Tetrahydrofolate dehydrogenase

Figure 14.1. Proposed electron-transport pathway in the acetogenic bacteria M. thermoacetica and M. thermoautotrophica. Cyt., cytochrome MTi/FD//, methylene tetrahydrofolate dehydrogenase Fd, ferredoxin Fp, flavoprotein H2ase, hydrogenase.

This enzyme [EC 1.5.1.3], also called tetrahydrofolate dehydrogenase, catalyzes the reversible reaction of 7,8-dihydrofolate with NADPH to produce 5,6,7,8-tetrahy-drofolate and NADP+. The enzyme isolated from mammals and some microorganisms can also slowly catalyze... 

Several other cellular target proteins for the reactive metabolite of paracetamol have also been detected and identified, namely, formyl tetrahydrofolate dehydrogenase, glyceralde-hyde-3-phosphate dehydrogenase (GAPDH), and GSH transferase, all cytosolic proteins. 

MTHFD Methylene tetrahydrofolate dehydrogenase (NAD-dependent), methylenetetrahydrofolate cyclohydrolase... 

Methylene-, methenyl-, and 10-formyl-tetrahydrofolates are freely interconvertible. The two activities involved - methylene-tetrahydrofolate dehydrogenase and methenyl-tetrahydrofolate cyclohydrolase - form a trifunctional enzyme with 10-formyl-tetrahydrofolate synthetase (Paukert et al., 1976). This means that single-carbon fragments entering the folate pool in any form other than as methyl-tetrahydrofolate can be readily available for any of the biosynthetic reactions shown in Figure 10.4. 

The activity of 10-formyl-tetrahydrofolate dehydrogenase, which catalyzes the oxidation of 10-formyl tetrahydrofolate to CO2 and tetrahydrofolate, is reduced at times of low methionine availability as a means of conserving valuable one-carbon fragments. Therefore, there is no sink for one-carbon substituted tetrahydrofolate, and increasing amounts of folate are trapped as methyl-tetrahydrofolate that cannot be used because of the lack ofvitantin B12 (Krebs etal., 1976). 

In experimental animals and with isolated tissue preparations and organ cultures, the test can be refined by measuring the production of G02 from [ C]histidine in the presence and absence of added methionine. If the impairment of histidine metabolism is the result of primary folate deficiency, the addition of methionine wUl have no effect. By contrast, if the problem is trapping of folate as methyl-tetrahydrofolate, the addition of methionine will restore normal histidine oxidation as a result of restoring the inhibition of methylene-tetrahydrofolate reductase by S-adenosylmethionine and restoring the activity of 10-formyl-tetrahydrofolate dehydrogenase, thus permitting more normal folate metabolism (Section 10.3.4.1). 

Tetrahydrofolate dehydrogenase. 5,6,7,8-tetrahydrofolate + NADP(+) = 7,8-dihydrofolate + NADPH. 

The folate analogues are powerful inhibitors of tetrahydrofolate dehydrogenase (or folate reductase), the enzyme which converts folate to the active tetrahydro derivative. Reduced forms of aminopterin also inhibit tetrahydrofolate dehydrogenase, dihydroaminopterin being about as... 

It was evident that the by-product of thymidylate synthesis, Hj-folate, could be reduced to H4-foIate through activity of tetrahydrofolate dehydrogenase ... 

The sequential action of these three enzymes to form a thymidylate-synthesizing cycle was visualized, in which H4-folate was regenerated (by tetrahydrofolate dehydrogenase), recharged with a one-carbon unit (by serine hydroxymethyltransferase), and returned to the reaction. 

Thus, the formation of thymidylate involves the formation of a carbon-carbon bond and a reduction 5,10-methyIene H4-folate is both donor of the one-carbon unit and reductant. It is apparent that a catalytic amount of H4-folate will serve in the cycle. This is consistent with the observation that in S. faecalis extracts the synthesis of thymidylate in the presence of excess H4-folate was unaffected by aminopterin, a potent inhibitor of tetrahydrofolate dehydrogenase when this reaction was conducted with catalytic amounts of H4-folate and excess NADPH, thymidylate synthesis was blocked by aminopterin (5). 

It was found in early studies that the folic acid analogues, methotrexate (amethopterin) and aminopterin very effectively blocked the incorporation of labeled deoxyuridine and of labeled formate into DNA thymine however, the incorporation of thymidine was not blocked. It was apparent, therefore, that the analogues interfered with the introduction of the methyl group into thymine, a process known to involve H -folate. When it became established that the antifolic agents were exceedingly potent inhibitors of tetrahydrofolate dehydrogenase (see Chapter 5), the mechanism of their inhibition of DNA synthesis was apparent. 

As noted in Chapter 5, aminopterin and methotrexate are bound very firmly to tetrahydrofolate dehydrogenase, with inhibition constants in the order of 10 M the bound analogue can be released by dialysis against folic acid. 

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