Pyridoxal phosphate kynureninase

Transamination Reactions of Other Pyridoxal Phosphate Enzymes Inaddition to theirmainreactions, anumberofpyridoxalphosphate-dependent enzymes also catalyze the half-reaction of transamination. Such enzymes include serine hydroxymethyltransferase (Section 10.3.1.1), several decarboxylases, and kynureninase (Section 8.3.3.2). 

Kynureninase Kynureninase is a pyridoxal phosphate (vitamin Be) -dependent enzyme that catalyzes the hydrolysis of 3-hydroxykynurenine to... 

The enzyme cleaving kynurenine to form anthranilic acid and alanine (W14) is kynureninase which also requires pyridoxal phosphate as coenzyme (B19). 

Quite recently Weber and Wiss (W8) studied the influence of vitamin Be depletion on various pyridoxal phosphate enzymes and found that rat liver kynureninase is much more affected by vitamin Be deficiency than kynurenine transaminase. In fact liver kynureninase of rats on a Be-deficient diet fell to 17%, whereas kynurenine transaminase was about 58% of the original activity after the same period. The different behavior of these two enzymes offers a way of studying closely the mechanism of the increased excretion of xanthurenic acid in pyridoxine... 

The detailed mechanism of pyridoxal phosphate participation in the kynureninase and kynurenine transaminase reactions is considered in detail later. Of interest in this connection is the finding that other amino... 

Pyridoxal phosphate is the coenzyme in a large number of amino acid reactions. At this point it is convenient to consider together 1,he mechanism of those pyridoxal-dependent reactions concerned with aromatic amino acids. The reactions concerned are (1) keto acid formation (e.g., from kynurenine, above), 2) decarboxylation (e.g., of 5-hydroxytrypto-phan to 5-hydroxytryptamine, p. 106), (3) scission of the side claain (e.g., 3-tyrosinase, p. 78 tryptophanase, p. 110 and kynureninase, above), and 4) synthesis (e.g., of tryptophan from indole and serine, p. 40). Many workers have considered the mechanism of one or more of these reactions (e.g., 24, 216, 361, 595), but a unified theory is primarily due to Snell and his colleagues (summarized in 593). Snell s experiments have been carried out largely in vitro, and it should be emphasized that in vivo it is the enzyme protein which probably directs the electromeric changes. 

Tryptophan is an essential amino acid involved in synthesis of several important compounds. Nicotinic acid (amide), a vitamin required in the synthesis of NAD+ and NADP+, can be synthesized from tryptophan (Figure 17-24). About 60 mg of tryptophan can give rise to 1 mg of nicotinamide. The synthesis begins with conversion of tryptophan to N-formylkynurenine by tryptophan pyrrolase, an inducible iron-porphyrin enzyme of liver. N-Formylkynurenine is converted to kynurenine by removal of formate, which enters the one-carbon pool. Kynurenine is hydroxylated to 3-hydroxykynurenine, which is converted to 3-hydroxyanthranilate, catalyzed by kynureninase, a pyridoxal phosphate-dependent enzyme. 3-Hydroxyanthranilate is then converted by a series of reactions to nicotinamide ribotide, the immedi-... 

M12. Mason, M., and Manning, B., Effects of steroid conjugates on availability of pyridoxal phosphate for kynureninase and kynurenine aminotransferase activity. Amer. J. Clin. Nutr. 24, 786-791 (1971). 

Braunstein and co-workers demonstrated that pyridoxine deficiency did not affect the conversion of tryptophan to kynurenine. They showed that the kynureninase activity of the liver of pyridoxine-deficient animals was greatly reduced and could be restored in vitro by the addition of pyridoxal phosphate—a result that has been confirmed by Dalgliesh et According to Knox, removal of the alanyl side chain becomes the limiting step in the metabolism of tryptophan in pyridoxine deficiency and permits the accumulation of kynurenine, hydroxykynurenine, and their conversion products. " Administration of extra tryptophan to the... 

The increased excretion of kynurenic and xanthurenic acids observed in pyridoxine deficiency is probably due to the preferential combination of the pyridoxal phosphate coenzyme with the transaminase. By preventing the loss of the side chain, as a result of a decreased activity of kynureninase in pyridoxine deficiency, cyclization is favored leading to increased formation of the two acids above. 

Kynureninase is a pyridoxal phosphate (vitamin Bg)-dependent enzyme, and its activity is extremely sensitive to vitamin depletion. Indeed, the ability to metabolize a test dose of tryptophan has been used to assess vitamin B nutritional status (section 11.9.5.1). Deficiency of vitamin B will lead to severe impairment of NAD synthesis from tryptophan. Kynureninase is also inhibited by oestrogen metabolites. 

Kynureninase (Figure 11.16) is a pyridoxal phosphate-dependent enzyme, and its activity falls markedly in vitamin deficiency, at least partly because it undergoes a slow mechanism-dependent inactivation that leaves catalytically inactive pyridoxamine phosphate at the active site of the enzyme. The enzyme can only be reactivated if there is an adequate supply of pyridoxal phosphate. This means that in vitamin deficiency there is a considerable accumulation of both hydroxykynurenine and kynurenine, sufficient to permit greater metabolic flux than usual through kynurenine transaminase, resulting in increased formation of kynurenic and xanthurenic acids. 

This is the excretion product of 3-hydroxykynurenic acid, an intermediate in the conversion of tryptophan to nicotinic acid. Pyridoxal phosphate is required as a cofactor for the enzyme, kynureninase, which catalyses the conversion of 3-hydroxykynurenic acid to 3-hydroxyanthrinilic acid. In patients with pyridoxine deficiency, 3-hydroxykynurenic acid accumulates and is excreted in the urine as xanthurenic acid. Xanthurenic acid can therefore be measured in urine (especially after giving an oral typtophan load) in order to detect pyridoxine deficiency. 

The oxidative pathway of tryptophan metabolism is shown in Figure 3. Kynureninase is a pyridoxal phosphate-dependent enzyme, and in deficiency its activity is lower than that of tryptophan dioxygenase, so that there is an accumulation of hydroxy-kynurenine and kynurenine, resulting in greater metabolic flux through kynurenine transaminase and increased formation of kynurenic and xanthurenic acids. Kynureninase is exquisitely sensitive to vitamin Bg deficiency because it undergoes a slow inactivation as a result of catalysing the half-reaction of transamination instead of its normal reaction. The resultant enzyme with pyridoxamine phosphate at the catalytic site is catalytically inactive and can only be reactivated if there is an adequate concentration of pyridoxal phosphate to displace the pyridoxamine phosphate. 

Deformylation of the cleavage product yields L-kynurenine, which is the substrate for hydrolytic C—C cleavage by kynureninase, a pyridoxal 5 -phosphate-dependent enzyme. The a-amino group of L-kynurenine is attached to the PLP cofactor, and deprotonation of the a-carbon forms a ketimine linkage, which provides an electron sink to assist C-C hydrolytic cleavage, as shown in Figure 27. 

More recently Wiss found that kynureninase inactivated by dialysis could be reactivated by pyridoxal-5-phosphate, but not by pyridoxal-2-phosphate. As pyridoxine deficiency has no observable effect on the conversion of hydroxyanthranilic acid to nicotinic acid, the action of this vitamin appears to be connected solely with the removal of the alanyl side chain at the kynurenine level. 

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