Figure 9.

In deregulated MTHFR mutant cells, SAM cycles in futile salvage pathways wherein both methionine and adenine are continuously utilized and recycled. A schematic for the metabolism of SAM in budding yeast. Major catabolism of SAM to SAH occurs by methylation of PE to PC, or SAM may also be consumed for the production of spermidine, spermine, and methylthioadenosine (MTA) via polyamine synthesis, the latter then enters a futile methionine salvage pathway producing adenine as a side-product. Relative intracellular levels of the two phospholipids intermediates involved in PE methylation and polyamines (spermidine and spermine) estimated from yeast transformants of MET13_WT or MET13_R357A in met13Δmet15Δ (ABC2613) that were grown overnight in minimal medium with amino acid supplements and GSH. These transformants were then reinoculated at 0.15 A₆₀₀ in fresh SD medium without any sulfur source. Methionine was added to the transformants after 3 h of secondary inoculation, and the samples were collected in triplicate at 1.0 OD. The graph here corresponds to the representative data set plotted using the average of three biological replicates and three technical replicates of each of these biological replicates along with ± S.D. values. Error bars indicate S.D. (n = 3). *, p < 0.05; **, p < 0.01; ***, p < 0.001; NS, not significant.