Liver folates

Under normal conditions, the predominant folates in liver are pentaglu-tamates, with small amounts of tetra- and hexaglutamates. The extent of poly-glutamylation is controlled to a great extent by the availability of folate in deficient animals, hexa- to octaglutamates predominate, whereas in supplemented animals, liver folate is mainly as the tri- to pentaglutamates (Cassady etal., 1980). 

Experimental animals that have been exposed to ititrous oxide to deplete vitamin B12 show an increase in the proportion of liver folate present as methyl-tetrahydrofolate (85% rather than the normal 45%), largely at the expense of unsubstituted tetrahydrofolate and increased urinary loss of methyl-tetrahydrofolate (Horne et al., 1989). Tissue retention of folate is impaired because methyl-tetrahydrofolate is a poor substrate for polyglutamyl-folate synthetase, compared with unsubstituted tetrahydrofolate (Section 10.2.2.1). As a result of this, vitamin B12 deficiency is frequently accompanied by biochemical evidence of functional folate deficiency, including impaired metabolism of histidine (excretion of formiminoglutamate Section 10.3.1.2) and impaired thymidylate synthetase activity (as shown by abnormally low dUMP suppression Section 10.3.3.3), although plasma concentrations of methyl-tetrahydrofolate are normal or elevated. 

Chronic therapy of experimental animals with primidone depletes liver folate pentaglutamates, suggesting inhibition of folate polyglutamate synthetase (Carl et al 1987). This would be expected to lead to increased excretion of folate metabolites. 

Cassady lA, Budge MM, Healy MJ, and Nixon PF (1980) An inverse relationship of rat liver folate polyglutamate chain length to nutritional folate sufficiency Biochimica et Biophysica Acta 633, 258-68. 

Folate status may be reliably assessed by direct measurement of serum and erythrocyte or whole blood concentrations, and its metabolic function as coen2yme assessed by metabolite concentrations, such as plasma homocysteine (see Chapters 20 and 26). Serum folate concentrations are considered indicative of recent intake and not of tissue stores, but serial measurements have been used to confirm adequate intake. Whole blood or erythrocyte folate concentrations are more indicative of tissue stores and have been shown to have a moderate correlation with liver folate concentrations taken through a biopsy. Because folate is taken up only by the developing erythrocyte in the bone marrow and not by the mature cell, erythrocyte concentrations reflect folate status over the 120-day lifespan of the ceU. Urine folate excretion is not considered to be a sensitive indicator of folate status. ... 

In historical terms, folates are among the most recently identified of the vitamins. Wills was the first to describe a form of anaemia associated with pregnancy and malnutrition which could be cured by yeast or liver extract (Wills, 1933 Wills et al, 1937). The active constituent of these dietary... 

POGRIBNY J P, BASNAKIAN A G, MILLER B J, LOPATINA N G, POIRIER L A and JAMES S J (1995) Breaks in genomic DNA and within the P5 3 gene are associated with hypomethy lation in livers of folate/methyl-deficient rats . Cancer Res, 55, 1894-901. 

Rule out vitamin B]2 and folate deficiency Rule out hypothyroidism with thyroid function tests Blood cell counts, serum electrolytes, liver function tests Other diagnostic tests... 

Recently a great deal of effort has been spent in studying the metabolism of leucovorin in vivo. This emphasis was prompted by the chemical stability of this folate and by the observation of a reduction in toxicity of methotrexate when it was given in conjunction with leucovorin. Fol-inic acid is found in human liver, but it is not the major circulating folate, which is 5-methyltetrahydrofolate. 

Mandatory studies Complete blood count Blood electrolytes Liver enzymes Urinalysis Vitamin B,2 level Folate level Syphilis (RPR/VDRL) Thyroid-stimulating hormone (TSH) Brain scan (CT or MRI) Electrocardiograph (EKG)... 

Folates Yeast, liver, fresh green vegetables Nucleotide metabolism... 

Eolates are present in nearly all natural foods. Those with the highest folate content include yeast, liver and other organ meats, fresh green vegetables and some fresh fruits. The term folic acid arose from its first identification in leafy vegetables (e.g. spinach). 

