Folate coenzymes and dependent enzymes introduce one carbon units at positions 2 (C(2)) and 8 (C(8)) of the purine ring during de novo biosynthesis. Formate is one source of one-carbon units. Although much is known about lower organisms, little data exists describing formate utilization for purine biosynthesis in humans.
Mass-spectrometric analysis of urinary uric acid, the final purine catabolite, following 1.0 g oral doses of sodium [(13)C] formate was performed and detected (13)C enrichment at C(2) and C(8) separately.
Three phenotypes were suggested. One incorporates (13)C 0.72 to 2.0% into C(2) versus only 0 to 0.07% into C(8). Another incorporates only 0 to 0.05% (13)C into C(2) or C(8). A third phenotype incorporates (13)C into C(8) (0.15%) but C(2) incorporation (0.44%) is still greater. In subjects who incorporated (13)C formate into C(2), peak enrichment occurred in voids from 8-12 h (24 h clock) suggesting a circadian rhythm.
Evidence that mammalian liver introduces C(8) and that C(2) is introduced in a non-hepatic site would explain our results. Our data are not similar to those in non-mammalian organisms or cells in culture and are not consistent with the hypothesis that formate from folate-dependent metabolism in mitochondria is a major one carbon source for purine biosynthesis. Timing of peak (13)C enrichment at C(2) corresponds to maximal DNA synthesis in human bone marrow. Phenotypes may explain the efficacy (or lack of) of certain anticancer and immunosuppressive drugs.
[Show abstract][Hide abstract] ABSTRACT: The ad-8 gene of Neurospora crassa, in addition to being used for the study of purine biology, has been extensively studied as a model for gene structure, mutagenesis and intralocus recombination. Because of this there is an extensive collection of well-characterized N. crassa ad-8 mutants in the Fungal Genetics Stock Center collection. Among these are spontaneous mutants and mutants induced with X-ray, UV or chemical mutagens. The specific lesions in these mutants have been genetically mapped at high resolution. We have sequenced the ad-8 locus from 13 of these mutants and identified the molecular nature of the mutation in each strain. We compare the historical fine-structure map to the DNA and amino acid sequence of each allele. The placement of the individual lesions in the fine-structure map was more accurate at the 5' end of the gene and no mutants were identified in the 3' untranslated region of this gene. We additionally analysed ad-8(+) alleles in 18 N. crassa strains subjected to whole-genome sequence analysis and describe the variability among Neurospora strains and among fungi and other organisms.
Journal of Genetics 08/2012; 91(2):199-204. DOI:10.1007/s12041-012-0175-1 · 1.09 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We determined whether ring-2 carbon of histidine is folate-dependently transferred to carbons 8 (C8) and/or 2 (C2) in urinary uric acid in humans. Two adults collected each urine void for four days. Aliquots of urine for the first day were used for baseline values; then the subjects ingested 0.7 g (3.3 mmol) of l-[ring-2-13C]histidine and collected urine for three experimental days. Aliquots were analyzed for percentage 13C-content at C2 and C8 by a liquid-chromatography-mass spectrometry method. Percentage enrichment was determined by subtracting time-of-day paired baseline percentage 13C-content from experimental percentage 13C-content for each void. C2 was predominantly 13C-enriched in the majority of voids. The percentage enrichments at C2 for two subjects were 0.14 (±0.028 [SEM], n = 26) and 0.18 (±0.049, n = 21), whereas at C8, they were 0.008 (±0.006) and -0.005 (±0.008), respectively. The mean C2-enrichments were significantly greater than zero (p < 0.01), whereas those of C8 were not (p > 0.2). The enrichment had a diurnal rhythm peaking in the morning. Our results may be useful in the estimation of the timing for the administration of drugs that interfere with purine nucleotide biosynthesis in the treatment of cancer and autoimmune disease.
[Show abstract][Hide abstract] ABSTRACT: Purine nucleotide biosynthesis de novo (PNB) requires 2 folate-dependent transformylases-5'-phosphoribosyl-glycinamide (GAR) and 5'-phosphoribosyl-5-aminoimidazole-4-carboxamide (AICAR) transformylases-to introduce carbon 8 (C8) and carbon 2 (C2) into the purine ring. Both transformylases utilize 10-formyltetrahydrofolate (10-formyl-H4folate), where the formyl-carbon sources include ring-2-C of histidine, 3-C of serine, 2-C of glycine, and formate. Our findings in human studies indicate that glycine provides the carbon for GAR transformylase (exclusively C8), whereas histidine and formate are the predominant carbon sources for AICAR transformylase (C2). Contrary to the previous notion, these carbon sources may not supply a general 10-formyl-H4folate pool, which was believed to equally provide carbons to C8 and C2. To explain these phenomena, we postulate that GAR transformylase is in a complex with the trifunctional folate-metabolizing enzyme (TFM) and serine hydroxymethyltransferase to channel carbons of glycine and serine to C8. There is no evidence for channeling carbons of histidine and formate to AICAR transformylase (C2). GAR transformylase may require the TFM to furnish 10-formyl-H4folate immediately after its production from serine to protect its oxidation to 10-formyldihydrofolate (10-formyl-H2folate), whereas AICAR transformylase can utilize both 10-formyl-H2folate and 10-formyl-H4folate. Human liver may supply AICAR to AICAR transformylase in erythrocytes/erythroblasts. Incorporation of ring-2-C of histidine and formate into C2 of urinary uric acid presented a circadian rhythm with a peak in the morning, which corresponds to the maximum DNA synthesis in the bone marrow, and it may be useful in the timing of the administration of drugs that block PNB for the treatment of cancer and autoimmune disease.
Advances in Nutrition 09/2015; 6(5):564-71. DOI:10.3945/an.115.008300 · 4.71 Impact Factor
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