Pyridoxal phosphate decarboxylase

Fig. 2. Biosynthetic pathway for epinephrine, norepinephrine, and dopamine. The enzymes cataly2ing the reaction are (1) tyrosine hydroxylase (TH), tetrahydrobiopterin and O2 are also involved (2) dopa decarboxylase (DDC) with pyridoxal phosphate (3) dopamine-P-oxidase (DBH) with ascorbate, O2 in the adrenal medulla, brain, and peripheral nerves and (4) phenethanolamine A/-methyltransferase (PNMT) with. Cadenosylmethionine in the adrenal...

By contrast, the cytoplasmic decarboxylation of dopa to dopamine by the enzyme dopa decarboxylase is about 100 times more rapid (Am 4x 10 " M) than its synthesis and indeed it is difficult to detect endogenous dopa in the CNS. This enzyme, which requires pyridoxal phosphate (vitamin B6) as co-factor, can decarboxylate other amino acids (e.g. tryptophan and tyrosine) and in view of its low substrate specificity is known as a general L-aromatic amino-acid decarboxylase. 

Histamine is synthesised by decarboxylation of histidine, its amino-acid precursor, by the specific enzyme histidine decarboxylase, which like glutaminic acid decarboxylase requires pyridoxal phosphate as co-factor. Histidine is a poor substrate for the L-amino-acid decarboxylase responsible for DA and NA synthesis. The synthesis of histamine in the brain can be increased by the administration of histidine, so its decarboxylase is presumably not saturated normally, but it can be inhibited by a fluoromethylhistidine. No high-affinity neuronal uptake has been demonstrated for histamine although after initial metabolism by histamine A-methyl transferase to 3-methylhistamine, it is deaminated by intraneuronal MAOb to 3-methylimidazole acetic acid (Fig. 13.4). A Ca +-dependent KCl-induced release of histamine has been demonstrated by microdialysis in the rat hypothalamus (Russell et al. 1990) but its overflow in some areas, such as the striatum, is neither increased by KCl nor reduced by tetradotoxin and probably comes from mast cells. 

Kamath AV, GL Vaaler, EE Snell (1991) Pyridoxal phosphate-dependent histidine decarboxylases. Cloning, sequencing, and expression of genes from Klebsiella planticola and Enterobacter aerogenes and properties of the overexpressed enzymes. J Biol Chem 266 9432-9437. 

The leucocyte method estimates pyridoxal phosphate in isolated leucocytes it is based on a coenzyme-catalyzed tyrosine decarboxylase system from S. faecalis (B32). Enough data are not yet on hand to evaluate this method. The determination of circulating or available vitamin Ba should offer a more direct approach. 

Scheme 18.45 Postulated inhibition mechanism of pyridoxal phosphate-dependent decarboxylases by a-allenic a-amino acids.

Figure 2.16. Pathways for the synthesis and metabolism of the catecholamines. A=phenylalanine hydroxylase+pteridine cofactor+Oj B tyrosine hydroxylase+ tetrahydropteridme+Fe+ +Oj C=dopa decarboxylase+pyridoxal phosphate D= dopamine beta-oxidase+ascorbate phosphate+Cu+ +Oj E=phenylethanolamine N-methyltransferase+S-adenosylmethionine l=monoamine oxidase and aldehyde dehydrogenase 2=catechol-0-methyltransferase+S-adenosylmethionine.

Following the synthesis of 5-hydroxytryptophan (5-HTP) by tryptophan hydroxylase, the enzyme aromatic amino acid decarboxylase (also known as 5-HTP or dopa decarboxylase) then decarboxylates the amino acid to 5-HT. L-Aromatic amino acid decarboxylase is approximately 60% bound in the nerve terminal and requires pyridoxal phosphate as an essential enzyme. 

Most people have heard of antihistamines, even if they have little concept of the nature of histamine. Histamine is the decarboxylation product from histidine, and is formed from the amino acid by the action of the enzyme histidine decarboxylase. The mechanism of this pyridoxal phosphate-dependent reaction will be studied in more detail later (see Section 15.7). 

This pyridoxal-phosphate-dependent enzyme [EC 4.1.1.29], also referred to as sulfinoalanine decarboxylase, catalyzes the conversion of 3-sulfino-L-alanine to... 

The principal pathways for the biogenesis and metabolism of histamine are well known. Histamine is formed by decarboxylation of the amino acid, L-histidine, a reaction catalyzed by the enzyme, histidine decarboxylase. This decarboxylase is found in both mammalian and non-mammalian species. The mammalian enzyme requires pyridoxal phosphate as a cofactor. The bacterial enzyme has a different pH optimum and utilizes pyruvate as a cofactor (26.27). 

Non-dependence of histidine decarboxylase from Lactobacillus 30a on pyridoxal phosphate suggested. 11... 

Histidine decarboxylase from Lactobacillus 30a isolated> free of pyridoxal phosphate. 12... 

Lactobacillus delbrueckii. In 1953, Rodwell suggested that the histidine decarboxylase of Lactobacillus 30a was not dependent upon pyridoxal phosphate (11). Rodwell based his suggestion upon the fact that the organism lost its ability to decarboxylate ornithine but retained high histidine decarboxylase activity when grown in media deficient in pyridoxine. It was not until 1965 that E. E. Snell and coworkers (12) isolated the enzyme and showed that it was, indeed, free of pyridoxal phosphate. Further advances in characterization of the enzyme were made by Riley and Snell (13) and Recsei and Snell (14) who demonstrated the existence of a pyruvoyl residue and the participation of the pyruvoyl residue in histidine catalysis by forming a Schiff base intermediate in a manner similar to pyridoxal phosphate dependent enzymes. Recent studies by Hackert et al. (15) established the subunit structure of the enzyme which is similar to the subunit structure of a pyruvoyl decarboxylase of a Micrococcus species (16). 

