Evidence that the decreased liver folate status following vitamin B-12 inactivation in the mouse is due to increased loss rather than impaired uptake

Correction of the DNA synthesis defect in vitamin B12 deficiency by tetrahydrofolate: evidence in favour of the methyl-folate trap hypothesis as the cause of megaloblastic anaemia in vitamin B12 deficiency

The critical disturbance of folate metabolism caused by vitamin B12 deficiency which results in megaloblastic anaemia remains controversial. Vitamin B12 is required in the methionine synthase reaction in which homocysteine is converted to methionine and methyl tetrahydrofolate (methyl THF) to THF. The 'methyl-folate trap' hypothesis suggested that failure of demethylation of methyl THF with consequent deficiency of folate co-enzymes derived from THF is the crucial lesion caused by vitamin B12 deficiency. A more recent theory suggested that reduced supply of methionine leads to reduced availability of 'activated formate' and hence of formyl THF and it is this defect that results in failure of folate co-enzyme synthesis. The present results, based on deoxyuridine suppression tests on 103 cases of megaloblastic anaemia, show that THF itself is equally capable of correcting the failure of thymidylate synthesis in vitamin B12 deficiency as in folate deficiency. Although not as effective as formyl THF in correcting the dU blocking test in vitamin B12 deficiency, this is equally so for the correction of the test by THF compared with formyl THF in folate deficiency. The results therefore favour the theory that it is in the supply of THF and not of 'active formate' or formyl THF that vitamin B12 plays a critical role in folate metabolism.

Characterization of cobalamin-dependent methionine synthase purified from the human malarial parasite, Plasmodium falciparum

Methionine synthase, which catalyzes the reaction, 5-methyltetrahydrofolate (5-CH3-H4PteGlu) + homocysteine----methionine + tetrahydrofolate, was detected and partially purified from the human malarial parasite, Plasmodium falciparum (K1 isolate). Partial purification was achieved using high-performance size-exclusion and anion-exchange chromatography. The apparent relative molecular weight of the enzyme was estimated as 105,000 daltons, and the apparent Km for 5-CH3-H4PteGlu was 24.2 microM. The enzyme was dependent on adenosylcobalamin or methylcobalamin but not on cobalamin, cyanocobalamin, or hydroxocobalamin in either the absence or presence of S-adenosylmethionine. Preincubation with nitrous oxide markedly inhibited the enzyme. Methionine synthase in P. falciparum may play a role in the supply of methionine and in folate salvage using exogenous 5-CH3-H4PteGlu for tetrahydrofolate metabolism.

Chronic cobalamin inactivation impairs folate polyglutamate synthesis in the rat

Nitrous oxide, by inactivating cobalamin in vivo, produces a suitable animal model for cobalamin 'deficiency.' The synthesis of folate polyglutamate with tetrahydrofolate as substrate is severely impaired in the N2O-treated rat, but is normal with formyltetrahydrofolate as substrate. Methionine restores the capacity of the N2O-treated rat to utilize tetrahydrofolate the minimum effective dose being 16 mumol. S-Adenosylmethionine was somewhat less effective than methionine but 5'methylthioadenosine, a product of S-adenosylmethionine metabolism, was significantly more effective than methionine in correcting the defect in folate polyglutamate synthesis. 5'Methylthioadenosine is metabolised to yield formate. It is suggested that these compounds have their effect in correcting folate polyglutamate synthesis by supplying formate for the formylation of tetrahydrofolate. Formyltetrahydrofolate, at least in the cobalamin-inactivated animal, is the required substrate for folate polyglutamate synthesis. Cobalamin is concerned with the maintenance of normal levels of methionine and this in turn is a major source of formate through S-adenosylmethionine and 5'methylthioadenosine.