Metabolism of 10-formyldihydrofolate in humans

Expression of Folate Pathway Genes in Stage III Colorectal Cancer Correlates with Recurrence Status Following Adjuvant Bolus 5-FU-Based Chemotherapy

Colorectal cancer is commonly treated with 5-fluorouracil and 5-formyltetrahydrofolate (leucovorin). Metabolic action of leucovorin requires several enzymatic steps that are dependent on expression of corresponding coding genes. To identify folate pathway genes with possible impact on leucovorin metabolism, a retrospective study was performed on 193 patients with stage III colorectal cancer. Relative expression of 22 genes putatively involved in leucovorin transport, polyglutamation and metabolism was determined in tumor and mucosa samples using quantitative real-time polymerase chain reaction. After surgery, patients received adjuvant 5-fluorouracil-based bolus chemotherapy with leucovorin during six months, and were followed for 3 to 5 years. Cox regression analysis showed that high tumoral expression of the genes SLC46A1/PCFT (proton-coupled folate transporter) and SLC19A1/RFC-1 (reduced folate carrier 1) correlated significantly (p < 0.001 and p < 0.01, respectively) with a decreased risk of recurrent disease, measured as disease-free survival (DFS). These two genes are involved in the transport of folates into the cells and each functions optimally at a different pH. We conclude that SLC46A1/PCFT and SLC19A1/RFC-1 are associated with DFS of patients with colorectal cancer and hypothesize that poor response to 5-fluorouracil plus leucovorin therapy in some patients may be linked to low expression of these genes. Such patients might need a more intensified therapeutic approach than those with high gene expression. Future prospective studies will determine if the expression of any of these genes can be used to predict response to leucovorin.

Occurrence, stability, and determination of formyl folates in foods

The B-vitamin folate has specific tasks as a one-carbon (C1) group supplier in the building and repair of DNA and RNA as well as in the methylation of homocysteine to methionine. Folate occurs in all living cells as a dynamic pool of several interconvertible forms carrying different C1 groups. Along the food chain, this dynamic pool of folates constantly changes due to either enzymatic or chemical interconversions during food processing and storage. These interconversions make it difficult to determine individual folate forms in foods. The formyl folates, the second most predominant forms of food folates, after 5-methyltetrahydrofolate, are particularly prone to interconvert at low pH. Today, this knowledge is often neglected, leading to risks for analytical underestimation of formyl folates. The purpose of the review is to explore the stability and interconversions of formyl folates in foods as well as to analyze the pitfalls in the determination of formyl folates.

Evidence for the hypothesis that 10-formyldihydrofolate is the in vivo substrate for aminoimidazolecarboxamide ribotide transformylase

We postulate that 10-formyl-7,8-dihydrofolate (10-HCO-H(2)folate), not 10-formyl-5,6,7,8-tetrahydrofolate (10-HCO-H(4)folate), is the predominant in vivo substrate for mammalian aminoimidazolecarboxamide ribotide (AICAR) transformylase, an enzyme in purine nucleotide biosynthesis de novo, which introduces carbon 2 (C(2)) into the purine ring. 10-HCO-H(2)folate exists in vivo as labeled 10-formyl-folic acid (10-HCO-folic acid: an oxidation product of 10-HCO-H(4)folate and 10-HCO-H(2)folate) and is found after doses of labeled folic acid in humans or laboratory animals. The bioactivity of the unnatural isomer, [6R]-5-formyltetrahydrofolate, in humans is explained by its in vivo conversion to 10-HCO-H(2)folate. The structure and active site of AICAR transformylase are not consistent with other enzymes that utilize 10-HCO-H(4)folate. Because 10-HCO-H(4)folate is rapidly oxidized in vitro to 10-HCO-H(2)folate by cytochrome C alone and in mitochondria, it is hypothesized that this process takes place in vivo. In vitro data indicate that 10-HCO-H(2)folate is kinetically preferred over 10-HCO-H(4)folate by AICAR transformylase and that this enzyme may not have access to sufficient supplies of 10-HCO-H(4)folate. Methotrexate blockage of the AICAR transformylase process in patients with rheumatoid arthritis suggests that dihydrofolate (H(2)folate) reductase is involved and is consistent with H(2)folate and 10-HCO-H(2)folate being the product and substrate for AICAR transformylase. The labeling of purine C(2) by an oral dose of [6RS]-5-H[(13)C]O-H(4)folate in a human subject is consistent with 10-H[(13)C]O-H(2)folate formation from unnatural isomer, [6R]-5-H[(13)C]O-H(4)folate, and it being a substrate for AICAR transformylase. In vitro exchange reactions of purine C(2) using H(4)folate coenzymes are not duplicated in vivo and is consistent with H(2)folate coenzymes being used in vivo by AICAR transformylase.

Folate and its preventive potential in colorectal carcinogenesis

Based on a 15-year old hypothesis, it is believed that an adequate ingestion of folate vitamins decreases, whereas a nutritional depletion of folate increases the risk of colorectal cancer. The present article reviews the efforts to provide biochemical and epidemiological evidence for folate as a chemopreventive agent against colorectal carcinogenesis. BIOLOGICAL EVIDENCE: Tetrahydrofolates govern the intracellular one-carbon metabolism and account for proper DNA biosynthesis and macromolecular modification. Numerous experimental studies traced different molecular pathways and tried to link folate depletion with DNA instability and/or mutagenesis. However, none of the proposed underlying molecular mechanisms appear clearly defined. EPIDEMIOLOGICAL EVIDENCE: Numerous case-control and prospective studies have been conducted on folate and colorectal cancer, which all together miss a clinical bottom line. The recommendation of folate intake to prevent colorectal cancer is therefore not evidence-based.