Oxidized folates

Oxidized folate is not only metabolically dead buyt may even be neurotoxic. For example, a patient with epilepsy who has not had a convulsion in years because dilantin has produced complete control, can be thrown into an immediate convulsion with a megadose of folic acid, because folic acid and dilantin compete for absorption at the brain cell surface, and too much oxidized folic acid will block the ability of the brain cell to take up dilantin, similar to the competition between dilantin and folic acid for uptake by the gutcell (22). 

S12). A second serum binder for folate has been described (W9, R6). Althou this binder has a high affinity for folate, its capacity is low moreover it binds oxidized folate well but 5-methyltetrahydrofolate poorly. Folate attached to this protein is not removed by charcoal, indicating that the bond is firm. Vitamin bound to tbis protein is not given up to cells and possibly represents a storage form. 

Mechanistic aspects of the action of folate-requiring enzymes involve one-carbon unit transfer at the oxidation level of formaldehyde, formate and methyl (78ACR314, 8OMI2I6OO) and are exemplified in pyrimidine and purine biosynthesis. A more complex mechanism has to be suggested for the methyl transfer from 5-methyl-THF (322) to homocysteine, since this transmethylation reaction is cobalamine-dependent to form methionine in E. coli. 

Folic acid derivatives (folates) are acceptors and donors of one-carbon units for all oxidation levels of carbon except that of CO2 (where biotin is the relevant carrier). The active coenzyme form of folic acid is tetrahydrofolate (THF). THF is formed via two successive reductions of folate by dihydrofolate reductase (Figure 18.35). One-carbon units in three different oxidation states may be bound to tetrahydrofolate at the and/or nitrogens (Table 18.6). These one-carbon units... 

The third reason for favoring a non-radical pathway is based on studies of a mutant version of the CFeSP. This mutant was generated by changing a cysteine residue to an alanine, which converts the 4Fe-4S cluster of the CFeSP into a 3Fe-4S cluster (14). This mutation causes the redox potential of the 3Fe-4S cluster to increase by about 500 mV. The mutant is incapable of coupling the reduction of the cobalt center to the oxidation of CO by CODH. Correspondingly, it is unable to participate in acetate synthesis from CH3-H4 folate, CO, and CoA unless chemical reductants are present. If mechanism 3 (discussed earlier) is correct, then the methyl transfer from the methylated corrinoid protein to CODH should be crippled. However, this reaction occurred at equal rates with the wild-type protein and the CFeSP variant. We feel that this result rules out the possibility of a radical methyl transfer mechanics and offers strong support for mechanism 1. 

While the fluid mosaic model of membrane stmcture has stood up well to detailed scrutiny, additional features of membrane structure and function are constantly emerging. Two structures of particular current interest, located in surface membranes, are tipid rafts and caveolae. The former are dynamic areas of the exo-plasmic leaflet of the lipid bilayer enriched in cholesterol and sphingolipids they are involved in signal transduction and possibly other processes. Caveolae may derive from lipid rafts. Many if not all of them contain the protein caveolin-1, which may be involved in their formation from rafts. Caveolae are observable by electron microscopy as flask-shaped indentations of the cell membrane. Proteins detected in caveolae include various components of the signal-transduction system (eg, the insutin receptor and some G proteins), the folate receptor, and endothetial nitric oxide synthase (eNOS). Caveolae and lipid rafts are active areas of research, and ideas concerning them and their possible roles in various diseases are rapidly evolving. 

Sonvico F, Mornet S, Vasseur S, Dubernet C, Jaillard D, Degrouard J, Hoebeke J, Duguet E, Colombo P, Couvreur P (2005) Folate-conjugated iron oxide nanoparticles for solid tumor targeting as potential specific magnetic hyperthermia mediators synthesis, physicochemical characterization, and in vitro experiments. Bioconjugate Chemistry 16 1181-1188. 

The chemical structure of folate (or folic acid) is shown in Figure 5.8. In humans, folate usually occurs as polyglutamate derivatives. The active form of folate is THF, sometimes shown as FH4) is derived from folate via two reductase reactions. THF functions as a carrier of one-carbon groups in varying oxidation states (Table 5.1). 

