Folate cycle

Methotrexate inhibits folate metabolism by preventing methylenetetrahydrofolate reductase from converting 5,10-methylenetetrahydrofolate to 5-methyltetrahydrofolate thus inhibiting thymidylate synthase conversion of dUMP to dTMP. DNA replication is effectively decreased by the diminution of dTMP availability. As shown in Fig. 2, multiple enzymes mediate the folate cycle. Thus, genetic variation in these enzymes may... 

Costea I, Moghrabi A, Laverdiere C et al. Folate cycle gene variants and chemotherapy toxicity in pediatric patients with aeute lymphoblastic leukemia. Haematologica 2006. 

In the folate cycle, which is linked to the methionine cycle, homocysteine is remethylated to methionine by the vitamin B -dependent enzyme methionine synthase (MS), thereby completing the cycle. 5-Methyltetrahydrofolate (CH3-THF) acts as a methyl donor in this reaction, which produces methionine and tetrahydrofolate (THF). 

Continuing the folate cycle, THF reacts with serine to produce 5,10-methylenetetrahydrofolate, a reaction catalyzed by the vitamin B6-dependent enzyme serine/glycine hydrox-ymethyltransferase. 

Measurement of blood tHcy is usually performed for one of three reasons (1) to screen for inborn errors of methionine metabolism (2) as an adjunctive test for cobalamin deficiency (3) to aid in the prediction of cardiovascular risk. Hyperhomocysteinemia, defined as an elevated level of tHcy in blood, can be caused by dietary factors such as a deficiency of B vitamins, genetic abnormalities of enzymes involved in homocysteine metabolism, or kidney disease. All of the major metabolic pathways involved in homocysteine metabolism (the methionine cycle, the transsulfuration pathway, and the folate cycle) are active in the kidney. It is not known, however, whether elevation of plasma tHcy in patients with kidney disease is caused by decreased elimination of homocysteine in the kidneys or by an effect of kidney disease on homocysteine metabolism in other tissues. Additional factors that also influence plasma levels of tHcy include diabetes, age, sex, lifestyle, and thyroid disease (Table 21-1). 

Pyridoxine (vitamin B6) Transsulfuration pathway cofactor for cystathionine-(3-synthase and cystathionine "/-lyase Folate cycle methylation of tetrahydrofolate cofactor for serine hydroxymethyltransferase... 

Riboflavin (vitamin B2) Folate cycle reduction of 5,10-methyltetrahydrofolate cofactor for methylene-tetrahydrofolate reductase... 

It was suggested by Hillman (H6) that an enterohepatic cycle has an important role in folate homeostasis. This was based on the observation that alcohol appeared to reduce the level of folate supplied to serum and tissue by interfering with folate clearance into bile. Studies in rats of this aspect of the folate cycle confirm that the enterohepatic pathway has a major role in folate homeostasis. It has been shown that when small amounts of radio-labeled methylpteroylmonoglutamate and pteroylmonoglutamate are placed in the rat jejunum they are both rapidly absorbed and transported to the liver. They are then cleared into the bile predominantly in the methyl form. The original methyl form appears in the bile unchanged 10 min after administration and the pteroylmonoglutamate appears in the bile after conversion to the methyl form in a similar time period. This confirms that only a very short period of time is required for methylation. Bile folate is reabsorbed for... 

If one analyzes the flow of carbon in the folate cycle, the equilibrium lies in the direction of the N -methyl FH4 form. This appears to be the most stable form of carbon attached to the vitamin. However, in only one reaction can the methyl group be removed from N -methyl FH4, and that is the methionine synthase reaction, which requires vitamin B12. Thus, if vitamin B12 is deficient, or if the methionine synthase enzyme is defective, N -methyl FH4 will accumulate. Eventually most folate forms in the body will become trapped in the N -methyl form. A functional folate deficiency results because the carbons cannot be removed from the folate. The appearance of a functional folate deficiency caused by a lack of vitamin B12 is known as the methyl-trap hypothesis, and its clinical implications are discussed in following sections. 

The remethylation cycle allows the conversion of homocysteine back to methionine by two pathways. The first and major pathway is catalyzed by the enzyme, methionine synthase, and links the folate cycle with homocysteine metabolism. Methionine synthase requires the cofactor, meth-ylcobalamin. The second pathway utilizes the enzyme, betaine-homocysteine methyltransfer-ase [8]. This pathway remethylates homocysteine using a methyl group derived from betaine, formed via oxidation of choline, and is presumably responsible for up to 50 % of homocysteine remethylation [10]. Both methionine and homocysteine play important roles in protein synthesis, folding, and function. 

