than sixfold the initial level. DFO and SIH inhibited the hemin stimulatory effect when administered simultaneously with hemin. Both suppression and stimulation with these chelators took place without a significant decrease or increase in the amount of enzyme. These results indicate that there was an inadequate supply of Fe2+ in the cells to support full activity of tryptophan hydroxylase.

Why tryptophan hydroxylase is difficult to purify: a reactive oxygen-derived species-mediated phenomenon that may be implicated in human pathology

1. Attempts and apparently successful procedures to obtain reasonable quantities of electrophoretically homogenous mammalian brain-derived tryptophan hydroxylase, (TPH), have been described, starting in the early 1970s. This work has been carried out with the primary objective to obtain specific antisera to this enzyme to map out serotonergic pathways in the nervous system. 2. By using a multitude of techniques, antisera have indeed been fabricated and employed. However, it is doubtful if pure, native TPH has ever been produced. Indeed, there is strong evidence that more than one isoform of TPH exists in the rat brain. Thus, these antisera are probably directed against TPH-derived polypeptides and not the holoenzyme(s). 3. The difficulty in the purification of TPH lies not only in its subjectivity to proteolysis, but more importantly in its probable capacity to produce superoxide leading to hydrogen perioxide formation. This, in turn, may undergo Fenton chemistry with iron at the active site of the protein to produce hydroxyl radicals that directly attack and destroy the enzyme molecule. Evidence for such a mechanism is presented together with possible protocols that might be used to produce pure stable holo TPH(s). 4. It is hypothesized that similar oxidative events may take place in vivo under certain conditions leading to pathological results. Strategies to block these events are suggested.

Radioenzymatic assay for tryptophan hydroxylase: [3H]H2O release assessed by charcoal adsorption

The rate-limiting reaction in the biosynthesis of the neurotransmitter, serotonin, is catalyzed by the enzyme, tryptophan hydroxylase. Studies on the characteristics of this enzyme have been hampered by its relative instability and paucity in the brain. We have modified a charcoal adsorption radioenzymatic assay used for the measurement of tyrosine hydroxylase to assess rat brain tryptophan hydroxylase activity. This protocol is based on the principle that aromatic amino acid hydroxylases are mixed-function oxygenases and will utilize O2 and reduced pterin to convert tritiated amino acid substrate to product and tritiated H2O. All product and unreacted substrate are adsorbed by acidified charcoal. The [3H]H2O is analyzed by liquid scintillation spectrometry and is indicative (stoichiometrically) of the amount of product formed and, thus, the activity of the enzyme. This assay has a high signal-to-noise ratio and is sensitive enough to determine enzymatic activity in homogenates of individual raphe nuclei. In addition, its simplicity in design allows for the simultaneous testing of large numbers of samples. The enzyme activity and kinetic determinations derived from this protocol agree with those of other investigators using more lengthy, involved procedures.

Modulation of tryptophan hydroxylase activity by phospholipids: stimulation followed by inactivation

The activity of tryptophan hydroxylase from the rat brainstem was stimulated rapidly three- to fourfold by the addition of phosphatidylinositol or phosphatidylserine. However, the activity of the enzyme once stimulated was decreased gradually by subsequent incubation with the phospholipid at 37 degrees C, reaching a level below the original activity after 1 h of incubation. The presence of ferrous ion almost perfectly protected the enzyme against this phospholipid inactivation. The activity of the enzyme inactivated by incubation with the phospholipid was not only restored, but also increased further by incubation at 37 degrees C with ferrous ion and dithiothreitol. Gel filtration analysis revealed that the enzyme stimulated by phosphatidylinositol was eluted in a void volume together with the phospholipid vesicles, but the enzyme inactivated by incubation with phosphatidylinositol was eluted at a later region apart from the vesicles. These results, taken together, suggest the possible involvement of cellular membranes in the regulation of tryptophan hydroxylase in the central nervous system.

In vitro reactivation of rat cortical tryptophan hydroxylase following in vivo inactivation by methylenedioxymethamphetamine

The activity of tryptophan hydroxylase (EC 1.14.16.4) from rat brain was significantly decreased 1 h following a single systemic injection of 3,4-methylenedioxymethamphetamine (MDMA) when assessed ex vivo by radioenzymatic assay or in vivo by the quantitation of 5-hydroxytryptophan accumulation following central L-aromatic amino acid decarboxylase inhibition. Recovery of enzymatic activity in vivo, which occurred within 24 h of low-dose MDMA treatment, appeared not to involve synthesis of new enzyme protein, because the return of enzymatic activity was not prevented by prior cycloheximide. Acutely MDMA-depressed cortical tryptophan hydroxylase activity could be completely restored in vitro by a prolonged (20-24 h) anaerobic incubation in the presence of dithiothreitol and Fe2+ at 25 degrees C; partial reconstitution occurred when 2-mercapto-ethanol was substituted for dithiothreitol. Cortical tryptophan hydroxylase acutely inactivated by methamphetamine or p-chloroamphetamine could be similarly reactivated. MDMA-inactivated cortical tryptophan hydroxylase derived from rats killed later than 3 days after drug treatment could not be significantly reactivated under the conditions described above, indicating the development of irreversible enzymatic damage. Kinetic analysis of enzyme reactivation revealed an approximate doubling of enzyme Vmax with no change in enzyme affinity for either substrate, tryptophan, or pterin cofactor. These studies suggest that MDMA and its congeners inactivate central tryptophan hydroxylase by inducing oxidation of key enzyme sulfhydryl groups. The reactivation capacity of drug-inactivated enzyme at various times after MDMA treatment may provide a means of assessing the development of MDMA-induced neurotoxicity.

