Tryptophan peroxidase-oxidase

No cofactors have been reported in the tryptophan peroxidase-oxidase reaction, but the marked reduction in the conversion of tryptophan to nicotinic acid in thiamine deficiency has been found in all probability to be due to interference with the reaction at the tryptophan peroxidase-oxidase stage (173 c/. diagram 19). The evidence is still inadequate to show how enzyme function and vitamin are related. Biotin may also be concerned in the reaction (800 but see 175a). 

The enzyme called formylase by Knox and Mehler (490, 591) and ky-nurenine formamidase by Jakoby (437) is present in liver in a considerable excess relative to tryptophan peroxidase-oxidase (e.g., 491), and formylky-nurenine is therefore not normally found in tissues or excreted in urine (e.g., 171). Partially purified tryptophan peroxidase-oxidase, from which formylase activity has been removed, accumulates formylkynurenine, shown (591) to be identical with synthetic (947 or better, 172) material. Formylase occurs widely in bacteria, and has been partially purified from Neurospora (437). In both higher and lower organisms the enzyme shows considerable specificity. 

Tryptophan is, however, not the only agent which can bring about an increase in tryptophan peroxidase-oxidase. A smaller effect can be produced by substances which initiate the stress reaction of the adrenal-pituitary system (478). High X-irradiation produces a similar effect in normal, but not in adrenalectomized, animals (869). Cortisone reverses the effect of adrenalectomy (868), and glucocorticoids (e.g., cortisone and hydrocortisone) can themselves cause an increase in the enzyme (484). How these changes are brought about is still obscure their elucidation... 

Hydroxykynurenine excretion in pathological states was also first reported by Japanese workers (573), who identified it as the substance causing the diazo reaction and the Weiss urochromogen reaction in urines from cases of severe tuberculosis. This was confirmed in the author s laboratory (178), where it was also shown that the excretion is unrelated to tuberculosis as such. Hydroxykynurenine excretion occurs in a large proportion of patients with fevers of varying etiology and is in all probability due to the increased rate of breakdown of body proteins in fever. Presumably the protein breakdown induces an adaptive increase in tryptophan peroxidase-oxidase, and the capacity of the available kynureninase, which comes later in the metabolic chain and is not an adaptive enzyme (480), is exceeded. 

Hydroxytryptophan was not metabolized by a tryptophan-adapted strain of Pseudomonas (217) and was not attacked by the tryptophan peroxidase-oxidase system (217, 884). The enteramine and kynurenine pathways are quite distinct, as is supported by the facts that synthetic 5-hydroxykynurenine (124, 574), the expected product of tryptophan peroxidase-oxidase action, does not act as an ommochrome precursor in insects or as a nicotinic acid precursor in Neurospora (124). 

When synthesis of this compound was accomplished, > experiments with it made it clear that it is not a normal tryptophan metabolite. The metabolism of oxindolylalanine was found to be quite different from that of tryptophan or kynurenine in rat liver slices, or in the intact animal. The paper chromatographs of the urines from normal and pyridoxine-deficient rats fed oxindolylalanine were quite different from those obtained when tryptophan was fed. Furthermore, the tryptophan peroxidase-oxidase enzyme system does not act on this compound, nor was it metabolized by the bacillus. Pseudomonas fluorescens, which had been adapted to tryptophan or kynurenine. The identity of the first intermediate of tryptophan oxidation, therefore, is still unknown. 

Tanaka and Knox (40) have recently found that peroxide is not directly involved in the formation of formylk3Tiurenine, but acts in converting an inactive ferric enzyme in the presence of tryptophan into an active ferrous protein. It is of interest that cyanide and catalase inhibit the inactive enzyme but not the active ferrous form. On the other hand, carbon monoxide inhibits only the active enzyme. The changes have been summarized by Tanaka and Knox according to the scheme given in Fig. 3. These authors have suggested, since the reaction involves a direct oxygenation of the substrate, that the enzyme be termed tryptophan pyrrolase rather than the previously used nomenclature of tryptophan-peroxidase-oxidase. 

