FIGURE 4.

The percentage of dietary one-carbon groups that appears in formate. The maximum production of formate from dietary precursors was calculated from the dietary intake of serine, glycine, methionine, tryptophan, histidine, and choline, measured over the last 2 days of the experiment. These estimates were corrected by subtracting the quantity of each amino acid required for net protein synthesis (full details are provided under “Results”). They took into account the fact that a maximum of four one-carbon groups could be supplied by the catabolism of choline, two from serine, and one each from glycine, histidine, methionine, and tryptophan. The rate of endogenous formate production, measured isotopically, was then expressed as a percentage of the maximum rate of production of formate from these dietary precursors.