Utilization of folacin
Folates may be absorbed along the entire length of the small intestine; however, there is evidence that the jejunum is the primary site for absorption.
Much ofthe vitamin occurs in the diet in the form of polyglutamates. Before it can be absorbed, excess glutamates must be removed from the side chain of the molecule by folate conjugases found in intestinal mucosa. The activity of the brushborder conjugase appears to be critical in the hydrolysis ofpteroylpolyglutamates. Thus the absorption of food folates appears to be controlled by the conjugase activity, which may, in turn, be affected by conjugase inhibitors in food, for example, yeast. Zinc deficiency reduces the absorption of the polyglutamate but not the monoglutamate forms offolacin, suggesting a role in the conjugase activity. The mucosal uptake of the monoglytamyl forms probably involves a specific carrier; inability to absorb folates is known as a genetic defect. Folate uptake is stimulated by presence of glucose, but no energy requirement for the process has been established. The maximum cellular uptake also depends on a very narrow intraluminal pH, close to 6.0, and is decreased by both acidification and alkalinization of the intestinal contents. Intracellular metabolism results in the release of methyl THFA and some THFA to the serosal fluid.100 Decreased mucosal uptake ofpteroylmonoglutamate has been demonstrated in patients with untreated sprue.
The chief form of folacin in plasma is methyl-THFA, which is loosely bound to proteins, mainly albumin, and serves as a source of folacin to bone marrow cells, reticulocytes, and other cells. Inside the cell it is either first demethylated to THFA, which can be converted to all active forms of folate, or it is directly used in polyglutamate synthesis by the pteroylpolyglutamate synthetase (PPS). Conversion to polyglutamate form is believed to be essential to the various coenzyme functions of folacin and probably also for tissue retention, although some of the cellular vitamin exists as monoglutamate. It is still uncertain whether the PPS can use methyl-THFA as a substrate or requires THFA or another specific form of folate as substrate for polyglutamate formation. Vitamin BIZ is known to be essential for the demethylation of N°-me thy 1THFA to THFA. It has been postulated that in vitamin BIZ deficiency, the active pool ofTHFA is reduced due to an inability to demethylate methyl-THFA. This, in turn, would result in secondary cellular deficiency of folacin due to trapping of the vitamin in the methylated formknown as the methyl trap theory of the folacin-BIz interrelationship. Whatever the mechanism, tissue folacin concentration (the polyglutamate form) is reduced in vitamin BIZ deficiency, but the serum folacin level is normal or elevated. In folate deficiency both erythrocyte and serum vitamin levels are reduced, whereas only the erythrocyte folacin is low in vitamin BIZ deficiency; this fact is used as part of the differential diagnosis in patients ,vith megaloblastic anemia.
Serum and erythrocyte folate levels are used most frequently to assess folacin nutriture. Normal serum levels of folacin range from 7 to 16 nanograms C1 ng = 10-9 g) per ml of serum. A new test for clinical use has been introduced recently, known as the deoxyuridine (dU) suppression test. 101 It is based on the fact that with adequate folacin, synthesis ofthymidylic acid takes place mainly by the N°' 10-methylene- THF A-dependent conversion of deoxyuridine monophosphate (dUMP) to thymidylic acid. With inadequate THFA, recycling of "salvaged" thymidine (from thymidylate degradation) by direct incorporation into thymidylate increases. This salvage pathway is inhibited by an ample supply of thymidylate formed from dUMP. In the test, the effect of added dU on the incorporation of radioactive thymidine into thymidylate and DNA by bone marrow or peripheral lymphocytes is measured in vitro. In cells from people with adequate folacin and vitamin BIZ status, the added dU is converted to thymidylate, thereby suppressing the direct incorporation of thymidine. Failure of dU to suppress thymidine incorporation indicates inadequacy of the active coenzyme. If the cause is folacin deficiency, incubation of the cells with folate but not with vitamin BIZ results in suppression. In Biz-deficient cells, partial response maybe obtained with folate, but complete suppression is produced only when vitamin BIZ also is added. Studies of the test suggest that use of peripheral lymphocytes, which have a long life span, permits diagnosis even after the hematologic symptoms have been corrected, up to 2 to 3 months after therapy. The test may be especially useful in patients whose diagnosis of megaloblastic anemia is complicated by presence of iron deficiency.lol
The liver appears to playa major role in the metabolism and excretion of folacin and may be responsible for the utilization of some forms of the vitamin. When folic acid (stable oxidized form found in pharmaceutical preparations) is administered orally, it is readily reduced to THFA then methylated in the mucosal cell. When administered intravenously, the same functions are performed by the liver, which rapidly removes folic acid from the circulation; intravenously administered methyl-THFA is removed by other tissues as well. Considerable amounts of THFA are secreted by the liver into the bile, as both methyl- and other derivatives. A significant portion is believed to be reabsorbed, thus participating in the enterohepatic circulation of folate. Normal enterohepatic circulation offolate appears to be essential for the maintenance of the serum folate level, which falls rapidly if the enterohepatic circulation is disturbed by cannulation of the bile duct.
Ingestion of alcohol also is associated with a rapid fall in serum folate. Recent research suggests that this may result from interference by ethanol with the transfer of non methyl folate to the bile by the liver, thereby altering the enterohepatic circulation of folate. Reduced serum level, in turn, interferes with the delivery of folate to the bone marrow and other tissues. However, other effects of acute ingestion of alcohol may contribute to the folate deficiency commonly observed in alcoholics after binge drinking. These include reduced intake, malabsorption and, perhaps, creation of the methyl trap in the liver. Oxidation of ethanol increases the hepa tic NADH:NAD ratio, which could result in increased reduction of NN methylene-THFA to methyl-THFA by the NADH-dependent reductase. In addition, chronic alcohol consumption appears to result in intestinal malabsorption and in decreased capacity by the liver to retain folates.
Small amounts offolate also are excreted in the urine, mainly as methyl-THFA. Ascorbic acid is believed to be essential in maintaining an active pool of reduced folates in the body. A person with scurvy was found to excrete mostly NlO-formyl folic acid in the urine prior to treatment with ascorbic acid and N-methyl THFA after treatment.
