HAT medium is commonly used in cell culture techniques, like hybridoma production, to select for specific cell populations. It contains Hypoxanthine, Aminopterin, and Thymidine.

Aminopterin Blocks De Novo Synthesis

Cells require nucleotides (building blocks for DNA and RNA) for survival and proliferation. They can synthesize these nucleotides via two main pathways:

• De novo pathway: A complex pathway that synthesizes nucleotides from simple precursors.
• Salvage pathway: A pathway that recycles pre-existing bases and nucleosides, requiring specific salvage enzymes.

Mechanism of Aminopterin Action

Aminopterin is added to the HAT medium specifically to target and block the de novo pathway of nucleotide synthesis.

• Aminopterin acts as a potent inhibitor of the enzyme dihydrofolate reductase (DHFR).
• DHFR is essential for regenerating tetrahydrofolate, a crucial cofactor required by several enzymes in the de novo pathway.
• By inhibiting DHFR, Aminopterin effectively halts the cell's ability to synthesize nucleotides via the de novo route.

This forces cells to rely solely on the salvage pathway. The selection works because only cells possessing a functional HGPRT (Hypoxanthine-guanine phosphoribosyltransferase) enzyme can utilize the hypoxanthine provided in the HAT medium via the salvage pathway. Cells lacking HGPRT cannot produce nucleotides through either pathway and therefore die.