Citrovorum factor formation

Inhibition of nudeobase synthesis (2). Tet-rahydrofolic acid (THF) is required for the synthesis of both purine bases and thymidine. Formation of THF from folic acid involves dihydrofolate reductase (p. 274). The folate analogues aminopterin and methotrexate (amethopterin) inhibit enzyme activity. Cellular stores of THF are depleted. The effect of these antimetabolites can be reversed by administration of folinic acid (5-formyl-THF, leucovorin, citrovorum factor). Hydroxyurea (hydroxycarbamide) inhibits ribonucleotide reductase that normally converts ribonucleotides into deoxyribonucleotides subsequently used as DNA building blocks. 

There are several one-carbon derivatives of folate (of different redox states) that function as one-carbon carriers in different metabolic processes. In all of these reactions, the one-carbon moiety is carried in a covalent linkage to one or both of the nitrogen atoms at the 5- and 10-positions of the pteroic acid portion of tetrahydrofolate. Six known forms of carrier are shown in Figure 27-4. Folinic acid (N -formyl FH4), also called leucovorin or citrovorum factor, is chemically stable and is used clinically to prevent or reverse the toxic effect of folate antimetabolites, such as methotrexate and pyrimethamine. The formation and interconversion of some metabolites of... 

Amino-4-desoxyfolic acid, IX, 101 formate and, IX, 103 thymidine and, IX, 103 4-Amino-4methylfolic acid, antileukemic activity of, IX, 101 formate and, IX, 103 2-Amino-6,8-dihydroxypurine(8-hy-droxyguanine), IX, 138 6-Amino-2,8-dihydroxypurine(2,8-dihydroxyadenine), IX, 138 4-Aminofolic acid, IX, 133-135 antagonism to citrovorum factor, VIII, 46 IX, 12... 

As mentioned previously, the citrovorum factor had a pronounced effect on the synthesis of inosinic acid de novo from elementary precursors. However, when inosinic acid was included in the reaction medium and bicarbonate omitted (conditions which favored the demonstration of the enzymatic exchange and limited de novo synthesis), it was found that the addition of the citrovorum factor markedly stimulated the incorporation of formate into position 2 of inosinic acid the incorporation of glycine-l-C and of HC OOH into positions 4 and 8, respectively, was only slightly increased. The interpretation of this experiment was that a primary site of action of the citrovorum factor was the incorporation of formate into position 2 of the acid. 

In order to explain the enzymatic exchange reaction, it was assumed that inosinic acid was enzymatically degraded to 4-amino-5-imi-dazolecarboxamide ribotide, and that the latter compound reacted vith radioactive formate, yielding inosinic acid labeled in the 2-position. It was further postulated that the citrovorum factor was the coenzjune of... 

This subject is discussed in reviews by E. L. Smith (1951,1954), Welch and Heinle (1951), Vilter and Mueller (1952), Welch and Nichol (1952), and Nieweg et al. (1954). Both vitamin B12 and the folic acid group of substances (which includes citrovorum factor) are concerned in the formation and transfer of one-carbon intermediates and in the synthesis of... 

These have been reviewed by Jukes (1952). Suggestions that vitamin Bis participates in the formation of citrovorum factor from pteroylglutamic acid are reviewed by Welch and Nichol (1952). In one patient who died shortly after treatment with vitamin Bis, only 60% of the foUc acid substances in the liver were present as citrovorum factor, whereas in two normal patients nearly all the hepatic activity was present as citrovorum factor (Girdwood, 1952a). It is clear, however, that the megaloblastic marrow of untreated pernicious anemia cannot be due solely to a failure to convert pteroylglutamic acid to citrovorum factor. 

An important chemical property of leucovorin is its ready conversion to 5,10-roethenyl-FH4 (anhydroleucovorin, 5,10-CH=FH4+) by the action of dilute acid. This transformation may also be brought about enzymically by an enzyme citrovorum factor cyclodehydrase which was separated from sheep liver by Peters and D. M. Greenberg (SB). 5,10-CH=FH4+ is also active as a single-carbon donor in crude enzyme systems and may be in equilibrium with 10-formyl-FH4 (S3) which may in turn be formed enzymically from FH4 and formate. 

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