Dihydrofolate reductases

Dihydrofolate reductase (EC 1.5.1.3) catalyzes the reduction of the 5,6 double bond in the H2-folate to form Hi-folate. The activity also converts H2-biopterin 

The separation of the substrate from the product was achieved by reversed-phase HPLC using an ODS column. The mobile phase consisted of an aqueous solution containing 0.5% acetonitrile and 0.1% tetrahydrofuran (v/v). The eluent was monitored fluorometrically with 350 and 450 nm excitation and emission wavelengths, respectively. 

The activity was prepared from adult rat brain after homogenization, centrifugation, and desalting on G-25 Sephadex. 

D-e/yf/iro-7,8-Dihydroneopterin triphosphate synthetase, or GTP cyclohydrolase I (EC 3.5.4.16), catalyzes the formation of D-eryr/iro-dihydroneopterin triphosphate (NH2TP) from GTP. This activity is required for the synthesis of tetrahydrobiopterin. The HPLC assay developed for this activity involves the direct measurement of neopterin phosphates after separation from GTP and its other hydrolytic products. 

The hydrolase activity was obtained from homogenized rat liver, centrifuged, and applied to a Sephadex G-2S column. The eluate was used as the hydrolase. 

Inhibitors of dihydrofolate reductase. Methotrexate, a structural analogue of dihydrofolate, is effective against intact mammalian cells but ineffective against protozoa and some bacteria owing to permeability barriers. Trimethoprim and pyrimethamine (2,4-diaminopyrimidines) are effective against microorganisms. The former is antibacterial and antimalarial the latter is primarily antimalarial. 

Starting from the X-ray structures of one bacterial and one human DHFR, the GRID/PCA analysis shows that PG 1 distinguishes between the two target proteins, clustering the objects into two groups, while PG 2 ranks the probes. 

A third region at the top of the active site showed markedly different interactions with the probes. While most differences can be attributed to interactions with the side chains of an unconserved amino acid, another area in this region is produced by different backbone carbonyl orientations. Due to the presumably lower conformational flexibility of the protein backbone compared to side chain atoms, the authors speculate that this area would be especially promising for the design of selective ligands. 

As part of a study on the pharmacological activities of P-carbolines [132-134], a stereoselective imine reduction reaction was developed by Espinoza-Moraga et al. 

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Gerber, P. R., Mark, A. E., van Gunsteren, W. F. An approximate but efficient method to calculate free energy trends by computer simulation Application to dihydrofolate reductase-inhibitor complexes. J. Comp. Aid. Mol. Desgn 7 (1993) 305-323... 

Fig. 7.15 The variation in torsion angles can be effectively represented as a series of dials, where the time corresponds to the distance from the centre of the dial. Data from a molecular dynamics simulation of an intermolecular complex between the enzyme dihydrofolate reductase and a triazine inhibitor [Leach and Klein 1995].

T A and H Kalayeh 1991. Applications of Neural Networks in Quantitative Structure-Activity ationships of Dihydrofolate Reductase Inhibitors, journal of Medicinal Chemistry 34 2824-2836. ik M and R C Glen 1992. Applications of Rule-induction in the Derivation of Quantitative icture-Activity Relationships. Journal of Computer-Aided Molecular Design 6 349-383. 

The sulfa dmgs are stiH important as antimicrobials, although they have been replaced in many systemic infections by the natural and semisynthetic antibiotics. They are of great value in third world countries where problems of storage and lack of medical personnel make appropriate use of antibiotics difficult. They are especially useful in urinary tract infections, particularly the combination of sulfamethoxazole with trimethoprim. Their effectiveness has been enhanced by co-adniinistration with dihydrofolate reductase inhibitors, and the combination of sulfamethoxazole with trimethoprim is of value in treatment of a number of specific microbial infections. The introduction of this combination (cotrimoxazole) in the late 1960s (1973 in the United States) resulted in increased use of sulfonamides. 

Sulfonamides in combination with dihydrofolate reductase inhibitors are of continuing value. Pyrimethamine [58-14-0] (5) in combination with sulfonamides is employed for toxoplasmosis (7), and a trimethoprim (6)-sulfamethoxa2ole preparation is used not only for urinary tract infections but also for bmceUosis, cholera, and malaria. 

The second type of antifolates bind preferentially with, and thus selectively inhibit, the enzyme dihydrofolate reductase contained in the plasmodia. This interferes with the abiUty of the malaria parasites to convert dihydrofolate to tetrahydrofoUc acid. In the erythrocyte host, however, dihydrofolate... 

Product formation kinetics in mammalian cells has been studied extensively for hybridomas. Most monoclonal antibodies are produced at an enhanced rate during the Gq phase of the cell cycle (8—10). A model for antibody production based on this cell cycle dependence and traditional Monod kinetics for cell growth has been proposed (11). However, it is not clear if this cell cycle dependence carries over to recombinant CHO cells. In fact it has been reported that dihydrofolate reductase, the gene for which is co-amplified with the gene for the recombinant protein in CHO cells, synthesis is associated with the S phase of the cell cycle (12). Hence it is possible that the product formation kinetics in recombinant CHO cells is different from that of hybridomas. 

In view of the well-documented inhibition of dihydrofolate reductase by aminopterin (325), methotrexate (326) and related compounds it is generally accepted that this inhibitory effect constitutes the primary metabolic action of folate analogues and results in a block in the conversion of folate and dihydrofolate (DHF) to THF and its derivatives. As a consequence of this block, tissues become deficient in the THF derivatives, and this deficiency has many consequences similar to those resulting from nutritional folate deficiency. The crucial effect, however, is a depression of thymidylate synthesis with a consequent failure in DNA synthesis and arrest of cell division that has lethal results in rapidly proliferating tissues such as intestinal mucosa and bone marrow (B-69MI21604, B-69MI21605). 

AJ Hopfinger. A QSAR investigation of dihydrofolate reductase inhibition by Baker triazmes based upon molecular shape analysis. I Am Chem Soc 102 7196-7206, 1980. 

GM Crippen. Quantitative structure-activity relationships by distance geometry Systematic analysis of dihydrofolate reductase inhibitors. I Med Chem 23 599-606, 1980. 

TA Andrea, H Kalayeh. Applications of neural networks in quantitative structure-activity relationships of dihydrofolate reductase inhibitors. J Med Chem 34 2824-2836, 1991. 

The NAD- and NADP-dependent dehydrogenases catalyze at least six different types of reactions simple hydride transfer, deamination of an amino acid to form an a-keto acid, oxidation of /3-hydroxy acids followed by decarboxylation of the /3-keto acid intermediate, oxidation of aldehydes, reduction of isolated double bonds, and the oxidation of carbon-nitrogen bonds (as with dihydrofolate reductase). 

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... 

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