Dihydrofolic acid reductase

Pharmacology The mechanism of action in RA is unknown it may affect immune function. Methotrexate inhibits dihydrofolic acid reductase and interferes with DNA synthesis, repair, and cellular replication. 

The answer is c. (Hardman, pp 1058-1059. Katzung, pp 793-795.) Trimethoprim inhibits dihydrofolic acid reductase. Sulfamethoxazole inhibits p-aminobenzoic acid (PABA) from being incorporated into folic acid by competitive inhibition of dihydropteroate synthase. Either action inhibits the synthesis of tetrahydrofolic acid. 

Trimethoprim, a trimethoxybenzylpyrimidine, selectively inhibits bacterial dihydrofolic acid reductase, which converts dihydrofolic acid to tetrahydrofolic acid, a step leading to the synthesis of purines and ultimately to DNA (Figure 46-2). Trimethoprim is about 50,000 times less efficient in inhibition of mammalian dihydrofolic acid reductase. Pyrimethamine, another benzylpyrimidine, selectively inhibits dihydrofolic acid reductase of protozoa compared with that of mammalian cells. As noted above, trimethoprim or pyrimethamine in combination with a sulfonamide blocks sequential steps in folate synthesis, resulting in marked enhancement (synergism) of the activity of both drugs. The combination often is bactericidal, compared with the bacteriostatic activity of a sulfonamide alone. 

In acute and chronic urinary tract infection, the combination of trimethoprim and sulfamethoxazole (Bactrim, Septra) exerts a truly synergistic effect on bacteria. The sulfonamide inhibits the utilization of p-amino-benzoic acid in the synthesis of folic acid (Figure 2.3), whereas trimethoprim, by inhibiting dihydrofolic acid reductase, blocks the conversion of dihydrofolic acid to tetrahydrofolic acid, which is essential to bacteria in the denovo synthesis of purines, pyrimidines, and certain amino acids. Because mammalian organisms do not synthesize folic acid and therefore need it as a vitamin in their daily diets, trimethoprim-sulfamethoxazole does not interfere with the metabolism of mammalian cells. 

Dihydrofolic acid reductase Trimethoprin, pyrimethamine, methotrexate... 

A complete understanding of sulfonamide action evolved over a 20-year period. The biosynthesis of the various folates in living cells had to be elucidated their functions in the scheme of things had to be worked out. The following discourse will consider the effects of sulfonamides, as well as that of another group of important enzyme inhibitors—the dihydrofolic acid reductase inhibitors. Figure 2-4 outlines the stratagem as it is presently understood. 

The combination of trimethoprim and sulfamethoxazole (usually five parts sulfamethoxazole to one part trimethoprim) interferes with the synthesis of active folic acid by means of two separate reactions. In the first, sulfonamides compete with PABA and prevent its conversion to dihydro-folic acid. In the second, trimethoprim, by inhibiting the activity of dihydrofolic acid reductase, prevents the conversion of dihydrofolic acid into tetrahydrofolic acid, which is necessary for the synthesis of DNA. These reactions are summarized in Figure 90. 

The activity and use of sulfonamides has been potentiated by addition of synergists that also inhibit bacterial enzymatic synthesis. The best known is trimethoprim 5-[(3,4,5-trimethoxyphenyl)methyl -2,4-pyrimidinediamine 47181 182 that binds dihydrofolic acid reductase. 43 and 47 are listed as antimalarial drugs183. 

B) Cyclophosphamide is an irreversible inhibitor of dihydrofolic acid reductase... 

Supra-additive interactions and potentiation appear to be much less common than antagonism and the simple additive interactions described above. Supra-additive (synergistic) interaction is said to occur if the result of interaction is greater than the sum of the drugs used alone the best example is the therapeutic synergism of certain antibiotic combinations such as sulfonamides and dihydrofolic acid reductase inhibitors such as trimethoprim. Potentiation is said to occur when a drug s effect is increased by another agent that has no such effect. The best example of this type of interaction is the therapeutic interaction of beta-lactamase inhibitors such as clavulanic acid with lactamase-susceptible penicillins. 

Dihydrofolic acid reductase inhibitor used as antineoplastic drug as well as for neatment of severe psoriasis and adult rheumatoid arthritis. 

More recently it has been observed that 2-amino-4-hydroxy-6,7-dimethyltetra-hydropteridine, which lacks N °, is unable to substitute for FH as a coenzyme in the utilization of formaldehyde for the enzymic formation of serine and of thymine-methyl (48a). The corresponding dimethyldihydropteridine can serve as a substrate for dihydrofolic acid reductase. 

Identical dihydrofolate reductase enzymes, isolated from P. berghei and P. knowlesi, have been found to be sensitive to pyrimethamine. There is no evidence that inhibition of the enzymes leads to inhibition of ONA synthesis.21 22 other dihydrofolic acid reductase inhibitors, cyclogua-nil, W.R. 38839 (I), trimethoprim, methotrexate and W.R. 40070 (II), the latter three at higher concentrations, acted in a manner similar to pyri-... 

En2ymatic reduction of folic acid leads to the 7,8-dihydrofolic acid (H2 folate) (2), a key substance in biosynthesis. Further reduction, cataly2ed by the en2yme dihydrofohc acid reductase, provides (65)-5,6,7,8-tetrahydrofohc acid (H folate) (3). The folate (3) is the key biological intermediate for the formation of other folates (4—8) (Table 2). 

