Pyridoxine kinase

This enzyme [EC 2.7.1.35] (also known as pyridoxine kinase, pyridoxamine kinase, and vitamin kinase) catalyzes the reaction of ATP with pyridoxal to produce ADP and pyridoxal 5 -phosphate. Pyridoxine, pyridoxamine, and various other derivatives can also act as substrates. 

The phosphorylated vitamers are dephosphorylated by membrane-bound alkaline phosphatase in the intestinal mucosa pyridoxal, pyridoxamine, and pyridoxine are all absorbed rapidly by carrier-mediated diffusion. Intestinal mucosal cells have pyridoxine kinase and pyridoxine phosphate oxidase (see Figure 9.1), so that there is net accumulation of pyridoxal phosphate by metabolic trapping. Much of the ingested pyridoxine is released into the portal circulation as pyridoxal, after dephosphorylation at the serosal surface. 

Tissue uptake of vitamin Be is again by carrier-mediated diffusion of pyridoxal (and other unphosphorylated vitamers), followed by metabolic trapping by phosphorylation. Circulating pyridoxal and pyridoxamine phosphates are hydrolyzed by extracellular alkaline phosphatase. All tissues have pyridoxine kinase activity, but pyridoxine phosphate oxidase is found mainly in the liver, kidney, and brain. 

Vitamin Bi is an essential co-factor for several enzymes of carbohydrate metabolism such as transketolase, pyruvate dehydrogenase (PDH), pyruvate decarboxylase and a-ketoglutarate dehydrogenase. To become the active co-factor thiamin pyrophosphate (TPP), thiamin has to be salvaged by thiamin pyrophosphokinase or synthesized de novo. In Escherichia coli and Saccharomyces cerevisiae thiamin biosynthesis proceeds via two branches that have to be combined. In the pyrimidine branch, 4-amino-5-hydroxymethy-2-methylpyrimidine (PIMP) is phosphorylated to 4-amino-2-methyl-5-hydroxymethyl pyrimidine diphosphate (PIMP-PP) by the enzyme HMP/HMP-P kinase (ThiD) however, the step can also be catalyzed by pyridoxine kinase (PdxK), an enzyme also responsible for the activation of vitamin B6 (see below). The second precursor of thiamin biosynthesis, 5-(2-hydroxyethyl)-4-methylthiazole (THZ), is activated by THZ kinase (ThiM) to 4-methyl-5-(2-phosphoethyl)-thiazole (THZ-P), and then the thia-zole and pyrimidine moieties, HMP-PP and THZ-P, are combined to form thiamin phosphate (ThiP) by thiamin phosphate synthase (ThiE). The final step, pyrophosphorylation, yields TPP and is carried out by thiamin pyrophosphorylase (TPK). 

Platzer, E. G., and Kassis, J. A. (1978). Pyridoxine kinase in Plasmodium lophurae and duckling erythrocytes.. Protozool. 25,556-559. 

Three enzymes play an active role in the metabolism of vitamin B6 in human erythrocytes. Pyridoxal kinase uses ATP to phosphorylate pyridoxine, pyri-doxamine, and pyridoxal. Pyridoxamine oxidase oxidizes pyridoxamine-5 -phosphate and pyridoxine-5 -phosphate to pyridoxal-5 -phosphate. The phosphatase activity produces pyridoxal from pyridoxal-5 -phosphate. The assay of the three enzymes required separation of the semicarbazone derivatives of pyridoxal-5 -phosphate and pyridoxal. The mobile phase used by Ubbink and Schnell (1988) contained 2.5% acetonitrile. Detection was by fluorescence. 

All three forms of vitamin B6 [pyridoxal, pyridoxine, and pyridoxamine] are phosphorylated by a single kinase that uses ATP as the phosphate donor. This assay describes the use of pyridoxamine as the substrate. 

Figure 9.1. Interconversion of the vitamin Be vitamers. Pyridoxal kinase, EC 2.7.1.38 pyridoxine oxidase, EC 1.1.1.65 pyridoxamine phosphate oxidase, EC 1.4.3.5 and pyridoxal oxidase, EC 1.1.3.12. Relative molecular masses (Mr) pyridoxine, 168.3 (hydrochloride, 205.6) pyridoxal, 167.2 pyridoxamine, 168.3 (dihydrochloride, 241.1) pyridoxal phosphate, 247.1 pyridoxamine phosphate, 248.2 and 4-pyridoxlc acid, 183.2.

Gregory JF 3rd (1980a) Effects of epsilon-pyridoxyllysine and related compounds onliver and brain pyridoxal kinase and liver pyridoxamine (pyridoxine) 5 -phosphate oxidase. Journal of Biological Chemistry 255, 2355-9. 

