[(Science: biochemistry) molecule that acts as a carrier of one carbon units in intermediary metabolism. It contains residues of p aminobenzoate, glutamate and a substituted pteridine. The latter cannot be synthesised by mammals, which must obtain tetrahydrofolate as a vitamin or from intestinal microorganisms. One carbon units are carried at three different levels of oxidation, as methyl, methylene or formimino groups. Important biosyntheses dependent on tetrahydrofolate include those of methionine, thymine and purines. analogues of dihydrofolate, such as aminopterin and methotrexate block the action of tetrahydrofolate by inhibiting its regeneration from dihydrofolate. a B vitamin that is essential for cell growth and reproduction.