DHFA

Standard heats of reaction of each component, dHfA, dHfB, dHfc, and dHfD. 

The folate pathway is one in which selective chemotherapeutic intervention has been very successful, and a number of drugs acting through this pathway are in current use. This is because of the fact that mammals receive FA from their diet and convert it into dihydrofolic acid (DHFA) and tetrahydrofolic acid (THFA), which give rise to folate cofactors. The bacteria and protozoans, on the other hand can not effectively utilize FA to get their requirements of DHFA and THFA. Consequently, these organisms synthesize DHFA de novo (for details, see Chapter 13). Further the affinity of the enzymes involved from different sources (mammalian, bacterial and protozoan) for different classes of inhibitors is quite different, which has resulted in the development of drugs with selective action. 

The key enzymes involved in the biosynthesis of DHFA and THFA are dihy-dropteroate synthetase, dihydrofolate synthetase and dihydrofolate reductase [1-3]. Drugs that block the synthesis of dihydropteroic acid are known as dihydropteroate synthetase inhibitors (PABA antagonists) and those which control the reduction of DHFA to THFA are called dihydrofolate reductase (DHFR) inhibitors. Collectively these drugs are known as antifolates. 

For the assay of dihydrofolate reductases (see p 612) suspend 66.5mg of DHFA in lOmL of O.OOIM HCl containing lOmM dithiothreitol (DTT stock made from 154mg in lOmL H2O making O.IM), shake well and freeze in 400 total volume of 2mL) to give a clear solution. To estimate the concentration of DHFA in this solution, dilute... 

Fig. 9.3 Biosyntheses and functions of folic acid coenzymes. [DHFA and THFA = di-(and tetra-)hydrofolic acid, respectively.]...

Dihydrofolic acid (7,8-dihydropteroyl-L-glutamic acid, DHFA) [4033-27-6] M 443.4, pKj 2.0 (basic... 

Unlike normal peroxidase reactions, the complex breaks down after reduction of Fe +. Hydrogen peroxide and half-oxidized DHFA (HM) leave the complex, which is recharged by oxygen. 

If one assumes that DHFA is oxidized to two oxalate molecules this would reconcile the stated stoichiometry with the cycle of Lemberg and Legge. 

In addition to the oxidase activity described, DHFA can promote aerobic hydroxylation by horse-radish peroxidase also. This catalytic action is attributed to complex 3, the oxyhemoglobin type of structure (Mason et al., 1957). 

Reactions (78) and (79) are driven by the reducing force of DHFA. The reason why Mason prefers reaction sets (76) and (77) is that the peroxide-peroxidase system does not hydroxylate in the same way. This is true indeed. 

Mayrargue-Kodsa et al. (1958) have studied the influence of horseradish peroxidase on various phenolic acids. The main reaction is that of polymerization of the monophenol by hydrogen removal (free radical formation). Diphenols are oxidized as if the enzyme were a polyphenolox-idase. In the aforementioned hydroxylating system, catalase and Mn++-ions are inhibitors. The former might destroy complex 3. Mn++ might favor the DHFA-oxidase reaction, competing with the hydroxylase activity. 

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