Vitamin Bn deficiency Deficiency, although rare, results in two serious problems megaloblastic anaemia (which is identical to that caused by folate deficiency) and a specific neuropathy called Bi2-associated neuropathy or cobalamin-deficiency-associated neuropathy (previously called, subacute combined degeneration of the cord). A normal healthy adult can survive more than a decade without dietary vitamin B12 without any signs of deficiency since it is synthesised by microorganisms in the colon and then absorbed. However, pernicious anaemia develops fairly rapidly in patients who have a defective vitamin B12 absorption system due to a lack of intrinsic factor. It results in death in 3 days. Minot and Murphy discovered that giving patients liver, which contains the intrinsic factor, and which is lightly cooked to avoid denaturation, cured the anaemia. For this discovery they were awarded the Nobel Prize in Medicine in 1934. 

Interestingly, after intravenous administration of a radiolabelled folate conjugate ( -In-dium-diethylenetriaminepenta acid (DTPA)-folate) in the rat, the conjugate was rapidly excreted in the urine. Moreover, after intravenous administration to athymic mice with a human tumour cell implant, the radiotracer was not only taken up by the subcutaneous tumour but was also taken up by the kidneys in significant quantities [63], indicating substantial renal selectivity of the folate conjugate. In addition to the kidney, the liver also has a high concentration of the folate-receptor [64]. 

Folic acid appears in the plasma approximately 15 to 30 minutes after an oral dose peak levels are generally reached within 1 hour. After IV administration, the drug is rapidly cleared from the plasma. Folic acid is metabolized in the liver. Normal serum levels of total folate have been reported to be 5 to 15 ng/mL normal CSF levels are approximately 16 to 21 ng/mL. In general, folate serum levels less than 5 ng/mL indicate folate deficiency, and levels less than 2 ng/mL usually result in megaloblastic anemia. A majority of the metabolic products appeared in the urine after 6 hours excretion was generally complete within 24 hours. 

Figure 1.1. Opposite) Sulpha drugs and their mode of action. The first sulpha drug to be used medically was the red dye prontosil rubrum (a). In the early 1930s, experiments illustrated that the administration of this dye to mice infected with haemolytic streptococci prevented the death of the mice. This drug, while effective in vivo, was devoid of in vitro antibacterial activity. It was first used clinically in 1935 under the name Streptozon. It was subsequently shown that prontosil rubrum was enzymatically reduced by the liver, forming sulphanilamide, the actual active antimicrobial agent (b). Sulphanilamide induces its effect by acting as an anti-metabolite with respect to /iflra-aminobenzoic acid (PABA) (c). PABA is an essential component of tetrahydrofolic acid (THF) (d). THF serves as an essential co-factor for several cellular enzymes. Sulphanilamide (at sufficiently high concentrations) inhibits manufacture of THF by competing with PABA. This effectively inhibits essential THF-dependent enzyme reactions within the cell. Unlike humans, who can derive folates from their diets, most bacteria must synthesize it de novo, as they cannot absorb it intact from their surroundings...

Fig. 1. Folate-cobalamin interaction in the synthesis of purines and pyrimidines and, therefore, of DNA. (1) In gastrointestinal mucosa cells (2) in the liver (3) in peripheral tissues. C, cobalamine DAC, desoxyadenosylcobalamine HC, hydroxy cobalamine MC, methylcobalamine F, folic acid MTHF, methyltetrahydrofolic acid THF, tetrahydrofolic acid DHF, dihydrofolic acid dUMP, deoxyuridinemonophosphate dTMP, deoxythymidine-monophosphate. (Adapted from Far-...

About 10-25%, i.e. 50-200 pg, of the daily dietary intake of folic acid in yeasts, liver, and green vegetables is absorbed via active and passive transport in the proximal jejunum. As humans do not have dihydropteroate synthetase, which synthesizes folic acid in bacteria, we require folic acid in the diet. Only small amounts of folate can be stored in the body and dietary deficiency for only a few days can result in symptomatic folate deficiency. 

Drowsiness, headache, dizziness, vertigo, seizures, confusion, psychosis, paresis, tremor, vitamin deficiency folate deficiency, cardiac arrhythmias, increased liver enzymes... 

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