The Methyl Folate Trap Hypothesis

1 The Methyl Folate Trap Hypothesis The reduction of meth-ylene-tetreihydrofolate to methyl-tetreihydrofolate is irreversible (Section 

and the major source of folate for tissues is methyl-tetrahydrofolate. The only metabolic role of methyl-tetrahydrofolate is the methylation of homocysteine to methionine, and this is the only way in which methyl-tetrahydrofolate can be demethylated to yield free tetrahydrofolate in tissues. Methionine synthetase thus provides the link between the physiological functions of folate and vitamin B12. 

Impairment of methionine synthetcise activity, for example, in vitamin B12 deficiency or after prolonged exposure to nitrous oxide (Section 10.9.7), will result in the accumulation of methyl-tetrcihydrofolate. This can neither be utilized for tmy other one-carbon trtmsfer reactions nor demethylated to provide free tetrahydrofolate. 

Experimental tmimeds that have been exposed to nitrous oxide to deplete vittimin B12 show an increase in the proportion of liver folate present as methyl-tetrtihydrofolate (85% rather than the normcd 45%), largely at the expense of unsubstituted tetrahydrofolate and increased urinary loss of methyl-tetrahydrofolate (Horne et al., 1989). Tissue retention of folate is impaired because methyl-tetrahydrofolate is a poor substrate for polyglutamyl-folate synthetase, compared with unsubstituted tetrahydrofolate (Section 10.2.2.1). As a result of this, vitamin B12 deficiency is frequently accompemied by biochemical evidence of functional folate deficiency, including impeured metabolism of histidine (excretion of formiminoglutamate Section 10.3.1.2) and impaired thymidylate synthetase activity (as shown by abnormally low dUMP suppression Section 10.3.3.3), although plasma concentrations of methyl-tetrahydrofolate are normal or elevated. 

This functional deficiency of folate is exacerbated by the associated low concentrations of methionine and S-adenosyl methionine, edthough most tissues (apeut from the centred nervous system) edso have beteune-homocysteine methyltransfereise that may be adequate to maintedn tissue pools of methionine. Under normed conditions S-adenosyl methionine inhibits methylene-tetrediydrofolate reductase and prevents the formation of further methyl-tetrediydrofolate. Relief of this inhibition results in increased reduction of one-cetrbon substituted tetrediydrofolates to methyl-tetrahydrofolate. 

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). 

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The cause of megaloblastosis is depressed DNA synthesis, as a result of impaired methylation of dCDP to TDP, catalyzed by thymidylate synthetase, but more or less normal synthesis of RNA. As discussed in Section 10.3.3, thymidylate synthetase uses methylene tetrahydrofolate as the methyl donor it is obvious that folic acid deficiency will result in unpaired thymidylate synthesis. It is less easy to see how vitamin B12 deficiency results in impaired thymidylate synthesis without invoking the methyl folate trap hypothesis (Section 10.3.4.1). The main circulating form of folic acid is methyl-tetrahydrofolate before this can be used for other reactions in tissues, it must be demethylated to yield free folic acid. The only reaction that achieves this is the reaction of methionine synthetase (Section 10.8.1). Thus, vitamin B12 deficiency results in a functional deficiency of folate. 

The hemopoietic problems associated with a B12 deficiency are identical to those observed in a folate deficiency and, in fact, result from a folate deficiency secondary to (i.e., caused by) the B12 deficiency (i.e., the methyl trap hypothesis). As the FH4 pool is exhausted, deficiencies of the tetrahydrofolate derivatives needed for purine and dTMP biosynthesis develop, leading to the characteristic megaloblastic anemia. 

If one analyzes the flow of carbon in the folate cycle, the equilibrium lies in the direction of the N -methyl FH4 form. This appears to be the most stable form of carbon attached to the vitamin. However, in only one reaction can the methyl group be removed from N -methyl FH4, and that is the methionine synthase reaction, which requires vitamin B12. Thus, if vitamin B12 is deficient, or if the methionine synthase enzyme is defective, N -methyl FH4 will accumulate. Eventually most folate forms in the body will become trapped in the N -methyl form. A functional folate deficiency results because the carbons cannot be removed from the folate. The appearance of a functional folate deficiency caused by a lack of vitamin B12 is known as the methyl-trap hypothesis, and its clinical implications are discussed in following sections. 

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