Function of Dihydrofolate Reductase (DHFR)
Dihydrofolate reductase (DHFR) catalyzes the NADPH-dependent reduction of dihydrofolate to tetrahydrofolate (THF), which is the essential active form of folate required for DNA synthesis, cell division, and methylation reactions throughout the body. 1, 2
Primary Enzymatic Function
- DHFR converts dihydrofolate into tetrahydrofolate (THF) using NADPH as a cofactor, representing a critical step in folate metabolism 3, 2
- This enzyme is expressed primarily in the liver, though its activity is relatively limited, which has led some to suggest that synthetic folic acid (PteGlu) is less biologically active than naturally occurring folates 1
- The conversion to THF is essential because THF serves as the fundamental active cofactor form that functions in one-carbon metabolism as a methyl group donor 4
Critical Metabolic Roles
DNA Synthesis and Cell Replication:
- THF and its one-carbon adducts are required for de novo synthesis of purines and thymidylate, which are building blocks of DNA 2
- The folate-dependent enzyme thymidylate synthase, which requires 5,10-methylenetetrahydrofolate (5,10-MTHF), catalyzes the conversion of uracil into thymine 1
- Low intracellular concentrations of 5,10-MTHF result in uracil buildup, imbalance of the cellular deoxyuridine monophosphate:deoxythymidine monophosphate ratio, and misincorporation of uracil into DNA, leading to DNA deletions and chromosomal instability 1
Amino Acid Metabolism:
- THF is required for the metabolism of glycine, methionine, and serine 2
- In the cytoplasm, one-carbon metabolism is required for the remethylation of homocysteine to methionine 1
- In mitochondria, one-carbon metabolism is required for the synthesis of formylated methionyl-tRNA 1
Methylation Reactions:
- THF plays a critical role as a cofactor in methylation reactions throughout the body 1
- The transfer of one-carbon units appears to be the only function of folate coenzymes in the body 1
Clinical Significance
Consequences of DHFR Deficiency:
- Germline mutations causing DHFR deficiency result in megaloblastic anemia and/or pancytopenia, severe cerebral folate deficiency, and cerebral tetrahydrobiopterin deficiency 5
- DHFR deficiency can be corrected by treatment with folinic acid, which bypasses the deficient enzyme 5
- The link between DHFR and cerebral tetrahydrobiopterin metabolism affects formation of dopamine, serotonin, and norepinephrine, providing insight into neurological conditions including depression, Alzheimer disease, and Parkinson disease 5
Therapeutic Target:
- Because DHFR inhibition disrupts purine and thymidylate biosynthesis and DNA replication leading to cell death, it has been an attractive target for chemotherapy 2
- Clinically used DHFR inhibitors include methotrexate for cancer treatment and trimethoprim for bacterial infections 6
- The effectiveness of antifolate medications demonstrates the pivotal role DHFR plays in cell proliferation by depleting THF and slowing DNA synthesis 3
Common Pitfalls
- Failing to recognize that DHFR activity is relatively limited in the liver, making the conversion of synthetic folic acid less efficient than utilizing naturally occurring folates or already-active forms like 5-methyltetrahydrofolate 1
- Not understanding that DHFR polymorphisms can affect mRNA levels and enzyme expression, influencing folate status, disease susceptibility, and response to medications like methotrexate 3
- Overlooking the interdependence with vitamin B12, as B12 deficiency leads to functional folate deficiency through accumulation of 5-MTHF (the "folate trap"), even when DHFR function is normal 4