13C NMR analysis of the use of alternative donors to the tetrahydrofolate-dependent one-carbon pools in Saccharomyces cerevisiae.

Serine is generally accepted as the major one-carbon donor in folate-mediated one-carbon metabolism in most cells. Previous work from our laboratory with the yeast Saccharomyces cerevisiae has demonstrated that glycine and formate can also provide one-carbon units. Under normal growth conditions, it is likely that cells utilize serine, glycine, formate, and perhaps other one-carbon donors simultaneously, but to differing degrees. In the present work, we have used 13C NMR to monitor how yeast cells distribute alternative, competing one-carbon sources into various pools. Cells were grown with [2-13C]glycine and unlabeled formate or folinic acid (leucovorin, 5-formyl-tetrahydrofolate) as competing one-carbon sources. The relative contribution of each one-carbon donor to the three oxidation states of the tetrahydrofolate-bound one-carbon pool [5-methyl-tetrahydrofolate (CH3-THF), 5,10-methylene-THF (CH2-THF), and 10-formyl-THF (10-CHO-THF)] was determined by analysis of two metabolic end products of one-carbon metabolism, choline and adenine. Glycine-derived 13C-labeled one-carbon units are incorporated into these two metabolites; dilution of the 13C indicates competition by the unlabeled one-carbon source. The results reveal that the contribution from formate, folinic acid, and glycine is different for each of the one-carbon pools. Formate competed most dramatically at the 10-CHO-THF pool, with decreasing competition into the CH2-THF and CH3-THF pools. In a mutant strain lacking cytosolic CH2-THF dehydrogenase activity, a distinct shift toward the use of glycine instead of formate as the source of one-carbon units for the more reduced pools (CH2-THF and CH3-THF) was observed, while 10-CHO-THF pools were not affected. In contrast, the formyl group of folinic acid competed almost exclusively at the 10-CHO-THF level, with barely detectable dilution of the CH2-THF and CH3-THF pools in wild-type cells. The mutant strain exhibited essentially identical results, confirming that 5-formyl-THF enters the active one-carbon pool at the level of 10-CHO-THF, presumably via 5,10-methenyl-THF. Furthermore, donation of one-carbon units by folinic acid was observed only when cells were depleted of THF by treatment with the dihydrofolate reductase inhibitor methotrexate. These results reveal that the state of equilibrium between one-carbon pools in a growing cell depends on the source of the one-carbon units. This work illustrates the power of 13C NMR for examining the in vivo utilization of alternative one-carbon donors under a variety of conditions.