Folic reductase

Folic acid (Fig. 6) is the precursor of a number of coenzymes vital for the synthesis of many important molecules. These derivatives of folic acid, referred to collectively as active formate and active formaldehyde , are responsible for the donation of one carbon fragments in the enzymatic synthesis of a number of essential molecules. In the formation of methionine from homocysteine, the folic acid coenzyme donates the S-methyl group, and in the conversion of glycine to serine it is necessary for the formation of the hydroxymethyl group. Folic add is converted into its active coenzyme forms by an initial two step reduction to tetra-hydrofolic add (Fig. 6) by means of two enzymes, folic reductase and dihydrofolic reductase. Conversion of tetrahydrofolic acid (THF) to an active coenzyme folinic acid subsequently occurs by ad tion of an Ns formyl group (Fig. 6). The formation of similar compounds such as an Nio formyl derivative, or the bridged Ns,Nio-methylenetetrahydrofolic acid, also leads to active coenzymes. 

Folic acid antagonists prevent the reduction of folic acid to tetrahydrofolic acid by competing with it for the folic reductase enzymes. In fact, the majority of... 

Progress has been made in the study of a number of analogues of folic acid in folic reductase systems, and has given some indication of the types of molecular alteration that might lead to antagonists. 

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 derivatives (folates) are acceptors and donors of one-carbon units for all oxidation levels of carbon except that of CO2 (where biotin is the relevant carrier). The active coenzyme form of folic acid is tetrahydrofolate (THF). THF is formed via two successive reductions of folate by dihydrofolate reductase (Figure 18.35). One-carbon units in three different oxidation states may be bound to tetrahydrofolate at the and/or nitrogens (Table 18.6). These one-carbon units... 

FIGURE 18.35 Formation of THF from folic acid by the dihydrofolate reductase reaction. The R group on these folate molecules symbolizes the one to seven (or more) glutamate units that folates characteristically contain. All of these glutamates are bound in y-carboxyl amide linkages (as in the folic acid structure shown in the box A Deeper Look Folic Acid, Pterins, and Insect VFingis). The one-carbon units carried by THF are bound at N, or at or as a single carbon attached to both... 

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. 

Cardiovascular Keep doses of NSAIDs and glucocorticoids low, consider initiation of folic acid to reduce homocysteine level elevations induced by methotrexate, consider initiation of low-dose aspirin and/or HMG-CoA reductase inhibitors (statins), and encourage smokers to discontinue tobacco use and assist with the development of a tobacco-cessation plan.11,12... 

Additional agents, including selenium, folic acid, and HMG-CoA reductase inhibitors (statins), show promise as chemopreventive agents in colon cancer, and preliminary and confirmatory studies evaluating their effectiveness have been completed or are ongoing, although none of these agents have been approved for the prevention of colon cancer.46... 

Folic acid antagonist inhibits dihydrofolate reductase (DHFR) blocks reduction of folate to tetrahydrofolate inhibits de novo purine synthesis results in arrest of DNA, RNA, and protein synthesis... 

The answer is c. (Hardman, pp 1058-1059. Katzung, pp 793-795.) Trimethoprim inhibits dihydro folic 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. 

Patients sustain convulsions and neurological deterioration. The urine contains low levels of the metabolites of serotonin, norepinephrine and dopamine. The reductase also plays a role in the maintenance of tetrahydrofolate levels in brain, and some patients have had low folate levels in the serum and CNS. Treatment has been attempted with tryptophan and carbidopa to improve serotonin homeostasis and with folinic acid to replete diminished stores of reduced folic acid. This therapy is sometimes effective. Diagnosis involves assay of DHPR in skin fibroblasts or amniotic cells. Phenylalanine hydroxylase activity is normal. 

One form of remethylation deficit involves defective metabolism of folic acid, a key cofactor in the conversion of homocysteine to methionine. Methylenetetra-hydrofolate reductase (Fig. 40-4 reaction 11) reduces... 

Recently, the enzymatic formation of folinic acid has been utilized to synthesize radioactively labeled products.34 The preparation of 5-formyl tetrahydrofolate, 9,3, 5 -3H and 5-formyl-14C-tetrahydrofolate starts with tritiated folic acid, which is reduced to dihydrofolate, incubated in the presence of formaldehyde, dihydrofolate reductase, and NADPH, and finally incubated with 5,10-methylenetetrahydrofolate dihydrogenase. The product,... 

The chemical structure of folate (or folic acid) is shown in Figure 5.8. In humans, folate usually occurs as polyglutamate derivatives. The active form of folate is THF, sometimes shown as FH4) is derived from folate via two reductase reactions. THF functions as a carrier of one-carbon groups in varying oxidation states (Table 5.1). 