Non-pyridoxal Phosphate Dependent. Figure 2 depicts the postulated mechanism for a non-pyridoxal phosphate catal) zed decarboxylation of histidine to histamine involving a pyruvoyl residue instead of pyridoxal -5 - phosphate (20). Histidine decarboxylases from Lactobacillus 30a and a Micrococcus sp. have been shown to contain a covalently bound pyruvoyl residue on the active site. The pyruvoyl group is covalently bound to the amino group of a phenylalanine residue on the enzyme, and is derived from a serine residue (21) of an inactive proenzyme (22). The pyruvoyl residue acts in a manner similar to pyridoxal phosphate in the decarboxylation reaction. 

Figure 1. Pyridoxal Phosphate Dependent Decarboxylase Mechanism...

Pyridoxine (vitamin Bg, 18) (Fig. 13) assists in the balancing of sodium and potassium as well as promoting red blood cell production. A lack of pyridoxine can cause anemia, nerve damage, seizures, skin problems, and sores in the mouth. It is required for the production of the monoamine neurotransmitters serotonin, dopamine, norepinephrine, and epinephrine, as it is the precursor to pyridoxal phosphate, which is the cofactor for the aromatic amino acid decarboxylase enzyme. 

Due to the absence of a hydrogen atom on the a-carbon, the a-fluoroalkyl amino acids (except, of course, the fluoroalanines, vide supra) cannot undergo an elimination of HR Consequently, they are more stable than fluoroalanines and other jS-fluoro amino acids previously described. On the other hand, similar to proteogenic amino acids, jS-fluoro amino acids and a-fluoroalkyl amino acids are generally substrates of pyridoxal phosphate depending on enzymes such as racemases and decarboxylases. When an amino acid is a substrate of such enzymes, the enzyme induces the development of a negative charge on the a-carbon, which can initiate a /(-elimination process. This reaction affords an electrophilic species (Michael acceptor type), which is able to add a nucleophilic residue of the enzyme. This notion of mechanism-based inhibitor is detailed in Chapter 7. 

Among the numerous enzymes that utilize pyridoxal phosphate (PLP) as cofactor, the amino acid racemases, amino acid decarboxylases (e.g., aromatic amino acids, ornithine, glutamic acid), aminotransferases (y-aminobutyrate transaminase), and a-oxamine synthases, have been the main targets in the search for fluorinated mechanism-based inhibitors. Pharmaceutical companies have played a very active role in this promising research (control of the metabolism of amino acids and neuroamines is very important at the physiological level). 

L-Canaline is an ineffective antimetabolite of L-ornithine since it has little ability to antagonize ornithine-dependent reactions. On the other hand, it forms a covalently bound Schiff-base complex with the pyridoxal phosphate moiety of Bg-containing enzymes. As such it is a potent inhibitor of many decarboxylases and aminotransferases that utilize this vitamin. 

Canaline reacts with the pyridoxal phosphate moiety of enzymes possessing this vitamin Bg-containing decarboxylases and transaminases typically are inhibited strongly by this non-protein amino acid (, ). When canaline reacts with free pyridoxal... 

Vitamin B6 occurs naturally in three related forms pyridoxine (6.26 the alcohol form), pyridoxal (6.27 aldehyde) and pyridoxamine (6.28 amine). All are structurally related to pyridine. The active co-enzyme form of this vitamin is pyridoxal phosphate (PLP 6.29), which is a co-factor for transaminases which catalyse the transfer of amino groups (6.29). PLP is also important for amino acid decarboxylases and functions in the metabolism of glycogen and the synthesis of sphingolipids in the nervous system. In addition, PLP is involved in the formation of niacin from tryptophan (section 6.3.3) and in the initial synthesis of haem. 

Tissues of the mammalian central nervous system contain a pyridoxal phosphate-dependent glutamate decarboxylase that catalyzes conversion of Glu to y-aminobutyrate (GABA), an inhibitory synaptic transmitter. GABA is degraded by trans-imination with a-oxoglutarate as the acceptor to yield succinic semialdehyde, which then is oxidized to succinate by an NAD-linked dehydrogenase. 

Mechanisms of action of pyridoxal phosphate (a) in glutamate-oxaloacetate transaminase, and (b) in aspartate /3-decarboxylase. 

Would you expect phosphatidylserine decarboxylase (fig. 19.3) to be a pyridoxal phosphate enzyme ... 

Tyrosine is converted to dopa by the rate-limiting enzyme tyrosine hydroxylase, which requires tetrahydrobiopterin, and is inhibited by a-methyltyrosine. Dopa is decarboxylated to dopamine by L-aromatic amino acid decarboxylase, which requires pyridoxal phosphate (vitamin B6) as a coenzyme. Carbidopa, which is used with levodopa in the treatment of parkinsonism, inhibits this enzyme. Dopamine is converted to norepinephrine by dopamine P-hydroxylase, which requires ascorbic acid (vitamin C), and is inhibited by diethyldithiocarbamate. Norepinephrine is converted to epinephrine by phenylethanolamine A -methyltransferase (PNMT), requiring S-adeno-sylmethionine. The activity of PNMT is stimulated by corticosteroids. 

---