Gastrointestinal tract, folate

Severe cyanocobalamin (vitamin B12) deficiency results in pernicious anemia that is characterized by megaloblastic anemia and neuropathies. The symptoms of this deficiency can be masked by high intake of folate. Vitamin B12 is recycled by an effective enterohep-atic circulation and thus has a very long half-hfe. Absorption of vitamin B12 from the gastrointestinal tract requires the presence of gastric intrinsic factor. This factor binds to the vitamin, forming a complex that... 

The most widely known chemotherapeutic agent directed against TS is 5-fluorour-acil (5-FU). 5-FU was first used clinically almost 50 years ago, yet still remains a mainstay for the treatment of carcinoma of the breast and gastrointestinal tract. In cells, 5-FU is metabolized to 5-FdUMP, which forms a stable inhibitory ternary complex with the co-substrate N5N10-methylene-5,6,7,8-tetrahydrofolate (CH2H4-folate) and TS. In this complex, a covalent bond links the thiol of cysteine 195 of human TS to C6 of deoxyuracil monophosphate (dUMP) and the methylene carbon of the co-substrate is joined to C5 of the nucleotide [55]. The fluorine at C5, unlike the proton, cannot... 

It is absorbed from the gastrointestinal tract by an active process also used by folates. It is eliminated from the plasma by cellular uptake and by renal excretion of the unmetabolised drug (t) 5 h). 

C4. Cooper, B. A., Physiology of absorption of mono utamyl folates from the gastrointestinal tract. In Folic Acid Biochemistry and Physiology in Relation to Human Nutrition Requirement (C. E. Butterworth, ed.)p. 188. Nad. Acad. Sci., Washington, D.C.,... 

---