Figure 15.2 Structural formula of tetrahydrofolate and representation of derivatives involved in single carbon transfer. The tetrahydrofolate is always part of a complex with several glutamate residues. The parent compound, pteroylglutamate (folate) lacks four hydrogen atoms, one each from carbon atoms 5, 6, 7 and 8. Tetrahydrofolate can exist in any one of three oxidation states, as shown they are interconvertible through oxidereduction reactions. Each plays a individual and different role is synthesis of key compounds (See below).

Figure 22.6 How various factors increase the risk of atherosclerosis, thrombosis and myocardial infarction. The diagram provides suggestions as to how various factors increase the risk of development of the trio of cardiovascular problems. The factors include an excessive intake of total fat, which increases activity of clotting factors, especially factor VIII an excessive intake of saturated or trans fatty acids that change the structure of the plasma membrane of cells, such as endothelial cells, which increases the risk of platelet aggregation or susceptibility of the membrane to injury excessive intake of salt - which increases blood pressure, as does smoking and low physical activity a high intake of fat or cholesterol or a low intake of antioxidants, vitamin 6 2 and folic acid, which can lead either to direct chemical damage (e.g. oxidation) to the structure of LDL or an increase in the serum level of LDL, which also increases the risk of chemical damage to LDL. A low intake of folate and vitamin B12 also decreases metabolism of homocysteine, so that the plasma concentration increases, which can damage the endothelial membrane due to formation of thiolactone.

Folate is the generic name used to refer to a family of vitamers with related biological activity. Instead, folic acid (pteroylglutamin acid, PGA) (Fignre 19.16) refers to the most oxidized, stable, and easily adsorbable synthetic form (monoglutamate). It is commonly nsed in food supplements and in food fortification because of its stability and becomes biologically active after redaction. 

Some commonly used detectors are UV (at 280 nm), ELD, ED and microbiological assay of collected fractions. UV presents a low sensitivity, but all folate derivatives respond to this detection. ELD is used even if some compounds, like folic acid, do not fluoresce. Therefore, a postcolumn derivatization, involving hypochlorite to cleave folic acid, di- and tetra-hydrofolic acid oxidatively to fluorescence pterins [571], has been introduced. Eewer reports on the use of LC-MS for folate detection are available in the literature [578-580]. 

The stereochemical course of the reaction catalyzed by dihydroneopterin aldolase has been established <2002JBC28841>. By carrying out the reaction in deuterium oxide and using multinuclear NMR spectroscopy of folate derived from the reaction product, 6-hydroxymethylpterin, it was shown that the late-stage enol intermediate undergoes protonation to form 6-hydroxymethylpterin with deuterium predominantly in the A-configuration. 

The other major class of antimalarials are the folate synthesis antagonists. There is a considerable difference in the drug sensitivity and affinity of dihydrofolate reductase enzyme (DHFR) between humans and the Plasmodium parasite. The parasite can therefore be eliminated successfully without excessive toxic effects to the human host. DHFR inhibitors block the reaction that transforms deoxyuridine monophosphate (dUMP) to deoxythymidine monophosphate (dTMP) at the end of the pyrimidine-synthetic pathway. This reaction, a methylation, requires N °-methylene-tetrahydrofolate as a carbon carrier, which is oxidized to dihydrofolate. If the dihydrofolate cannot then be reduced back to tetrahydrofolate (THF), this essential step in DNA synthesis will come to a standstill. 

Figure 14.2 Chemical structure of folates. Folate molecules consist of pteridine, para-aminobenzoate (pABA), and glutamate moieties. Plants usually contain polyglutamylated forms of folates that are made by the addition of up to about six glutamate residues (which form the y-glutamate tail) attached to the first glutamate, each linked by amide bonds to the preceding molecule of glutamate through the y-carboxyl of the latter. Cl units at various levels of oxidation can be attached to NS and/or N1 0, as indicated by Ri and R2.

Folate is a relatively unstable nutrient processing and storage conditions that promote oxidation are of particular concern since some of the forms of folate found in foods are easily oxidized. The reduced forms of folate (dihydro- and tetrahydrofolate) are oxidized to p-aminobenzoylglutamic acid and pterin-6-carboxylic acid, with a concomitant loss in vitamin activity. 5-Methyl-H4 folate can also be oxidized. Antioxidants (particularly ascorbic acid in the context of milk) can protect folate against destruction. The rate of the oxidative degradation of folate in foods depends on the derivative present and the food itself, particularly its pH, buffering capacity and concentration of catalytic trace elements and antioxidants. 

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