Metabolic precursors exogenously supplied

Vitamins are micronutrients that are essential for the metabolism of all living organisms. They are found as precursors of intracellular co-enzymes that are necessary to regulate vital biochemical reactions in the cell. In view of the inability of mammalian cells to synthesize these vital biomolecules, an exogenous supply of most vitamins appears inevitable to prevent nutritional deficiencies. Although most vitamins are present in a variety of foods, vitamin deficiencies still exist in many countries including industrialized nations. 

C. Pharmacodynamics Vitamin B is essential in two reactions conversion of methyl-malonyl-CoA to succinyl-CoA and conversion of homocysteine to methionine. The second reaction is linked to folic acid metabolism and synthesis of deoxythymidylate (dTMP Figure 33-2, reaction 2), a precursor required for DNA synthesis. In vitamin B,2 deficiency, folates accumulate as AP-methyltetrahydrofolate the supply of tetrahydrofolate is depleted and the production of red blood cells slows. Administration of folic acid to patients with vitamin Bj deficiency helps refill the tetrahydrofolate pool (Figure 33-2, reaction 3) and partially or fully corrects the anemia. However, the exogenous folic acid does not correct the neurologic defects of vitamin Bj2 deficiency. 

Vitamins An essential organic micronutiient that must be supplied exogenously and in many cases in the precursor to a metabolically derived coenzyme. 

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