Cobalamin Dependent Methionine Synthesis and Methyl‐Folate‐Trap in Human Vitamin B₁₂ Deficiency

Abstract

The activity of methionine synthetase (MS) is important for the rapid growth of human haematopoietic cells and cultured lymphoblastoid cells. The MS reaction is the only known metabolic step in which both vitamin B12 and folate are essential in a single enzyme reaction. In vitamin B12 deficiency the MS activity in bone marrow cells is significantly lower than that in normal bone marrow. Free tetrahydrofolic acid (H4PteGlu) is normally liberated from its metabolically inactive storage form, 5-methyl-H4PteGlu (CH3H4PteGlu), in the cobalamin-dependent MS reaction. Thus, in vitamin B12 deficiency H4PteGlu is not available in sufficient concentration to maintain the de novo synthesis of thymidylate and purines, and accords with the methyl-folate-trap hypothesis. After treatment with amethopterin (Methotrexate), the incorporation of 3H-deoxyuridine into cellular DNA is reduced. In proliferating normal cells this effect of methotrexate can be prevented (and the cells rescued) with CH3-H4PteGlu or with CHO-H4PteGlu (5-formyl-H4PteGlu; Leucovorin). On the other hand, in vitamin B12 deficient bone marrow cells this so-called rescue-effect could only be achieved with CHO-H4PteGlu and not with CH3-H4PteGlu. These observations also support the hypothesis of the methyl-folate-trap in vitamin B12 deficiency. Decreased MS activity in vitamin B12 deficiency seems to be the essential metabolic fault, which is responsible for secondary alterations of folate metabolims. Thus, measurement of MS activity may allow direct functional assessment of vitamin B12 deficiency, at least with regard to DNA metabolism.

In Vitro DNA Synthesis by Bone Marrow Cells and PHA‐stimulated Lymphocytes. Suppression by Nonradioactive Thymidine of the Incorporation of 3H‐Deoxyuridine into DNA: Enhancement of Incorporation when Inadequate Vitamin B12 or Folate is Corrected

“…Sauer & Wilmanns (1977) demonstrated there was less incorporation of radioactive deoxyuridine than thymidine into megaloblastic than normal cell DNA, raising the question as to whether direct 'radioactive deoxyuridine incorporation' might be preferable to the indirect 'dU suppression' tests. Factors favouring 'dU suppression' tests are the much greater (Sauer & Wilmanns, 1977; and the current study) incorporation of radioactive thymidine than deoxyuridine into DNA, resulting in more striking changes when the missing vitamin is supplied, and the fact that thymidine incorporation correlates well with thymidine kinase (which has been better studied than the kinase promoting d U entry).…”

Bacterial Conversion of Folinic Acid Is Required for Antifolate Resistance

“…However, the precise enzymatic mechanism leading to these phenotypes remains unclear. 5-CH 3 -H 4 PteGlu n is a major reduced H 4 PteGlu species in one-carbon metabolism, and impaired metabolism of 5-CH 3 -H 4 PteGlu n is associated with the so-called methylfolate trap phenomenon (57,58), which may also contribute to antifolate susceptibility. Increased cellular content of glycine in the absence of MTHFS might also attribute to the general antibiotic resistance (50,52).…”

Effect of Omega-3 and Vitamins E + C Supplements on the Concentration of Serum B-Vitamins and Plasma Redox Aminothiol Antioxidant Status in Elderly Men after Strength Training for Three Months

“…Since, we did not observe any reduction in the dietary intake of vitamin B12 or any increase in folate intake in subjects on placebo, the reduction in the concentration of serum vitamin B12 might be due to strength training ( table 3 ). It has been demonstrated that in patients with a deficiency of vitamin B12 (cobalamin), the concentration of serum folate increases the so-called methyl-trap hypothesis [43,44] . Intracellular 5-methyltetrahydrofolate (5-MTHF) is a methyl-donor, and intracellular vitamin B12 is a methyl-acceptor during remethylation of homocys- teine to methionine.…”