Pyridoxal-phosphatase

Ketimine 121,744s. See also Schiff base from pyridoxal phosphate 742 as electron acceptor 746, 747 a-Ketoacid. See 2-Oxoacid Ketoamine 434s Ketodeoxyoctonate. See KDO Ketone(s), acidity of 46 Khorana, H. Gobind 84 Kidney cells, alkaline phosphatase in 645... 

Other compounds, not shown in Table VI, which are hydrolyzed by alkaline phosphatase are TPN (28), poly A (28), phosphocellulose (28), pyrophosphoserine (102, 103), phosphoserine (102-104), pyridoxine phosphate (104), pyridoxal phosphate (104), phosphothreonine (104), and phosphocholine (104) These compounds are all hydrolyzed at approximately the same rate. 

Whyte MP, Landt M, Ryan LM, Mulivor RA, Henthorn PS, Fedde KN, Mahuren JD, Coburn SP. 1995. Alkaline phosphatase placental and tissue-nonspecific isoenzymes hydrolyze phosphoethanolamine, inorganic pyrophosphate, and pyridoxal 5 -phosphate. Substrate accumulation in carriers of hypophos-phatasia corrects during pregnancy. J Clin Invest 95 1440-5. 

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. 

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. 

Extrahepatic tissues take up both pyridoxal and pyridoxal phosphate from the plasma. Pyridoxal phosphate is hydrolyzed to pyridoxal, which can cross cell membranes, by extracellular alkaline phosphatase, which is then trapped intracellularly by phosphorylation. 

In subjects with hypophosphatasia, the rare genetic lack of extracellular alkaline phosphatase, plasma concentrations of pyridoxal phosphate are very much higher than normal (up to 4 /rmol per L, compared with a normal range of about 100 nmol per L), and intracellular concentrations of pyridoxal phosphate are lower than normal (Narisawa et al., 2001). 

Tissue concentrations of pyridoxal phosphate are controlled by the balance between phosphorylation and dephosphorylation. The activity of phosphatases acting on pyridoxal phosphate is greater than that of the kinase in most tissues, although this may be an artifact of determining alkaline phosphatase activity at its pH optimum rather than at a more physiological pH, when the two activities are approximately equal. This means that pyridoxal phosphate that is not bound to enzymes is readily dephosphorylated. 

Another metabolic disorder that is hereditary and little known is hypophosphatasia. Hypophosphatasia is an inherited metabolic (chemical) bone disease that results from low levels of an enzyme called alkaline phosphatase (ALP). ALP is normally present in large amounts in bones and the liver. In hypophosphatasia, abnormalities in the gene that makes ALP lead to the production of inactive ALP. Subsequently, several chemicals, including phosphoethanolamine, pyridoxal 57-phosphate (a form of vitamin B ) and inorganic pyrophosphate, accumulate in the body and are found in large amounts in the blood and urine. It appears that the accumulation of inorganic pyrophosphate is the cause of the characteristic defective calcification of bones seen in infants and children (rickets) and in adults (osteomalacia). 

Release of free vitamin, mainly pyridoxal, occurs when physiological nonsaturating levels of vitamin are absorbed. Here the phosphates are hydrolyzed by nonspecific alkaline phosphatase located on the plasma membrane of cells. Some PLP is also released into the circulation by the liver. Because the reactive aldehyde is capable of forming Schiff bases with amino groups, PLP in plasma is more tightly com-plexed to proteins—mostly albumin—than is pyridoxal, which forms an intramolecular hemiacetal between the... 

Uptake and Metabolism. The vitamin Bg family consists of pyridoxine, pyridoxal, pyridoxamine, pyridoxine phosphate, pyridoxal phosphate (PLP), and pyridoxamine phosphate (Fig. 8.33). The commercial form is pyridoxine. Pyridoxal phosphate is the coenzyme form. It and pyridoxamine phosphate are from animal tissues. Pyridoxine is from plant tissues. All phosphorylated forms are hydrolyzed in the intestinal tract by phosphatases before being absorbed passively. Conversion to the phosphorylated forms occurs in the liver. Notice that niacin (NAD) and riboflavin (FMN, FAD) are required for interconversion among the vitamin Bq family. The phosphorylated forms are transported to the cells where needed. The major excretory product is 4-pyr-idoxic acid. 

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,... 

S4 Saraswathi, S., and Bachhawat, B. K., Phosphatases from hmnan brain. I. Purification and properties of pyridoxal phosphate phosphatase. J. Neurochem. 10, 127-133 (1983). 

Smith GP, Peters TJ (1981) SubceUular localization and properties of pyridoxal phosphate phosphatases of human polymorphonuclear leukocytes and their relationship to acid and alkaline phosphatase. Biochim Biophys Acta 661 287-294... 

Wilson PD, Smith GP, Peters TJ (1983) Pyridoxal 5 -phosphate a possible physiological substrate for alkaline phosphatase in human neutrophils. Histochem J 15 257-264... 

If SIP is not secreted or dephosphorylated by specific SIP phosphatases, as well as by more general lipid phosphatases, it is cleaved irreversibly to ethanolamine phosphate and rrans-2-hexadecenal by SIP lyase (J. Zhou, 1998 P.P. Van Veldhoven, 20(X)). As shown first in the 1970s by W. Stoffel and coworkers, the phosphoethanolamine can be utilized for the synthesis of phosphatidylethanolamine (Chapter 8), and fran5-2-hexadecenal can be reduced to the alcohol and incorporated into alkyl ether lipids. Under certain conditions, degradation of sphingoid bases can account for as much as one-third of the ethanolamine in phosphatidylethanolamine (E.R. Smith, 1995). It is interesting that both the first enzyme of sphingoid base metabolism (SPT) and the last enzyme, the lyase, are pyridoxal 5 -phosphate-dependent. 

Fig. 1. The metabolic interconversion of various forms of vitamin Bg. Reactions 1, 2 and 3 are catalysed by pyridoxal kinase, reactions 4, S and 6 by various phosphatases, reactions 7, 8 and 10 by pyridoxal-P oxidases, reactions 9 and 11 by certain aminotransferases, and reactions 12 and 13 by various pyridoxal dehydrogenases (taken from ref. 2).

Fedde, K.N. and Whyte, M.P. (1990) Alkaline phosphatase (tissue-nonspecific isoenzyme) is a phosphoethanolamine and pyridoxal-5 -phosphate ectophosphate normal and hypo-phosphatasia fibroblast study. American Journal ofEiuman Genetics 47, 757-775. 

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 importance of alkaline phosphatase in regulating pyridoxal phosphate concentrations in plasma is indicated by the decreased pyridoxal phosphate observed in liver disease diaracterized by increased alkaline phosphatase (Labadarios et al, 1977) and the high concentrations of pyridoxal phosphate found in hypophosphatasia (low alkaline phosphatase) (Whyte et oL, 1985). The relative importance of pyridoxal and pyridoxal phosphate in vitamin B6 transport to peripheral tissues remains uncertain. Limited evidence in pigs and goats suggests that tissues take up approximately equal amounts of pyridoxal and pyridoxal phosphate (Coburn et al, 1992a). 

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