Folate and pterin metabolism by cancer cells in culture.

Malignant cells grown in culture excrete into their growth medium a folate catabolite that can be seen as a blue-fluorescent region on paper chromatograms of such media. This folate catabolite has now been identified by paper chromatography, thin-layer chromatography, and combined gas chromatography-mass spectrometry as 6- hydroxymethylpterin and not as pterin-6-carboxaldehyde as previously reported. Moreover, when pterin-6-carboxaldehyde was added to the growth medium of logarithmically growing malignant cells, it was primarily reduced to 6-hydroxymethylpterin. In contrast pterin-6-carboxylate was the principal product formed from added pterin-6-carboxaldehyde by normal established cell lines in culture. These results have been interpreted as indicative of a possible mechanism of folate catabolism in malignant cells. Folic acid or another folate derivative is oxidatively cleaved at the C-9-N-10 bond to yield pterin-6-carboxaldehyde as one of the products. This derivative is subsequently reduced to 6-hydroxymethylpterin, which is excreted into the growth medium.