Thymidine folic acid

Methotrexate belongs to the class of antimetabolites. As a derivative of folic acid it inhibits the enzyme dihydrofolate reductase resulting in a decreased production of thymidine and purine bases essential for RNA and DNA synthesis. This interruption of the cellular metabolism and mitosis leads to cell death. 

Folic acid Thymidylate synthase Thymidine (pyrimidine) MCC alcoholics and pregnancy (body... 

Folic acid is a vitamin, as we developed in chapter 15. It is a complex molecule that serves as an essential precursor for coenzymes involved in the metabolism of one-carbon units. For example, folic acid-derived coenzymes are critically involved in the biosynthesis of thymidine for nucleic acid synthesis and methionine for protein biosynthesis. The synthesis of both demands donation of a methyl group and they come from folic acid-derived coenzymes. 

Inhibition of nucleobase synthesis (2). Tetrahydrofolic acid (THF) is required for the synthesis of both purine bases and thymidine. Formation of THF from folic acid involves dihydrofolate reductase (p. 272). The folate analogues aminopterin and methotrexate (ame-thopterin) inhibit enzyme activity as false substrates. As cellular stores of THF are depleted, synthesis of DNA and RNA building blocks ceases. The effect of these antimetabolites can be reversed Ltillmann, Color Atlas of Pharmacology 2000 Thieme All rights reserved. Usage subject to terms and conditions of iicense. 

Both sulfonamides and trimethoprim (not a sulfonamide) sequentially interfere with folic acid synthesis by bacteria. Folic acid functions as a coenzyme in the transfer of one-carbon units required for the synthesis of thymidine, purines, and some amino acids and consists of three components a pteridine moiety, PABA, and glutamate (Fig. 44.1). The sulfonamides, as structural analogues, competitively block PABA incorporation sulfonamides inhibit the enzyme dihydropteroate synthase, which is necessary for PABA to be incorporated into dihydropteroic acid, an intermediate compound in the formation of folinic acid. Since the sulfonamides reversibly block the synthesis of folic acid, they are bacteriostatic drugs. Humans cannot synthesize folic acid and must acquire it in the diet thus, the sulfonamides selectively inhibit microbial growth. 

Fusion with the cells compensates for this deficiency. When fused and unfused cells are incubated in the presence of the folic acid antagonist aminopterin, the de novo synthesis of purines and pyrimidines for DNA is blocked. Cells deficient in HGPRT die, whereas hybrid cells are able to bypass aminopterin blockage by metabolism of hypoxanthine and thymidine added to the medium. In the generation of mouse hybridomas, an number of myelomas deficient in HGPRT are available, all originating from MOPC 21, a spontaneous myeloma from the BALB/c mouse strain. 

A metabolic pathway that has received considerable attention is the conversion of 2 -deoxyuridine 5 -monophosphate (dUMP, 6.60) to thymidine 5 -monophosphate (TMP, 6.61) (Scheme 6.13). Without an adequate supply of TMP, a cell or bacterium cannot create DNA for cell division. Therefore, blocking TMP synthesis is an attractive method for slowing the advancement of certain cancers and bacterial infections. Important molecules in the methylation of dUMP are the various folic acid derivatives folic acid (FA, 6.62), dihydrofolic acid (DHF, 6.63), tetrahydrofolic acid (THF, 6.64), and N5, A1 "-methylene tetrahydrofolic acid (MTHF, 6.65) (Figure 6.23). These structures... 

A depressed uptake of deoxyuridine brought about by a deficiency of vitamin B12 can be reversed as reflected by a reduction in labeled thymidine uptake by the addition of as little as 1 xg of the vitamin. Folic acid added to a concentration of 50 p.g/ml of the culture will correct abnormal results due to folate deficiency and may partially correct abnormal results due to vitamin B12 deficiency. Abnormal results may occur in bone marrow from patients with iron deficiency and from patients being treated with 5-fluorouracil (B7). 

Aminopterin and amethopterin are 4-amino analogues of folic acid (Fig. 11.5) and as such are potent inhibitors of the enzyme dihydrofolate reductase (EC 1.5.1.3) (Blakley, 1969). This enzyme catalyses the reduction of folic acid and dihydrofolic acid to tetrahy-drofolic acid which is the level of reduction of the active coenzyme involved in many different aspects of single carbon transfer. As is clear from Fig. 11.6, tetrahydrofolate is involved in the metabolism of (a) the amino acids glycine and methionine (b) the carbon atoms at positions 2 and 8 of the purine ring (c) the methyl group of thymidine and (d) indirectly in the synthesis of choline and histidine. 

Mutants lacking, or with much reduced levels of the enzyme thymidine kinase survive as this enyzme is not essential. Thus the cell can make dTMP from dUMP using folic acid as the one carbon donor (Fig. 13.1). 

