Pyridoxal kinase and

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. 

In healthy volunteers, theophylline reduced circulating pyridoxal phosphate (vitamin B6) concentrations, presumably by noncompetitive inhibition of pyridoxal kinase. Theophylline concentrations of approximately 10 gg/ml produced only partial inhibition, plasma pyridoxal kinase and pyridoxal concentrations being unaffected. The authors speculated that with theophylline overdose and greater inhibition, vitamin B6 deficiency might contribute to seizures (SEDA-14, 2). 

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. 

Fig. 1. Ethylene biosynthesis. The numbered enzymes are (1) methionine adenosyltransferase, (2) ACC (l-aminocyclopropane-l-carboxylic acid) synthase, (3) ethylene forming enzyme (EFE), (4) 5 -methylthio-adenosine nucleosidase, (5) 5 -methylthioribose kinase. Regulation of the synthesis of ACC synthase and EFE are important steps in the control of ethylene production. ACC synthase requires pyridoxal phosphate and is inhibited by aminoethoxy vinyl glycine EFE requires 02 and is inhibited under anaerobic conditions. Synthesis of both ACC synthase and EFE is stimulated during ripening, senescence, abscission, following mechanical wounding, and treatment with auxins.

Phosphorylase kinase, one of the Ser/Thr-kinases, is oligomeric with the stoichiometry (a/3yS)4.80> The y-subunit of the enzyme is homologous to other protein kinases and possesses protein kinase activity when separated from other subunits.811 Phosphorylase kinase was more effectively inactivated by AP3-PL and AP4-PL than PLP and AP2-PL. Ca2+ and Mg2+, activators for this enzyme, enhanced the degree of inactivation by all the pyridoxal compounds. Inactivation by AP3-PL and AP4-PL was markedly protected by adenylyl, /3,y-imidodiphosphate, a nonhydrolyzable analogue of ATP, and ADP. Because the a- and /3-subunits have regulatory ATP-binding sites and the kinase activity is regulated by these subunits,821 the incorporation of AP3-PL into the y-subunit and... 

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. 

TIQ 135 was reinvestigated by Kametani et al. (253). Pyridoamine, but not pyridoxal, prevented the accumulation of ketonic metabolites of aromatic amino acids by eliminating them as Schiff bases (254). Isoquinolines obtained from pyridoxal and dopamine, such as TIQ 135, did not inhibit the enzyme pyridoxal kinase (255). 

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.

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. 

Pyridoxal, pyridoxamine and pyfidoxine are collectively known as vitamin-B6. All three compounds are efficiently converted to the biologically active form of vitamin-B6, pyridoxal phosphate. The ATP requiring enzyme, pyridoxal kinase, catalyzes this conversion. 

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). 

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