Folate conjugase

Folate antagonists (eg, methotrexate and certain antiepileptics) are used ia treatment for various diseases, but their adininistration can lead to a functional folate deficiency. Folate utilization can be impaired by a depletion of ziac (see Zinc compounds). In humans, the intestinal bmsh border folate conjugase is a ziac metaHoenzyme (72). One study iadicates that the substantial consumption of alcohol, when combiaed with an iaadequate iatake of folate and methionine, may iacrease the risk of colon cancer (73). Based on this study, it is recommended to avoid excess alcohol consumption and iacrease folate iatake to lower the risk of colon cancer. 

T Tamura, Y Mizuno, KE Johnston, RA Jacob. Food folate assay with protease, a-amylase, and folate conjugase treatments. J Agric Food Chem 45 135-139, 1997. 

The quantitation of folate absorption under both normal and abnormal conditions remains difficult, especially in view of the variety of forms of folate that may be consumed. Absorption of each of these forms has not been studied in detail. Since OCAs have been postulated by some to interfere with folate conjugase (see below), it is especially important for this discussion to compare the efficiency of absorption of conjugated and unconjugated forms of folic acid. 

The hypothesis that contraceptive steroids might interfere with folate deconjugation in the small intestine was examined by Stephens et al. (S19). Homogenized human jejunal mucosa obtained by peroral biopsy and lysosomal preparations from guinea pig jejunum were utilized. Estradiol, estrone, and progesterone were found not to inhibit folate conjugase in these systems, or to inhibit transport of folate conjugase across lysosomal membranes. 

The biochemical functions of folate were first determined with bacteria. Folate plays a major role in carrying one-carbon units within cells. In the brush border of mucosal cells, the polyglutamyl chain is removed by the enzyme folate conjugase and folate monoglutamate is subsequently absorbed. Folate acts as both a donor and receiver of one-carbon moieties in a variety of reactions. 

The trienzyme extraction method is often used to determine the folate content in foods. The method involves a simultaneous incubation of foods with folate conjugase and ot-amylase, followed by the incubation with protease (Tamura and Hyun 2005). Folate conjugase treatment is intended to hydrolyse poly-glutamyl folate (the primary food folate form) to monoglutamyl forms, which can be assayed subsequently. a-Amylase and protease treatments, however, permit enzymatic digestion of carbohydrate and protein matrices of foods in order to liberate trapped folate (Tamura et al. 2002). More efficient extraction of food folates is provided by these treatments compared with the classical combination of heating and folate conjugase treatment. 

H-S Lim, AD Mackey, T Tamura, SC Wong, MF Picciano. Measurable human milk folate is increased by treatment with a-amylase and protease in addition to folate conjugase. Food Chem 63 401-407, 1999. 

Unaltered folic acid is readily and completely absorbed in the proximal jejunum. Dietary folates, however, consist primarily of polyglutamate forms of N 5-methyltetrahydrofolate. Before absorption, all but one of the glutamyl residues of the polyglutamates must be hydrolyzed by the enzyme -1-glutamyl transferase ("conjugase") within the brush border of the intestinal mucosa. 

Many of the water-soluble vitamins are present in foods bound to proteins, and their release may require either the action of gastric acid (as for vitamin B12, Section 10.7.1) or specific enzymic hydrolysis [e.g., the action of conjugase to hydrolyze folate conjugates (Section 10.2.1) and the hydrolysis of biocytinto release biotin (Section 11.2.3)]. 

Because conjugase is a zinc metallo-enzyme, zinc deficiency can impair the absorption of conjugated food folates, but not folate monoglutamate. 

Conjugase responds rapidly to zinc depletion and repletion, and it has been suggested that the absorption of a test dose of folate polyglutamates may provide a sensitive index of zinc nutritional status (Canton and Cremin, 1990). The absorption of folate monoglutamates (from pharmaceutical preparations or foods) is not affected. 

Free folate, released by conjugase action, is absorbed by a carrier-mediated mechanism in the jejunum. However, the folate in mUk is mainly bound to a specific binding protein (which has been used in radioligand binding assays for folate) the protein-folate complex is absorbed intact, mainly in the ileum, by a mechanism that is distinct from the j ejunal transport system for free folate. The biological availability of folate from miLk, or of folate from diets to which mUk has been added, is considerably greater than that of unbound folate, whereas that of folate from cereal foods, or of free folic acid taken with cereal foods, is lower. 

DiphenyUiydantoin and other anticonvulsants impair the intestinal absorption of folates. This may be by inhibition of intestinal conjugase however, the evidence from various studies is conflicting. 

---