ABSTRACT
The folates are the collective name for the 10 or so different forms of this vitamin that occur in nature (1). They all consist of a pteridine ring, which is attached to paminobenzoic acid, which in turn is attached to the amino acid glutamate (Fig. 1). In all of the naturally occurring forms of the vitamin, the pteridine ring is in the reduced state, as tetrahydrofolate, either on its own or with different so-called carbon one units attached to it, i.e., formyl (—CHO), methylene (—CH2-), methenyl (—CH=), or methyl (— CH3). The sole exception is dihydrofolate, formed as a result of the action of the enzyme thymidylate synthase, which, as discussed later, is involved in the conversion of the uracil-type base found in RNA to the thymine-type base found in DNA. In all cells not one but several glutamates are attached to the folates. These are linked to each other through the γ-carboxyl group. During intestinal absorption this polyglutamate chain is removed by an enzyme present in the brush border of the duodenal mucosal cells called folate polyglutamate conjugase. This enzyme converts all of the naturally occurring folate polyglutamates into monoglutamates.
