Folic acid is converted (in the presence of ascorbic acid) in the liver and plasma to its metabolically active form (tetrahydrofolic acid) by dihydrofolate reductase.
来源:Hazardous Substances Data Bank (HSDB)
代谢
在吸收1毫克或更少的叶酸后，叶酸在大部在肝脏被还原和甲基化成N-甲基四氢叶酸……。
Following absorption of 1 mg or less, folic acid is largely reduced and methylated in the liver to N-methyltetrahydrofolic acid... .
The folates are taken up by the liver and metabolized to polyglutamate derivatives (principally pteroylpentaglutamate), via the action of folypolyglutamate synthase. ... Folate polyglutamates are released from the liver to the systemic circulation and to the bile. When released from the liver into the circulation, the polyglutamate forms are hydrolyzed by gamma-glutamylhydrolase and reconverted to the monoglutamate forms.
IDENTIFICATION: Folic acid is an antianaemic vitamin. Origin of the substance: Folic acid was isolated from green leafy vegetables, liver, yeast and fruits. Synthetic folic acid is commercially available. Yellow to orange brown crystalline powder which is odorless. Readily soluble in alkali, hydroxides and carbonates. Insoluble in alcohol, acetone, chloroform and ether. Solutions are inactivated by ultraviolet light. Alkaline solutions are sensitive to oxidation and acid solutions are sensitive to heat. Indications: For the prevention and treatment of vitamin B deficiency. For the treatment of megaloblastic anemia and macrocytic anemia due to folic acid deficiency. Folic acid supplements may be required in low birth weight infants, infants breastfed by folic acid deficient mothers, or those with prolonged diarrhea and infection. Other conditions which may increase folic acid requirements include alcoholism, hepatic disease, hemolytic anemia, lactation, oral contraceptive use and pregnancy. It has been given to pregnant mothers to reduce the risk of birth defects. HUMAN EXPOSURE: Main risks and target organs: Folic acid is relatively non-toxic. However, there have been reports of reactions to parenteral injections. Allergic reactions to folic acid have been rarely reported. Summary of clinical effects: Severe allergic reactions are characterized by hypotension, shock, bronchospasm, nausea, vomiting, rash, erythema. Itching may also occur. Adverse gastrointestinal and central nervous system effects have been reported. Treatment with folic acid is usually well tolerated except for rare reports of allergic reactions. Bioavailability: Folic acid is rapidly absorbed from gastrointestinal tract following oral administration. Peak folate activity in blood is 30 to 60 minutes after oral administration. Contraindications: It should be given with caution to patients with abnormal renal function. It is also contra-indicated in patients who show hypersensitivity reactions to folic acid. Caution is advised in patients who may have folate dependent tumours. Folic acid should never be given alone or in conjunction with inadequate amounts of Vitamin B12 for the treatment of undiagnosed megaloblastic anaemia. Although folic acid may produce a haematopoietic response in patients with megaloblastic anaemia due to Vitamin B12, it fails to prevent the onset of subacute combined degeneration of the cord. Absorption by route of exposure: Oral: Folic acid is rapidly absorbed from the proximal part of the gastrointestinal tract following oral administration. It is mainly absorbed in the proximal portion of the small intestine. The naturally occurring folate polyglutamate is enzymatically hydrolyzed to monoglutamate forms in the gastrointestinal tract prior to absorption. The peak folate activity in blood after oral administration is within 30 to 60 minutes. Enterohepatic circulation of folate has been demonstrated. Distribution by route of exposure: Tetrahydrofolic acid and its derivatives are distributed in all body tissues. The liver contains half of the total body stores of folate and is the principal storage site. Metabolism: Folic acid once absorbed is acted upon by hepatic dihydrofolate reductase to convert to its metabolically active form which is tetrahydrofolic acid. Following absorption, folic acid is largely reduced and methylated in the liver to N-5 methyltetrahydrofolic acid, which is the main transporting and storage form of folate in the body. Larger doses may escape metabolism by the liver and appear in the blood mainly as folic acid. Elimination by route of exposure: Oral: Following oral administration of single doses of folic acid in health adults, only a trace amount of the drug appears in urine. Following administration of large doses, the renal tubular reabsorption maximum is exceeded and excess folate is excreted unchanged in urine. Small amounts of orally administered folic acid have been recovered from feces. Pharmacodynamics: Folic acid is transformed into different coenzymes that are responsible for various reactions of intracellular metabolism mainly conversion of homocysteine to methionine, conversion of serine to glycine, synthesis of thymidylate, histidine metabolism, synthesis of purines and utilization or generation of formate. In man, nucleoprotein synthesis and the maintenance of normal erythropoiesis requires exogenous folate. Folic acid is the precursor of tetrahydrofolic acid which is active and acts as a co-factor for 1-carbon transfer reactions in the biosynthesis of purines and thymidylates of nucleic acids. Adults: There is little data available on folic acid toxicity in humans. A case of two patients who showed exacerbation of psychotic behavior during treatment with folic acid has been reported. Cytomorphological effects of folic acid were studied using in-vitro establishment human oral epithelium. A concentration twice that used clinically did not induce marked cytotoxic