Effects of dietary methionine and vitamin B12 deficiency on folate metabolism

Cyanocobalamin [c-lactam] Inhibits Vitamin B12 and Causes Cytotoxicity in HL60 Cells: Methionine Protects Cells Completely

The [c-lactam] derivative of cobalamin antagonizes vitamin B12 in vivo. Therefore, we investigated its effects in tissue culture to develop a model in which to study vitamin B12-deficient hemopoiesis. HL60 cells were cultured in medium containing either methionine or L-homocysteine thiolactone, and various concentrations of 5-methyltetrahydrofolate or pteroylglutamic acid. In medium with L-homocysteine thiolactone, 5-methyltetrahydrofolate, and dialyzed serum, cyanocobalamin [c-lactam] caused cell death, reversible by additional vitamin B12 . Pteroylglutamic acid did not prevent this cytotoxic effect. Methionine completely protected cells against cyanocobalamin [c-lactam] for periods of up to 4 months of culture, irrespective of the folate source. Cyanocobalamin [c-lactam] reversibly impaired the incorporation of 5-[14CH3]-tetrahydrofolate and [1-14C] propionic acid by intact cells, consistent with inhibition of methionine synthase and methylmalonyl-CoA mutase. A substantial proportion of 5-[14CH3]-tetrahydrofolate uptake could not be suppressed by methionine and may, therefore, have occurred outside of the methionine synthase pathway. These findings are the first indication that cyanocobalamin [c-lactam] antagonizes vitamin B12 in vitro and causes cell death from methionine deficiency. The model should be valuable for investigating the biochemical pathology of vitamin B12-deficient hemopoiesis. The results suggest that methylfolate is not trapped when methionine synthase is inhibited in HL60 cells, but they do not disprove the methylfolate trap hypothesis as applied to normal blood cells.

Inter-relationships between single carbon units' metabolism and resting energy expenditure in weight-losing patients with small cell lung cancer. Effects of methionine supply and chemotherapy

The one-carbon unit metabolism was investigated in 8 weight-losing patients with small cell carcinoma of the lung (SCLC). At diagnosis, 6 of the 8 patients had elevated formiminoglutamic acid (FIGLU) excretion after a histidine load, suggesting a lack of one-carbon units. In accordance, a significant decrease of FIGLU excretion was observed in the patients after oral administration of DL-methionine for 4 days. The elevated FIGLU excretion was positively correlated to weight loss prior to diagnosis and negatively correlated to serum albumin at time of diagnosis. After 3 months of combination chemotherapy, FIGLU excretion was reduced in all patients except 1, who had progressive disease. Despite the elevated FIGLU excretions, all patients had normal blood folate levels. The resting energy expenditure (REE) was recorded in 7 patients, and a significant, positive correlation was observed between pretreatment FIGLU excretion and REE, although the REE measured in this group of patients was within the normal range. These data demonstrate an increased demand of "active" one-carbon units in energy consumption in a group of weight-losing cancer patients. The one-carbon unit deficit was reconditioned by oral administration of the one-carbon unit donor DL-methionine.

The role of methionine and vitamin B12 in folate incorporation by rat liver

The decreased folate uptake induced by exposure of rats to nitrous oxide can be partly reversed by methionine supplementation. This is consistent with the theory that N2O inhibits methylcobalamin-dependent methionine synthetase (EC 2.1.1.13) in rat liver and previous observations showing decreased folate incorporation in rats made nutritionally vitamin B12 deficient.

Interrelationships between Vitamin B12 and folic acid in myelomatosis: cobalamin coenzyme and tetrahydrofolic acid function

Cobalamin and folate metabolism was investigated in 43 patients with myelomatosis, in 8 control subjects of similar age and 22 younger controls. Plasma total cobalamin was lower in myeloma patients than in either of the control groups and methylcobalamin (Me-Cbl) was disproportionately reduced. Erythrocyte levels of total cobalamin were very similar in patients and elderly controls but were half the levels in younger controls. Erythrocyte levels of Me-Cbl were slightly higher in patients than in the dlderly controls. FIGLU excretion after L-histidine was elevated in 53% of the patients but values did not correlate with serum or erythrocyte folate or with plasma total cobalamin. FIGLU excretion decreased after DL-methionine or Me-Cbl only in patients whose FIGLU excretion was initially high. The results are discussed in the light of the 'methylfolate trap hypothesis' and suggest that some patients with myelomatosis have insufficient activity of methionine synthetase to meet the additional metabolic demand for one carbon compounds.

A methionine-reversible folate deficiency in rats following the acute administration of diethylnitrosamine and alpha-naphthylisothiocyanate

Acute doses of the hepatotoxic agents diethylnitrosamine (DEN) and alpha-naphthylisothiocyanate (ANIT) to young adult male rats led to the production of a folate deficiency as determined by an elevated urinary excretion of formiminoglutamic acid (FIGLU) 2 to 4 days following administration of the compounds. High dietary levels of methionine significantly reduced the elevated levels of FIGLU produced by the 2 chemicals. Dietary folate had no significant effect on the excretion of urinary FIGLU. Although the hepatic levels of S-adenosylmethionine (SAM) were significantly increased in rats fed the high dietary levels of methionine, an acute dose of DEN did not depress the hepatic levels of SAM. The results indicate that the methyl-reversible folate deficiency caused by hepatotoxic agents is not the direct consequence of altered hepatic levels of SAM.

