Reduced forms of folic acid are required for essential biochemical reactions that provide precursors for the synthesis of amino acids, purines, and DNA. Folate deficiency is relatively common, even though the deficiency is easily corrected by administration of folic acid. The consequences of folate deficiency go beyond the prob-lem of anemia because folate deficiency is implicated as a cause of congenital malformations in newborns and may play a role in vascular disease (see Box: Folic Acid Supplementation: A Public Health Dilemma).
Folic acid (pteroylglutamic acid) is composed of a heterocycle (pteridine), p-aminobenzoic acid, and glutamic acid (Figure 33–4). Various numbers of glutamic acid moieties are attached to the pteroyl portion of the molecule, resulting in monoglutamates, triglutamates, or polyglutamates. Folic acid undergoes reduction, catalyzed by the enzyme dihydrofolate reductase (“folate reductase”), to give dihydrofolic acid (Figure 33–3). Tetrahydrofolate is subsequently transformed to folate cofactors possessing one-car-bon units attached to the 5-nitrogen, to the 10-nitrogen, or to both positions (Figure 33–3). Folate cofactors are interconvertible by various enzymatic reactions and serve the important biochemical function of donating one-carbon units at various levels of oxida-tion. In most of these, tetrahydrofolate is regenerated and becomes available for reutilization.
The average American diet contains 500–700 mcg of folates daily, 50–200 mcg of which is usually absorbed, depending on metabolic requirements. Pregnant women may absorb as much as 300–400 mcg of folic acid daily. Various forms of folic acid are present in a wide variety of plant and animal tissues; the richest sources are yeast, liver, kidney, and green vegetables. Normally, 5–20 mg of folates is stored in the liver and other tissues. Folates are excreted in the urine and stool and are also destroyed by catabolism, so serum levels fall within a few days when intake is diminished. Because body stores of folates are relatively low and daily requirements high, folic acid deficiency and megaloblastic anemia can develop within 1–6 months after the intake of folic acid stops, depending on the patient’s nutritional status and the rate of folate utilization.
Unaltered folic acid is readily and completely absorbed in the proximal jejunum. Dietary folates, however, consist primarily of polyglutamate forms of N5-methyltetrahydrofolate. Before absorp-tion, all but one of the glutamyl residues of the polyglutamates must be hydrolyzed by the enzyme α-1-glutamyl transferase (“conjugase”) within the brush border of the intestinal mucosa. The monoglutamate N5-methyltetrahydrofolate is subsequently transported into the bloodstream by both active and passive transport and is then widely distributed throughout the body. Inside cells, N5-methyltetrahydrofolate is converted to tetrahydro-folate by the demethylation reaction that requires vitamin B12 (Figure 33–3).
Tetrahydrofolate cofactors participate in one-carbon transfer reac-tions. As described earlier in the discussion of vitamin B12, one of these essential reactions produces the dTMP needed for DNA synthe-sis. In this reaction, the enzyme thymidylate synthase catalyzes the transfer of the one-carbon unit of N5, N10-methylenetetrahydrofolate to deoxyuridine monophosphate (dUMP) to form dTMP (Figure 33–3, section 2). Unlike all the other enzymatic reactions that use folate cofactors, in this reaction the cofactor is oxidized to dihydrofolate, and for each mole of dTMP produced, 1 mole of tetrahydrofolate is consumed. In rapidly proliferating tissues, con-siderable amounts of tetrahydrofolate are consumed in this reac-tion, and continued DNA synthesis requires continued regeneration of tetrahydrofolate by reduction of dihydrofolate, catalyzed by the enzyme dihydrofolate reductase. The tetrahydrofolate thus pro-duced can then reform the cofactor N5, N10-methylenetetrahydro-folate by the action of serine transhydroxymethylase and thus allow for the continued synthesis of dTMP. The combined catalytic activities of dTMP synthase, dihydrofolate reductase, and serine transhydroxymethylase are referred to as the dTMP synthesis cycle. Enzymes in the dTMP cycle are the targets of two anticancer drugs; methotrexate inhibits dihydrofolate reductase, and a metabolite of 5-fluorouracil inhibits thymidylate synthase .
Cofactors of tetrahydrofolate participate in several other essential reactions. N5-Methylenetetrahydrofolate is required for the vitamin B 12-dependent reaction that generates methionine from homocysteine (Figure 33–2A; Figure 33–3, section 1). In addition, tetrahydrofolate cofactors donate one-carbon units dur-ing the de novo synthesis of essential purines. In these reactions, tetrahydrofolate is regenerated and can reenter the tetrahydrofo-late cofactor pool.
Folate deficiency results in a megaloblastic anemia that is micro-scopically indistinguishable from the anemia caused by vitamin B12 deficiency (see above). However, folate deficiency does not cause the characteristic neurologic syndrome seen in vitamin B12 deficiency. In patients with megaloblastic anemia, folate status is assessed with assays for serum folate or for red blood cell folate. Red blood cell folate levels are often of greater diagnostic value than serum levels, because serum folate levels tend to be labile and do not necessarily reflect tissue levels.
Folic acid deficiency is often caused by inadequate dietary intake of folates. Patients with alcohol dependence and patients with liver disease can develop folic acid deficiency because of poor diet and diminished hepatic storage of folates. Pregnant women and patients with hemolytic anemia have increased folate requirements and may become folic acid-deficient, especially if their diets are marginal. Evidence implicates maternal folic acid deficiency in the occurrence of fetal neural tube defects. (See Box: Folic Acid Supplementation: A Public Health Dilemma.) Patients with malabsorption syn-dromes also frequently develop folic acid deficiency. Patients who require renal dialysis are at risk of folic acid deficiency because folates are removed from the plasma during the dialysis procedure.
Folic acid deficiency can be caused by drugs. Methotrexate and, to a lesser extent, trimethoprim and pyrimethamine, inhibit dihydrofolate reductase and may result in a deficiency of folate cofactors and ultimately in megaloblastic anemia. Long-term therapy with phenytoin can also cause folate deficiency, but only rarely causes megaloblastic anemia.
of folic acid is rarely necessary, since oral folic acid is well absorbed even
in patients with malabsorption syndromes. A dose of 1 mg folic acid orally
daily is sufficient to reverse megaloblastic anemia, restore normal serum
folate levels, and replenish body stores of folates in almost all patients.
Therapy should be continued until the underlying cause of the deficiency is
removed or corrected. Therapy may be required indefinitely for patients with
malabsorption or dietary inadequacy. Folic acid supplementation to prevent
folic acid deficiency should be consid-ered in high-risk patients, including
pregnant women, patients with alcohol dependence, hemolytic anemia, liver
disease, or cer-tain skin diseases, and patients on renal dialysis.