Tryptophan oxygenase (tryptophan pyrrolase) plays an important role in the metabolism of tryptophan and has been prepared from animal tissues (Knox and Mehler, 1950) and bacteria (Hayaishi and Stanier, 1951). Enzymes from the two sources were found to be comparable in many respects. Knox and Mehler named the enzyme tryptophan peroxidase-oxidase, since they had found that catalase inhibits the reaction and that this inhibition is reversed by hydrogen peroxide, suggesting the intermediate formation and the subsequent utilization of peroxide as shown in Eqs. (21) and (22). 

The oxidative conversion of tryptophan to kynurenine was first observed in liver preparation by workers in the laboratory of Kotake (Itagaki and Nakayama, 1941). The low activity of the preparation described by Kotake was difficult to detect. Subsequently Knox and Mehler (1950) discovered the increased activity of livers from animals previously given large amounts of tryptophan. Liver served as the source of a slightly purified preparation that was described as tryptophan peroxidase-oxidase. A similar preparation was obtained from a Pseudomonas adapted to tryptophan (Hayaishi and Stanier, 1951). More recent evidence about the nature of the enzyme has forced the abandonment of the peroxidase-oxidase name and the original term suggested by Kotake, tryptophan pyrrolase, has been adopted as less descriptive but more correct. 

Some physiological or pathological stimuli induce the liver cell to synthesize or break down some protein selectively. Even during starvation the content of all liver protein does not drop simultaneously. For example, while the activities of catalase, xanthine oxidase, alkaline phosphatase, and acid phosphatase drop at various rates as starvation progresses, that of glucose-6-phosphatase increases. Hydrocortisone and tryptophan administration induces a massive increase in tryptophan peroxidase activity. In either case, at least part of the increase in enzyme activity results from de novo enzyme synthesis. If tryptophan administration is interrupted, the activity of the peroxidase returns to normal. During the induction, turnover rates of other proteins do not change. 

The first well-defined step in tryptophan catabolism is the splitting of the indole ring with the formation of formylkynurenine. This is accomplished by a labile enzyme system present only in liver which is composed of a tryptophan peroxidase and an oxidase which produces hydrogen peroxide as a product of its activity. This coupled oxidation system induces the reaction given in the following equation ... 

The biochemical model contains the pathways of the enzymatic reactions in the synthetic routes. Model can be constructed for each alkaloid. Eigure 75 presents biochemistry model of Catharantus alkaloids. The most important enzymes on this model are TDC (tryptophan decarbocylase), GlOH (geraniol 10-hydroxylase) and SS (strictoside synthase). NADPH+, PO (Peroxidase), O (oxidase) and NADH+ are all active in different Catharantus alkaloid formations. The biochemical models are subject to both qualitative and quantitative alkaloid... 

A comparative study on the reactivity of five indole derivatives (tryptamine, N-acetyltryptamine, tryptophan, melatonin, and serotonin), with the redox intermediates compound I ( 2) and compound n (kj) of the plant enzyme horseradish peroxidase and the two mammaUan enzymes lactoper-oxidase and myeloperoxidase, was performed using... 

Two enzyme systems have been discovered that initiate the catabolism of tryptophan and lead to the products shown in Fig. 13. These are tryptophan oxidase-peroxidase and kynureninase. Their roles in the catabolic process will be discussed below. 

Formation of kynurenine from tryptophan was discovered in liver extracts by Kotake and Masayama 289). These authors proposed the name tryptophan pyrrolase for the enzyme. Knox and Mehler 290) subsequently showed that the formation of kynurenine consisted of two enzyme reactions, an initial oxidation to formylkynurenine followed by hydrolysis to kynurenine. Because the reaction was stimulated by HsO produced in situ it was assumed that there was an intermediate formation and utilization of peroxide in the oxidation. For this reason the enzyme was renamed tryptophan oxidase-peroxidase by Knox. Experiments with O showed that molecular O2 was incorporated into the reaction products 291). One mole of O2 per mole of tryptophan was contained in the formylkynurenine. When H20 was tested, very little 0 was utilized. This observation led Tanaka and Knox 292) to return to the use of the original name, tryptophan pyrrolase. 

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