Antimetabolites (inhibition of purine and pyrimidine nucleotide synthesis) Methotrexate Folic acid antagonist, inhibits tetrahydrofolate reductase and therefore dTMP synthesis 6-Mercaptopurine Interferes with purine synthesis 5-Fluorouracil Inhibits dTMP synthesis ... 

Methotrexate is an antimetabolite of folic acid and has immunosuppressant properties. It inhibits the enzyme dihydrofolate reductase that is required for the synthesis of purines and pyrimidines. It is used in malignant disease, Crohn s disease, rheumatic disease and psoriasis. Folic acid is given with methotrexate to reduce the occurrence of side-effects particularly the risk of mucositis. 

These three compounds exert many similar effects in nucleotide metabolism of chicks and rats [167]. They cause an increase of the liver RNA content and of the nucleotide content of the acid-soluble fraction in chicks [168], as well as an increase in rate of turnover of these polynucleotide structures [169,170]. Further experiments in chicks indicate that orotic acid, vitamin B12 and methionine exert a certain action on the activity of liver deoxyribonuclease, but have no effect on ribonuclease. Their effect is believed to be on the biosynthetic process rather than on catabolism [171]. Both orotic acid and vitamin Bu increase the levels of dihydrofolate reductase (EC 1.5.1.4), formyltetrahydrofolate synthetase and serine hydroxymethyl transferase in the chicken liver when added in diet. It is believed that orotic acid may act directly on the enzymes involved in the synthesis and interconversion of one-carbon folic acid derivatives [172]. The protein incorporation of serine, but not of leucine or methionine, is increased in the presence of either orotic acid or vitamin B12 [173]. In addition, these two compounds also exert a similar effect on the increased formate incorporation into the RNA of liver cell fractions in chicks [174—176]. It is therefore postulated that there may be a common role of orotic acid and vitamin Bj2 at the level of the transcription process in m-RNA biosynthesis [174—176]. 

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. 

These are pyrimidine derivatives and are effective because of differences in susceptibility between the enzymes in humans and in the infective organism. Anticancer agents based on folic acid, e.g. methotrexate, inhibit dihydrofolate reductase, but they are less selective than the antimicrobial agents and rely on a stronger binding to the enzyme than the natural substrate has. They also block pyrimidine biosynthesis. Methotrexate treatment is potentially lethal to the patient, and is usually followed by rescue with folinic acid (A -formyl-tetrahydrofolic acid) to counteract the folate-antagonist action. The rationale is that folinic acid rescues normal cells more effectively than it does tumour cells. 

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 folate, more precisely DHF, from precursors. Selective interference with bacterial biosynthesis of THF can be achieved with sulfonamides and trimethoprim. 

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. 

Dihydrofolate reductase acts as an auxiliary enzyme for thymidylate synthase. It is involved in the regeneration of the coenzyme N, N -methylene-THF, initially reducing DHF to THF with NADPH as the reductant (see p. 418). The folic acid analogue methotrexate, a frequently used cytostatic agent, is an extremely effective competitive inhibitor of dihydrofolate reductase. It leads to the depletion of N, N -methylene-THF in the cells and thus to cessation of DNA synthesis. 

All of these compounds are inhibitors of dihydrofolate reductase in bacteria, plasmodia, and humans. Fortunately, they have a significantly higher affinity to bacterial and protozoal dihydrofolate reductase. Pyrimethamine, for example, inhibits dihydrofolate reductase in parasites in concentrations that are a several hundred times lower than that required to inhibit dihydrofolate reductase in humans. This is the basis of their selective toxicity. Selective toxicity can be elevated upon the host organism s production of folic acid, which parasites are not able to use. 

A while later, pyrimethamine (33.1.60) was suggested as a result of intensive research of antimetabolites of folic acid. Trimethoprim (33.1.51) is the result of later research. The structural similarity of these drugs with the pteridine fragment of folic acid undoubtedly determines their affinity with binding regions of dihydrofolate reductase. 

Drugs that may interact with folic acid include aminosalicylic acid, oral contraceptives, dihydrofolate reductase inhibitors (eg, methotrexate, trimethoprim), sulfasalazine, hydantoins. 

Pharmacology Leucovorin is one of several active, chemically reduced derivatives of folic acid. It is useful as an antidote to drugs that act as folic acid antagonists. Administration of leucovorin can counteract the therapeutic and toxic effects of folic acid antagonists such as methotrexate, which act by inhibiting dihydrofolate reductase. 

Trimethoprim (TMP)-Sulfamethmazole (SMX) [Co-Trimoxazole] (Bactrim, Septra) [Antibiotic/Folate Antagonist] Uses un Rx prophylaxis, otitis media, sinusitis, bronchitis Action SMX T synth of dihydro-folic acid TMP T dihydrofolate reductase to impair protein synth Dose Adul. 1 DS tab PO bid or 5-20 mg/kg/24 h (based on TMP) IV in 3-4 doses P. jiroveci ... 

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