Tetrahydrofolic acid (THF) is a coenzyme in the synthesis of purine bases and thymidine. These are constituents of DNA and RNA and are required for cell growth and replication. Lack of THF leads to inhibition of cell proliferation. Formation of THF from dihydrofolate (DHF) is catalyzed by the enzyme dihydrofolate reductase. DHF is made from folic acid, a vitamin that cannot be synthesized in the body but must be taken up from exogenous sources. Most bacteria do not have a requirement for folate, because they are capable of synthesizing it-more precisely DHF-ffom precursors. Selective interference with bacterial biosynthesis of THF can be achieved with sulfonamides and trimethoprim. 

Fig. 5.1. Two pathways in normal cells to synthesize DNA precursors. The mutant myeloma cell line, which has the capacity to grow indeflnitely in vitro and can confer this property on antibody-producing lymphocytes through cell fusion, lacks the salvage pathway (no thymidine kinase (TK) or hypoxanthine-guanine phosphori-bosyl transferase (HGPRT)). This cell line would, therefore, not grow in the presence of folic acid antagonists. Fused cells, however, may grow since the antibody-producing cells contribute the salvage pathway.

In vitamin B12 or folate deficiency anemia, megaloblastosis results from interference in fohc acid-and vitamin B -interdependent nucleic acid synthesis in the immature erythrocyte. The rate of RNA and cytoplasm production exceeds the rate of DNA production. The maturation process is retarded, resulting in immature large RBCs (macrocytosis). Synthesis of the RNA and DNA necessary for cell division depends on a series of reactions catalyzed by vitamin B12 and folic acid, as they have a role in the conversion of midine to thymidine. As shown in Fig. 99-4, dietary folates are absorbed in this process and converted (A) to 5-methyl tetrahydrofolate, which is then converted via a Bi2-dependent reaction (B) to tetrahydrofolate (C). After gaining a carbon, tetrahydrofolate is converted to a folate cofactor (D), 5,10-methyl-tetrahydrofolate, used by thymidylate synthetase (E) in the... 

Pemetrexed is a multitargeted antifolate that inhibits at least three biosynthetic pathways in thymidine and purine synthesis (see Table 124—11). In addition to inhibition of DHFR, it also inhibits thymidine synthase and glycinamide ribonucleotide formyltransferase, decreasing the risk of development of drug resistance. Supplementation of folic acid and vitamin B12 is required to decrease myelosuppression... 

What are called antifolate drugs pertain in general to blocking the biosynthesis of purines and pyrimidines, the heterocyclic bases used in the further synthesis of DNA and RNA, where folic acid is required as a coenzyme (or vitamin) for the enzyme dihydrofolate reductase. The previously mentioned compound called methotrexate or amethopterin (4-amino-A °-methyl folic acid), being a structural analog of folate or folic acid, locks up the enzyme dihydrofolate reductase, which in turn blocks the synthesis of a thymidine nucleotide necessary for cell division. 

The sulfonamides are bacteriostatic when administered to humans in achievable doses. They inhibit the enzyme dihydropteroate synthase, an important enzyme needed for the biosynthesis of folic acid derivatives and, ultimately, the thymidine required for DNA. They do this by competing at the active site with... 

A further step in the pathway leading from the pteroates to folic acid and on to DNA bases requires the enzyme dihydrofolate reductase. Exogenous folic acid must be reduced stepwise to dihydrofolic acid and then to tetrahydrofolic acid, an important cofactor essential for supplying a 1-carbon unit in thymidine biosynthesis and, ultimately, for DNA synthesis (Fig. 38.5). The same enzyme also must reduce endogenously produced dihydrofolate. Inhibition of this key... 

Although the plasmodia that cause malaria synthesize pyrimidines de novo, few other protozoa do so. Instead they take uracil or uridine from the host, and convert some of it to thymidine (Kidder, 1967). Plasmodia synthesize folic acid and cannot use exogenous folate, but flagellates and ciliates require exogenous folate (Kidder, 1967). 

By the thymineless death method a situation is created in which the parental cells die and the mutant cells do not grow but are able to stay alive in the selective environment. Since mammalian cells utilize thymidine instead of thymine, a state of thymidine starvation is created by exposing cells to a folic acid antagonist, such as aminopterin. Under these conditions, the prototrophs continue to grow until death, while the auxotrophs do not grow but remain alive and can thus be rescued. This method has been successfully applied to isolation of glutamine-requiring mutant lines from HeLa cells (De Mars and Hooper, 1960) and Chinese hamster cells (Chu et aLy 196%). 

It was found in early studies that the folic acid analogues, methotrexate (amethopterin) and aminopterin very effectively blocked the incorporation of labeled deoxyuridine and of labeled formate into DNA thymine however, the incorporation of thymidine was not blocked. It was apparent, therefore, that the analogues interfered with the introduction of the methyl group into thymine, a process known to involve H -folate. When it became established that the antifolic agents were exceedingly potent inhibitors of tetrahydrofolate dehydrogenase (see Chapter 5), the mechanism of their inhibition of DNA synthesis was apparent. 

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