Folate, absorption biosynthesis

Many drugs interact with folate to affect its absorption, antagonize its biochemical activity, or increase its loss from the body. These drugs include ethanol, phenytoin, and oral contraceptives. Salicylates can compete with foUc acid for plasma protein binding. Methotrexate, a cytotoxic agent, is a folate antagonist that inhibits the biosynthesis of this coenzyme. 

A. Pernicious anemia occurs when the stomach does not produce adequate intrinsic factor for absorption of vitamin B12, which is required for the conversion of methylmalonyl CoA to succinyl CoA and homocysteine to methionine. A vitamin B12 deficiency results in the excretion of methylmalonic acid and an increased dietary requirement for methionine. The methyl group transferred from vitamin B12 to homocysteine to form methionine comes from 5 -methyl tetrahydrofolate, which accumulates in a vitamin B12 deficiency, causing a decrease in folate levels and symptoms of folate deficiency, including increased levels of FIGLU and decreased purine biosynthesis. 

---