The regulation of folate and methionine metabolism

1. The isolated perfused rat liver and suspensions of isolated rat hepatocytes fail to form glucose from histidine, in contrast with the liver in vivo. Both rat liver preparations readily metabolize histidine. The main end product is N-formiminoglutamate. In this respect the liver preparations behave like the liver of cobalamin- or folate-deficient mammals. 2. Additions of L-methionine in physiological concentrations (or of ethionine [2-amino-4-(ethylthio)butyric acid]) promotes the degradation of formiminoglutamate, as is already known to be the case in cobalamin of folate deficiency. Added methionine also promotes glucose formation from histidine. 3. Addition of methionine accelerates the oxidation of formate to bicarbonate by hepatocytes. 4. A feature common to cobalamin-deficient liver and the isolated liver preparations is taken to be a low tissue methionine concentration, to be expected in cobalamin deficiency through a decreased synthesis of methionine and caused in liver preparations by a washing out of amino acids during the handling of the tissue. 5. The available evidence is in accordance with the assumption that methionine does not directly increase the catalytic capacity of formyltetrahydrofolate dehydrogenase; rather, that an increased methionine concentration raises the concentration of S-adenosylmethionine, thus leading to the inhibition of methylenetetrahydrofolate reductase activity [Kutzbach & Stokstad (1967) Biochim. Biophys. Acta 139, 217-220; Kutzbach & Stokstad (1971) Methods Enzymol. 18B, 793-798], that this inhibition causes an increase in the concentration of methylenetetrahydrofolate and the C1 tetrahydrofolate derivatives in equilibrium with methylenetetrahydrofolate, including 10-formyltetrahydrofolate; that the increased concentration of the latter accelerates the formyltetrahydrofolate dehydrogenase reaction, because the normal concentration of the substrate is far below the Km value of the enzyme for the substrate. 6. The findings are relevant to the understanding of the regulation of both folate and methionine metabolism. When the methionine concentration is low, C1 units are preserved by the decreased activity of formyltetrahydrofolate dehydrogenase and are utilized for the synthesis of methionine, purines and pyrimidines. On the other hand when the concentration of methionine, and hence adenosylmethionine, is high and there is a surplus of C1 units as a result of excess of dietary supply, formyltetrahydrofolate dehydrogenase disposes of the excess. When ample dietary supply causes an excess of methionine, which has to be disposed of by degradation, the increased activity of formyltetrahydrofolate dehydrogenase decreases the supply of methyltetrahydrofolate. Thus homocysteine, instead of being remethylated, enters the pathway of degradation via cystathionine. 7. The findings throw light on the biochemical abnormalities associated with cobalamin deficiency (megaloblastic anaemia), especially on the 'methylfolate-trap hypothesis'. This is discussed. 8...

Effect of some anticonvulsant drugs on depletion of folate in rats on a vitamin B12-deficient diet

Rats fed on vitamin B12-deficient or vitamin B12-supplemented diets and treated with phenobarbitone by intraperitoneal injection for 5 d showed significant increases in the activity of glutamate formiminotransferase in liver. The only significant depletion in liver folate activity was in vitamin B12-supplemented rats that were starved for 48 h. When rats were given the same diets, with phenobarbitone and diphenylhydantoin added, for 12 weeks or more in two separate experiments, significant increases in transferase activity were found only in the livers of animals fed on the deficient diet. However, there was significant depletion of liver folate in animals taking the supplemented diet.

The metabolic consequences of vitamin B-12/methionine deficiency in rats

The synthesis of liver pteroylglutamates (folates) from injected [3-H]-pteroylglutamic acid was investigated in vitamin B-12 methionine-deficient rats and pair-fed controls using improved extraction and chromatographic procedures to identify the monoglutamyl derivatives present. Livers from deficient animals had significantly increased levels of radioactive 5-methyltetrahydropteroylglutamate detected after conjugase treatment of extracts and significantly decreased levels of radioactive tetrahydropteroylglutamate and formylated derivatives. However, that this is only a temporary effect and the result of a decreased rate of equilibration of the exogenous radioactivity was shown by measuring the fully equilibrated endogenous pools. This was done by microbiological assay of the endogenous liver folates with Lactobacillus casei. No significant difference was found between deficient animals and controls in the proportion of the endogenous microbiologically active derivatives present. These results in the rat do not support the idea that in vitamin B-12 deficiency the cellular folates accumulate as the reduced 5-methyl derivative, resulting in inadequate amount of the other cofactors to partipicate in nucleic acid biosynthesis.

Regulation of 5-methyltetrahydrofolate: homocysteine methyltransferase activity by methionine, vitamin B12, and folate in cultured baby hamster kidney cells

Rapid growth of BHK cells in methioninedeficient medium required supplementation with homocysteine, B(12), and over 40-fold greater levels of folic acid than growth in methionine-supplemented medium. The activity of the B(12)-dependent 5-methyltetrahydrofolate: homocysteine methyltransferase was studied in extracts of BHK cells grown in media containing various concentrations of the compounds of the enzyme reaction. The methyltransferase activity increased over 4-fold when B(12)-deficient deficient medium was supplemented with optimal levels of B(12); this increase was not prevented by puromycin. Addition of homocysteine to growth medium containing methionine, B(12), and folic acid was without effect. However, methyltransferase activity increased 2.5- to 4.0-fold further beyond the highest levels obtained in the presence of methionine, B(12), and folic acid when homocysteine was substituted for methionine in the growth medium. This increase was blocked by puromycin and was not due to removal of feedback inhibition of activity by the product methionine. These results suggest that methyltransferase activity may be regulated in part by derepression of the enzyme's synthesis on substitution of the substrate homocysteine for the product methionine.