Folic acid becomes sequentially reduced in the body by the enzyme dihydrofolate reductase to give dihydrofolic acid (FH2) and then tetrahydrofolic acid (FFi4). Reduction occurs in the pyrazine ring portion. 

Different antimalarials selectively kill the parasite s different developmental forms. The mechanism of action is known for some of them pyrimethamine and dapsone inhibit dihydrofolate reductase (p. 273), as does chlorguanide (proguanil) via its active metabolite. The sulfonamide sulfadoxine inhibits synthesis of dihydrofolic acid (p. 272). Chlo-roquine and quinine accumulate within the acidic vacuoles of blood schizonts and inhibit polymerization of heme, the latter substance being toxic for the schizonts. 

Successful fusion (2) is a rare event, but the frequency can be improved by adding polyethylene glycol (PEG). To obtain only successfully fused cells, incubation is required for an extended period in a primary culture with HAT medium (3), which contains hypoxan-thine, aminopterin, and thymidine. Amino-pterin, an analogue of dihydrofolic acid, competitively inhibits dihydrofolate reductase and thus inhibits the synthesis of dTMP (see p. 402). As dTMP is essential for DNA synthesis, myeloma cells cannot survive in the presence of aminopterin. Although spleen cells are able to circumvent the inhibitory effect of aminopterin by using hypoxanthine and thymidine, they have a limited lifespan and die. Only hybridomas survive culture in HAT medium, because they possess both the immortality of the myeloma cells and the spleen cells metabolic side pathway. 

Trimethoprim acts in the body by interfering with the action of hydrofolate reductase, an enzyme that reduces dihydrofolic acid to tetrahydrofolic acid. This process is necessary for purine biosynthesis of live organisms and DNA, respectively. Reducing the dihydrofolic acid to tetrahydrofolic acid is also catalyzed in humans by dihydrofolate reductase. However, trimethoprim has thousands of more inhibitory effects with respect to bacterial enzymes than with respect of analogons enzymes of mammals, which is the main benefit of trimethoprim. 

Pharmacology SMZ inhibits bacterial synthesis of dihydrofolic acid by competing with para-aminobenzoic acid. TMP blocks the production of tetrahydrofolic acid by inhibiting the enzyme dihydrofolate reductase. 

I I 3. The answer is c. (Hardman, pp 1243-1247.) Antimetabolites of folic acid such as methotrexate, which is an important cancer chemotherapeutic agent, exert their effect by inhibiting the catalytic activity of the enzyme dihydrofolate reductase. The enzyme functions to keep folic acid in a reduced state. The first step in the reaction is the reduction of folic acid to 7,8-dihydrofolic acid (FH2), which requires the cofactor nicotinamide adenine dinucleotide phosphate (NADPH). The second step is the conversion of FH2 to 5,6,7,8-tetrahydrofolic acid (FH ). This part of the reduction reaction requires nicotinamide adenine dinucleotide (NADH) or NADPH. The reduced forms of folic acid are involved in one-carbon transfer reactions that are required during the synthesis of purines and pyrimidine thymidylate. The affinity of methotrexate for dihydrofolate reductase is much greater than for the substrates of folic acid and FH2. The action of... 

It acts by inhibiting dihydrofolate reductase. It inhibits conversion of dihydrofolic acid to tetrahydrofolic which is essential for purine synthesis and amino acid interconversions. It primarily affects DNA synthesis but also RNA and protein synthesis. It has cell cycle specific action and kills cells in S phase. It is readily absorbed from gastrointestinal tract but larger doses are absorbed incompletely, little drug is metabolised and it is excreted largely unchanged in urine. 

Trimethoprim (Proloprim, Trimpex) interferes with the bacterial folic acid pathway by inhibiting the dihydrofolate reductase enzyme in susceptible bacteria (see Fig. 33-2). This enzyme converts dihydrofolic acid to tetrahydrofolic acid during the biosynthesis of folic acid cofactors. By inhibiting this enzyme, trimethoprim directly interferes with the production of folic acid cofactors, and subsequent production of vital bacterial nucleic acids is impaired. 

Methotrexate acts by inhibition of dihydrofolate reductase, the enzyme requisite for the reduction of dihydrofolic acid (3) to 5,6,7,8-tetrahydrofolic acid (4). In turn, (4) is a precursor to a series of enzyme cofactors (5-7) essential for the transfer of one carbon unit necessary for the biosynthesis of purines and pyrimidines and hence, ultimately, DNA. As an inhibitor of dihydrofolate reductase, methotrexate kills cells during the S phase of the cell cycle, when the cells are in the log phase of growth. Unfortunately, this cytotoxicity is non-selective, and rapidly proliferating normal cells, e.g., gastrointestinal epithelium cells and bone marrow, are dramatically affected as well. In addition, recent use of high dose methotrexate therapy with leucovorin rescue has led to additional clinical problems arising from a dose-related nephrotoxic metabolite, 7-hydroxy methotrexate (8). Finally, the very polar nature of methotrexate renders it virtually impenetrable to the blood-brain barrier, which can necessitate direct intrathecal injection in order to achieve therapeutic doses for the treatment of CNS tumours. 

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. 

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