Tryptophan 5-hydroxylase. Rapid purification from whole rat brain and production of a specific antiserum

Tryptophan 5-hydroxylase (EC 1.14.16.4; L-tryptophan tetrahydropteridine: oxygen oxidoreductase) was purified to electrophoretic homogeneity from whole brain supernatant using the following steps: pteridine-argarose affinity chromatography, hydrophobic and finally hydroxyapatite chromatography. Exogenous catalase was necessary throughout most of the purification procedure in order to protect the enzyme against inactivation. The iron chelator desferrioxamine at a concentration of 10 microM or higher brought about an irreversible loss of enzyme activity of a partially purified preparation containing an excess of catalase, whereas this same chelator at a lower concentration afforded considerable protection of the enzyme's activity during the final purification stage despite the quasi-total absence of catalase and the presence of an excess of ferrous iron. Antiserum raised in the rabbit to purified tryptophan 5-hydroxylase appears to be monospecific for the enzyme after immunoadsorption of anti-catalase antibodies which were present due to the trace of catalase which remained in the final enzyme preparation.

A sensitive radiometric assay for tryptophan hydroxylase applicable to crude extracts

We describe here a simple and convenient method for assay of tryptophan 5-monooxygenase (hydroxylase), applicable to enzyme in all states of purification. It is based on the enzyme-catalysed formation of 5-hydroxy-[4-3H]tryptophan from [5-3H]tryptophan, and the subsequent acid-dependent quantitative release of 3H as 3H2O; unreacted substrate is removed with activated charcoal. The assay is linear with respect to both protein concentration and time, and gives results similar to those in a standard fluorimetric assay.

Tryptophan 5-monooxygenase from mouse mastocytoma P815. A simple purification and general properties

Tryptophan 5-monooxygenase was purified 880-fold with a 48% yield from mouse mastocytoma cells (P815) by only a one-step purification procedure of pteridine affinity chromatography. The specific activity of the final preparation was 5280 nmol min-1 mg-1. It gave a single protein band on polyacrylamide gel electrophoresis in the absence and presence of sodium dodecylsulfate. The molecular weight of the enzyme was determined to be 270,000 by gel filtration and 280,000 by gradient polyacrylamide gel electrophoresis. Sodium dodecylsulfate/polyacrylamide gel electrophoresis revealed the enzyme to be composed of identical subunits with a molecular weight of 53,000. Tetrameric structure of the enzyme was suggested by cross-linking studies using dimethyl suberimidate as a bifunctional reagent. The isoelectric point of the enzyme was estimated to be 6.0. Amino acid analysis showed a residue composition similar to that reported for rat liver phenylalanine 4-monooxygenase. The enzyme activity was stimulated approximately fivefold by preincubation with dithiothreitol and Fe2+. The purified enzyme had an activity of phenylalanine hydroxylation and also a weak activity of tyrosine hydroxylation. The kinetic properties of the enzyme are also presented.

Purification and properties of tryptophan 5-monooxygenase from rat brain-stem

Tryptophan 5-monooxygenase was purified approximately 5,500-fold, to apparent homogeneity with a specific activity of 374 nmol min-1 mg-1 at 30 degrees C, from rat brain-stem using Sepharose CL-6B, DEAE-Sepharose CL-6B and pteridine-agarose chromatography. Two distinct active forms were separable by DEAE-Sepharose CL-6B and designated as form I and form II based on their order of elution from the gel column. The apparent molecular weight of form I was determined to be 300,000 by gel filtration on Ultrogel AcA 34 and 288,000 by gradient polyacrylamide gel electrophoresis. The enzyme gave a single band on sodium dodecylsulfate/polyacrylamide gel electrophoresis, the molecular weight of which was estimated to be 59,000, indicating that the enzyme might be composed of four identical subunits. The tetrameric structure of the enzyme was further suggested by cross-linking studies using dimethyl suberimidate as a bifunctional reagent. The enzyme activity was stimulated approximately 3.5-fold by the addition of Fe2+. Kinetic studies revealed that this activation was associated with an increase of V value. The purified enzyme had an activity of phenylalanine hydroxylation but not an activity of tyrosine hydroxylation.

Kinetic properties of bovine pineal tryptophan-5-monooxygenase activated by an endogenous activating substance

We previously reported that an endogenous activating substance different from bovine serum albumin, phospholipids and heparin, exists in the extract from bovine pineal glands and that this substance interacts with tryptophan-5-monooxygenase under reducing conditions with sulfhydryl reagents, to stimulate monooxygenase activity. The present paper reports that the activating substance is of peptide nature; that it is sensitive to trypsin-digestion; and that it does not change the apparent Km's for substrates, L-tryptophan and oxygen, and coenzyme, reduced biopterin or DMPH4; but that it increases the Vmax 1.5- to 2.3-fold. These results suggest that an activating protein, present in some particles of the cell structure, activates tryptophan-5-monooxygenase under the regulation of a sulfhydryl compound. The apparent Km's for reduced biopterin and DMPH4 were 77.2 microM and 294 microM, respectively. The apparent Km's for L-tryptophan and oxygen with reduced biopterin were 15.0 microM and 4.7%, respectively; with DMPH4, they were 11.0 microM and 8.5%, respectively. Significant inhibition of both L-tryptophan and oxygen was observed with reduced biopterin, but not with DMPH4 (at the tested concentrations of up to 0.5 mM and 20%, respectively).