13C enrichment of carbons 2 and 8 of purine by folate-dependent reactions after [13C]formate and [2-13C]glycine dosing in adult humans

The 10-formyl moiety of 10-formyltetrahydrofolate is the source of carbons at the positions 8 (C8) and 2 (C2) of the purine ring, originating from formate and a few amino acids. Uric acid is the final catabolic product of purines. In adult humans, we independently measured the 13C enrichment of the C2 and C8 positions of urinary uric acid after an oral dose of [13C]sodium formate and that of the C2 and C8 plus C5 positions after [2-13C]glycine. A liquid chromatography-mass spectrometric method was used to measure the 13C enrichment of uric acid in urine, which was collected for 3 to 4 days. Purine catabolism to uric acid does not alter the positions of carbons in the ring. After the formate dose, the 13C enrichment at C2 was greater than at C8, and a circadian rhythm was observed in the enrichment at C2. After the glycine dose, the C8 plus C5 positions were enriched, whereas no significant enrichment at C2 was found. These 13C enrichment patterns are not consistent with previous accepted metabolism. To our knowledge, this is the first study to investigate 13C enrichment from formate and glycine independently into the C2 and C8 positions of purine in the same subjects. Possible mechanisms explaining our findings are discussed. Oral [13C]formate or [2-13C]glycine dosing and urine collection can be used to study purine biosynthesis in humans.