Folate pathway, inhibition

Gene polymorphisms in MTX transporters and enzymes in the folate and adenosine pathways inhibited by MTX have been studied in RA patients. 

Pyrimethamine and proguanil selectively inhibit plasmodial dihydrofolate reductase, a key enzyme in the pathway for synthesis of folate. Sulfonamides and sulfones inhibit another enzyme in the folate pathway, dihydropteroate synthase. As described in Chapter 46 and shown in Figure 46-2, combinations of inhibitors of these two enzymes provide synergistic activity. 

Several antibacterial drugs inhibit bacterial nucleic acid synthesis by inhibiting the production of folic acid.17 Folic acid serves as an enzymatic cofactor in a number of reactions, including synthesis of bacterial nucleic acids and certain essential amino acids. The pathway for synthesis of these folic acid cofactors is illustrated in Figure 33-2. Certain antibacterial drugs block specific steps in the folate pathway, thus impairing the production of this enzymatic cofactor and ultimately impairing the production of nucleic acids and... 

The sulfonamides are a group of organic compounds with chemotherapeutic activity they are antimicrobial agents and not antibiotics. They have a common chemical nucleus that is closely related to PABA, an essential component in the folic acid pathway of nucleic acid synthesis. The sulfonamides are synergistic with the diaminopyrim-idines, which inhibit an essential step further along the folate pathway. The combination of a sulfonamide and a diaminopyrimidine is advantageous because it is relatively non-toxic to mammalian cells (less sulfonamide is administered) and is less likely to select for resistant bacteria. Only these so-called potentiated sulfonamides are used in equine medicine. These drugs are formulated in a ratio of one part diaminopyrimidine to five parts sulfonamide, but the optimal antimicrobial ratio at the tissue level is 1 20, which is achieved because the diaminopyrimidines are excreted more rapidly than the sulfonamides. 

The antimetabolites described in Tables 7.6 and 7.7 inhibit DNA, RNA and protein production by blocking the folate pathway. Tetrahydrofolate donates single carbon molecules to the synthesis of purines, pyrimidines and some amino acids (methionine, formyl-methionine and serine). 

Figure 7.15 Sulfonamides and trimethoprim inhibit different sites in the folate pathway. This results in decreased tetrahydrofolic acid cofactors which are essential in the formation of DNA, RNA and protein.

Two other validated targets for antimicrobial chemotherapy will be mentioned here, both of which are defined by pathway inhibition. The earliest antibiotics, the sulfonamides, are inhibitors of the folate biosynthetic pathway (24). Sulfamethoxazole and trimethoprim, targeting dihydropteroate synthase and dihydrofolate reductase, respectively, are each susceptible to rapid emergence of resistance but have been used successfully in combination. Recently, other enzymes in the pathway have begun to engender interest for target-based screening. Fatty acid biosynthesis is another pathway that has previously been... 

Danishpajooh, I.O., Gudi, T., Chen, Y., Kharitonov, V.G., Sharma, V.S., and Boss, G.R. (2001). Nitric oxide inhibits methionine synthase activity in vivo and disrupts carbon flow through the folate pathway. J. Biol. Chem. 276, 27296-27303. 

Inhibit Enzymes Many drugs are competitive inhibitors of key enzymes in pathways. The statin drugs (lovastatin, simvastatin), used to control blood cholesterol levels, competitively inhibit 3-hvdroxy-3-methylglutaryl coenzyme A (HMG CoA) reductase in cholesterol biosynthesis. Methotrexate, an antineoplastic drug, competitively inhibits dihydrofolate reductase, depriving the cell of active folate needed for purine and deoxythymidine synthesis, thus interfering with DNA replication during S phase. 

Resistance occurs as the result of one or more alterations in the cellular metabolism of the bacteria both mutation and plasmid-mediated resistance occurs. These changes, which can be irreversible, include alterations in the physical or enzymatic characteristics of the enzyme or enzymes that metabolize PABA and participate in the cellular synthesis of tetrahydrofolic acid. The appearance of alternative pathways for PABA synthesis within the bacteria or the development of an increased capacity to inactivate or eliminate the sulfonamide also may contribute to bacterial cell resistance. Bacteria that can use preformed folate are not inhibited by sulfonamides. 

Other useful targets for pharmaceutical agents are thymidylate synthase and dihydrofolate reductase, enzymes that provide the only cellular pathway for thymine synthesis (Fig. 22-49). One inhibitor that acts on thymidylate synthase, fluorouracil, is an important chemotherapeutic agent. Fluorouracil itself is not the enzyme inhibitor. In the cell, salvage pathways convert it to the deoxynucleoside monophosphate FdUMP, which binds to and inactivates the enzyme. Inhibition by FdUMP (Fig. 22-50) is a classic example of mechanism-based enzyme inactivation. Another prominent chemotherapeutic agent, methotrexate, is an inhibitor of dihydrofolate reductase. This folate analog acts as a competitive inhibitor the enzyme binds methotrexate with about 100 times higher affinity than dihydrofolate. Aminopterin is a related compound that acts similarly. 

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