Polysaccharides, 44, 58 Prokaryote Cell, 7 Proline, 439 Promoters, 391 Protamines, 149 Proteans, 151 Protein Biosynthesis, 448 Protein Catabolism, 428 Protein Maturation, 449 Proteins, 262 Purine Metabolism, 379 Purines, 113 Pyridoxine, 229 Pyrimidines, 113 Pyruvate Kinase, 288... 

Vitamin deficiency can result from treatment with certain drags. Thus, destruction of intestinal microorganisms by antibiotic therapy can produce symptoms of vitamin K deficiency. Isoniazid, used to treat tuberculosis, is a competitive inhibitor of pyridoxal kinase, which is needed to produce pyridoxal phosphate. Isoniazid can produce symptoms of pyridoxine deficiency. To prevent this, pyridoxine is often incorporated into isoniazid tablets. Methotrexate and related folate antagonists act by competitively inhibiting dihydrofolate reductase (Chapter 27). 

Metabolism of pyridoxine-related compounds in mammals. Enzymes 1, pyridoxal kinase (present in all mammalian tissues) 2, nonspecific (probably alkaline) phosphatases 3, pyridoxine oxidase (cofactor is FMN O2 is required subject to product inhibition) 4, aldehyde oxidase or aldehyde dehydrogenase 5, aminotransferase,... 

Modeling vitamin B6 metabolism is further complicated by the fact that the activity of the kinase, oxidase, and phosphatase enzymes varies between organs and species. A very simplified diagram of vitamin B6 metabolism is shown in Fig. 2. In the intestine any phosphoiylated forms are hydrolyzed. The free vitamers are readily taken up by diffusion into the intestinal wall where significant phosphorylation (Middleton, 1979) and other metabolism (Middleton, 1985) occurs. In mice small doses (up to 14 nmol) of pyridoxine (Sakurai et aL, 1988) and pyridoxamine (Sakurai et oL, 1992) were converted almost completely to pyridoxal before being released into the portal circulation. While it is dear that the intestinal microflora produce vitamin B6,... 

The major sales form of vitamin B6 is the hydrochlorid salt of the primary alcohol pyridoxine. Another vitamin B6 form introduced in the market is the dihydrochlo-rid salt of pyridoxamine. Both vitamin B6 forms are commercially produced via various straightforward chemical synthesis routes. The biologically active cofactor is the aldehyde pyridoxal-5 -phosphate, which is derived in human or animals from the vitamin B6 forms by oxidation or transamination before or after 5 phosphorylation by pyridoxal kinases. 

A second enzymatic site, which has been selected as a target to direct the chelating potential of the thiosemicarbazone molecule, is pyiidoxal phospho-kinase [65]. This catalyst is a zinc-requiring enzyme [66] that catalyzes the phosphorylation of pyridoxine at position 5 to form the active coenzyme form. In an effort to accomplish inhibition, 2-formyl-3-hydroxy-4,5-bis(hydroxy-methyOpyridine thiosemicarbazone (117) was synthesized [65]. This was accomplished by two different procedures. The common intermediate, 3-acetoxy-2,4,5-tris(acetoxymethyl)pyridine (111) [67], upon acid hydrolysis afforded 3-hydroxy-2,4,5-tris(hydroxymethyl)pyridine hydrochloride (112), which was treated with hydrogen chloride as a suspension in acetone [68). The amount of hydrogen chloride taken up by the suspension is a critical factor and. 

Kinases catalyzing the phosphorylation of pyridox-ine, pyridoxamine, and pyridoxal have been found in microorganisms and in mammalian tissues. The kinases require zinc and magnesium, and ATP acts as phosphate donor. The affinity for the substrate varies depending on the source pyridoxine and pyridoxamine are the preferred substrates with yeast enzyme, but the mammalian enzyme has a greater affinity for pyridoxal. 

The term vitamin Bg refers to a group of naturally occurring pyridine derivatives represented by pyridoxine (pyridoxol, PN), pyridoxal (PL), and pyridoxamine (PM), and their phosphorylated derivatives. They are collectively referred to as vitamin Bg vitamers. The natural free forms of the vitamers could be converted to the key coenzymatic form, pyridoxal phosphate (PLP), by the action of two enzymes, a kinase and an oxidase. There are more than 140 PLP-dependent enzymatic reactions, and they are distributed in all organisms. These enzymes comprise diverse groups such as the oxidoreductases, transferases, hydrolases, lyases, and isomerases. About... 

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