In the folate cycle, the THF reacts with serine synthetizing N5,io-methylene-THF (5,10-CH2-THF) in a reaction catalysed by serine hydroxymethyltransferase (SHMT). The 5,10-CH2-THF is reduced to 5-CH3-THF by the... 

Other candidate SNPs in different genes may also influence the folate metabolism network through transport systems, methylation cycle or folate cycle, and impact on health risk. Many SNPs have been identified. Some of these have been studied, although not all reported studies have eonfirmed association with health alteration. 

Antimetabolites interfere with normal metabolic pathways. They can be grouped into folate antagonists and analogues of purine or pyrimidine bases. Their action is limited to the S-phase of the cell cycle and therefore they target a smaller fraction of cells as compared with alkylating agents. 

Enzymatic reactions that use folates. Section 1 shows the vitamin B12 -dependent reaction that allows most dietary folates to enter the tetrahydrofolate cofactor pool and becomes the "folate trap" in vitamin B12 deficiency. Section2 shows the dTMP cycle. 

H FIGURE 22-49 Thymidylate synthesis and folate metabolism as targets of chemotherapy, (a) During thymidylate synthesis, /V5,N10-methylenetetrahydrofolate is converted to 7,8-dihydrofolate the N5,N10-methylenetetrahydrofolate is regenerated in two steps (see Fig. 22-44). This cycle is a major target of several chemotherapeutic agents, (b) Fluorouracil and methotrexate are important chemotherapeutic agents. In cells, fluorouracil is converted to FdUMP, which... 

The formation of phosphatidylserine and possibly other phospholipids in animal tissues may also be accomplished by exchange reactions (Eq. 21-10, step a). 82 83 At the same time, decarboxylation of phosphatidylserine back to phosphatidylethanolamine (Eq. 21-10, step b) also takes place, the net effect being a catalytic cycle for decarboxylation of serine to ethano-lamine. The latter can react with CTP to initiate synthesis of new phospholipid molecules or can be converted to phosphatidylcholine (step c). However, unless there is an excess of methionine and folate in the diet, choline is an essential human nutrient.184... 

Tetrahydrofolate cofactors participate in one-carbon transfer reactions. As described above in the section on vitamin B12, one of these essential reactions produces the dTMP needed for DNA synthesis. In this reaction, the enzyme thymidylate synthase catalyzes the transfer of the one-carbon unit of N 5,N 10-methylenetetrahydrofolate to deoxyuridine monophosphate (dUMP) to form dTMP (Figure 33-2, reaction 2). Unlike all of the other enzymatic reactions that utilize folate cofactors, in this reaction the cofactor is oxidized to dihydrofolate, and for each mole of dTMP produced, one mole of tetrahydrofolate is consumed. In rapidly proliferating tissues, considerable amounts of tetrahydrofolate can be consumed in this reaction, and continued DNA synthesis requires continued regeneration of tetrahydrofolate by reduction of dihydrofolate, catalyzed by the enzyme dihydrofolate reductase. The tetrahydrofolate thus produced can then reform the cofactor N 5,N 10-methylenetetrahydrofolate by the action of serine transhydroxy- methylase and thus allow for the continued synthesis of dTMP. The combined catalytic activities of dTMP synthase, dihydrofolate reductase, and serine transhydroxymethylase are often referred to as the dTMP synthesis cycle. Enzymes in the dTMP cycle are the targets of two anticancer drugs methotrexate inhibits dihydrofolate reductase, and a metabolite of 5-fluorouracil inhibits thymidylate synthase (see Chapter 55 Cancer Chemotherapy). 

Folate (vitamin B9) Methionine cycle methyl donor in the remethylation of homocysteine to methionine via methionine synthase... 

Cyanocobalamine is a component of several coenzymes and has an effect on nucleic acid formation through its action in cycling 5-methyl-tetrahydrofolate back into the folate pool. The most important dietary sources of the vitamin are animal products. Vitamin Bl2 is also produced by many microorganisms. It is not surprising that vitamin B12 deficiency of dietary origin only occurs in vegetarians. 

Steinberg SE, Campbell CL, and Hillman RS (1979) Kinetics of the normal folate entero-hepatic cycle. Journal of Clinical Investigation 64, 83-8. 

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