reaction in cultured cells. The most pronounced changes were cultures which showed degenerating cells showing edema, increased translucency of the cytoplasm, flattened cells and atypical filaments. Interactions: Folic acid therapy may increase phenytoin metabolism in folate deficient patients resulting in decreased phenytoin serum concentration. It has also been reported that concurrent administration of folic acid and chloramphenicol in folate deficient patients may result in antagonism of the hematopoietic response to folic acid. The use of ethotoin or mephenytoin concurrently with folic acid may decrease the effects of hydantoins by increasing hydantoin metabolism. Trimethoprim acts as a folate antagonist by inhibiting dihydrofolate reductase, so in patients receiving this drug leucovorin calcium must be given instead of folic acid. Folic acid may also interfere with the effects of pyrimethamine. Aminopterin (4 aminofolic acid) and methotrexate (4 amino- 10 methylfolic acid) antagonizes reduction of folic acid to tetrahydrofolic acid. Methotrexate continues to be used as an antineoplastic drug whose activity may be dependent on blocking certain syntheses, of purines, in which folic acid is required, thereby depriving neoplastic cells of compounds essential for their proliferation. Calcium leucovorin is used therapeutically as a potent antidote for the toxic effects of folic acid antagonists used as antineoplastic agents. Methotrexate or pyrimethamine or triamterene also acts as folate antagonist by inhibiting dihydrofolic reductase. Analgesics, anticonvulsants, antimalarials and corticosteroids may cause folic acid deficiency. Main adverse effects: Allergic reactions to folic acid have been rarely reported including erythema, rash, itching, general malaise and bronchospasm. Adverse gastrointestinal and central nervous system effects have been reported in patients receiving 15 mg of folic acid daily for one month. ANIMAL/PLANT STUDIES: Mode of action: Folic acid is relatively non-toxic. Toxicity studies in mice showed that folic acid could cause convulsions, ataxia and weakness. Histopathological studies in some strains of mice showed that toxic doses may also cause acute renal tubular necrosis. A possible relationship between folic acid neurotoxicity and cholinergic receptors in the pyriform cortex and amygdala has been shown.
Neither normal nor excessively high intakes of folate are associated with liver injury or liver test abnormalities. In long term clinical trials, serum enzyme and bilirubin elevations were no more frequent with folic acid therapy than with placebo. Use of high doses of folic acid (up to 15 mg daily) has not been associated with appreciable adverse reactions, ALT elevations or hepatotoxicity.
Pemetrexed therapy is associated with a low-to-moderate rate of serum enzyme elevations, but these are generally mild, transient and without accompanying symptoms or jaundice. Serum ALT or AST elevations above 5 times ULN occur in 1% to 6% of patients, but are usually self-limited in course and rarely require dose modification or discontinuation. No instances of clinically apparent acute liver injury attributed to pemetrexed have been reported. In addition, pemetrexed has not been linked to sinusoidal obstruction syndrome or to reactivation of hepatitis B, but it is rarely used in high doses in neoplastic disease or in conditioning regimens for bone marrow transplantation, situations in which other neoplastic agents are commonly associated with these complications.
Pralatrexate is associated with serum enzyme elevations during therapy, but these abnormalities are generally mild and self-limited, rising to above 5 times ULN in 2% to 6% of patients and rarely requiring dose adjustment. No instances of clinically apparent acute liver injury attributed to pralatrexate have been reported in the literature, but monitoring for liver toxicity is recommended. Pralatrexate has not been linked specifically to sinusoidal obstruction syndrome, but it is rarely used in high doses in neoplastic disease or in conditioning regimens for bone marrow transplantation, situations in which alkylating agents are commonly associated with this complication.
Folic acid is absorbed rapidly from the small intestine, primarily from the proximal portion. Naturally occurring conjugated folates are reduced enzymatically to folic acid in the gastrointestinal tract prior to absorption. Folic acid appears in the plasma approximately 15 to 30 minutes after an oral dose; peak levels are generally reached within 1 hour.
After a single oral dose of 100 mcg of folic acid in a limited number of normal adults, only a trace amount of the drug appeared in the urine. An oral dose of 5 mg in 1 study and a dose of 40 mcg/kg of body weight in another study resulted in approximately 50% of the dose appearing in the urine. After a single oral dose of 15 mg, up to 90% of the dose was recovered in the urine. A majority of the metabolic products appeared in the urine after 6 hours; excretion was generally complete within 24 hours. Small amounts of orally administered folic acid have also been recovered in the feces. Folic acid is also excreted in the milk of lactating mothers.
Folic acid is absorbed rapidly from the GI tract following oral administration oral administration; the vitamin is absorbed mainly in the proximal portion of the small intestine.
The monoglutamate forms of folate, including folic acid, are transported across the proximal small intestine via a saturable pH-dependent process. Higher doses of the pteroylmonoglutamates, including folic acid, are absorbed via a nonsaturable passive diffusion process. The efficiency of absorption of the pteroylmonoglutamates is greater than that of pteroylpolyglutamates.
