Folic acid interconversion

These three compounds exert many similar effects in nucleotide metabolism of chicks and rats [167]. They cause an increase of the liver RNA content and of the nucleotide content of the acid-soluble fraction in chicks [168], as well as an increase in rate of turnover of these polynucleotide structures [169,170]. Further experiments in chicks indicate that orotic acid, vitamin B12 and methionine exert a certain action on the activity of liver deoxyribonuclease, but have no effect on ribonuclease. Their effect is believed to be on the biosynthetic process rather than on catabolism [171]. Both orotic acid and vitamin Bu increase the levels of dihydrofolate reductase (EC 1.5.1.4), formyltetrahydrofolate synthetase and serine hydroxymethyl transferase in the chicken liver when added in diet. It is believed that orotic acid may act directly on the enzymes involved in the synthesis and interconversion of one-carbon folic acid derivatives [172]. The protein incorporation of serine, but not of leucine or methionine, is increased in the presence of either orotic acid or vitamin B12 [173]. In addition, these two compounds also exert a similar effect on the increased formate incorporation into the RNA of liver cell fractions in chicks [174—176]. It is therefore postulated that there may be a common role of orotic acid and vitamin Bj2 at the level of the transcription process in m-RNA biosynthesis [174—176]. 

Pteridines carrying a polyhydroxyalkyl sidechain are valuable synthons for the synthesis of functionalized derivatives. Various 6-(l,2,3-trihydroxypropyl)pteridines (216, 217, 232) have been oxidized by periodate to the corresponding pteridine-6-carbaldehydes (218, 225, 233) which are further substrates for the interconversion into oximes (219, 220, 225, 226, 234, 235), carbonitrile (221, 228, 236), hydroxymethyl (222, 229, 237) and carboxy derivatives (223, 230, 238) as well as folic acid analogues (224, 231, 239) (Scheme 34) <90Ml 718-08). 

In archaebacteria the folic acid pathway for C-l transformations is based upon methanopterin as an ancient precursor of these important cellular interconversions. Tetrahydromethanopterin (478) has been identified as the active coenzyme and carbon carrier (479-481) in methanogenesis <84JBC(259)9447> and functions et alia as formaldehyde activation factor <84PNA(8l)l976>. 

Purine biosynthesis de novo was one of the hrst areas of metabolism in which a folic acid derivative was specifically identified as a cofactor in an enzymatic reaction. The ability of pigeon liver extracts to add formate to phosphoribosyl glycineamide was impaired by treatment with charcoal, but was restored by addition of H4-folate. Although the complicated interconversions of the Hrfolate coenzymes (see Chapter 5) caused confusion for some time, the specific one-carbon donor for this reaction was eventually identified as 5,10-methenyl H4-folate. The phosphoribosyl glycineamide formyltransferase reaction itself is irreversible. 

As expected, there are microbiological hints that folic acid is concerned in tyrosine metabolism. Thus, Lampen et at. (1949) found that tyrosine, among other amino acids, spared the PABA requirement of an E. colt mutant. The usual folic-requiring microorganisms have not provided information on this point because they must be provided with phenylalanine or tyrosine because of other blocks in synthesis. Another complication is that the interconversion of these aromatic amino acids may not follow the same path in microorganisms and in higher animals e.g. tyrosine and phenylalanine in many bacteria may have a common precursor and hardly be interconvertible. 

The structures of the folic acid derivatives and the cyclohydrolase reaction are shown in Fig. 6. Interconversion of these compounds also took place spontaneously at a slow rate (111). AT -Tetrahydrofolic acid was not utilized in transformylation reactions unless it first was converted to an active form (96,108, Hi). 

Amino-4-imidazolecarboxamide ribonucleotide transfoimylase catalyzes the formylation of AICAR to form 5-formamido-4-imidazolecarboxa-mide ribonucleotide (FAICAR) (Fig. 11). Various formylated folic acid compounds donated their formyl groups to AICAR (76,96,107). However, when the transformylase enzyme was purified and freed from cyclohydrolase (see Section II, B, 1 for a discussion of interconversions of folic acid derivatives), a specific requirement for V -formyl-FH4 was demonstrated (109). The equation for the reaction is ... 

D. Interconversion of Biologically Active Forms of Folic Acid.722... 

Folic acid (pteroyhnonoglutamic acid or PGA) exists in different forms in nature. These forms are changed to at least five active coenzymes critically important for the formation of purines and pyrimidines needed for the synthesis of DNA and RNA, the formation of hanoglobin, the interconversion of amino acids such as homocysteine to methionine, and the synthesis of choline from ethanolamine. Vitamins B,2, Bg, and C are essential as coenzymes for the activity of folacin in many metabolic processes. In practical terms, folic acid is required for cell division and reproduction, and prevents neural tube defects in newborns and cardiovascular diseases in adults. The cardiovascular protective role is because folacin and vitamin Bjj lower levels of homocysteine. 

Thus a thiamine derivative plays a metabolic role as cocarboxylase, which has been found to be inactivated by a specific phosphatase of yeast (122,123). The inactivation was inhibited by thiamine itself and to a lesser degree by thiamine monophosphate and the pyrimidine constituent of the thiamine molecule. Synthesis and breakdown of thiamine by Phycomyces species have also been studied (9,45,98). Pyridoxine derivatives are now known to catalyze two t3T)cs of bacterial reactions, involving transamination and decarboxylation of amino acids (4,32,35,59). Interconversion between members of the group of substances of natural occurrence which are related to pyridoxine has been observed in microorganisms and appears likely to afford a series of changes comparable to those observed in nicotinic acid dreivatives. Production of folic acid from chemically defined precursors by bacterial suspensions has also been observed (110,111). 

The N-5 position is considerably more basic than the N-10 position, and this basicity is one of several factors that control certain preferences in the course of reactions involving tetrahydrofolate. Thus, for-mylation occurs more readily at N-10 while alkylation occurs more readily at N-5. Benkovic and Bullard (1973) have reviewed evidence for an iminium cation at N-5 as the active donor in formaldehyde oxidation-level transfers. Recently, Barrows et al. (1976) have further studied such a mechanism for folic acid. The interconversion of these forms of folate coenzymes by enzymatic means has been reviewed by Stokstad and Koch (1967), and the reader is directed there for further details. Folate coenzymes are involved in a wide variety of biochemical reactions. These include purine and pyrimidine synthesis, conversion of glycine to serine, and utilization and generation of formate. In addition, the catabolism of histidine, with the formation of formiminoglu-tamic acid (FIGLU), is an important cellular reaction involving folate. 

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