Folate Activity of Human Lymphocytes Determined by Measurement of Serine Synthesis

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Abstract
A method has been developed for measuring the folate activity in isolated human lymphocytes by quantitating the incorporation of 14C-formate into serine. Isotope experiments showed that serine was formed from glucose and mainly catabolized to CO2, NH3, and two one-carbon fragments. 10% of serine is incorporated into protein and 5% is converted to metabolic acids. Small amounts were metabolized to Krebs cycle intermediates and other amino acids and also over ethanolamine and choline to glycine. Forcing the incorporation of 14C-formate into serine by adding 6.7 mmol/l glucose and 20 mmol/l glycine to the incubation medium gave in 24 experiments an incorporation of 9.2 ± 0.4% (S.E.M.) of added formate per 109 lymphocytes per 4 h. The incorporation was not influenced by added pyridoxal phosphate or tetrahydrofolic acid. The effect of added serine, homocysteine, and methionine on the formate incorporation was established in model experiments, but does not appear to be of significance for the clinical applicability of the method. In 29 experiments with lymphocytes from patients with folate deficiency, the average 14C-formate incorporation into serine was 4.4 ± 0.4% with practically no overlapping in the range of normal values. The extent of 14C-formate incorporation into methionine, RNA, and protein was also determined and found decreased in folate deficiency. The incorporated 14C-formate was recovered with 63% in free serine, 5% as protein-bound serine, 14% in methionine, and 18% in RNA. The same relative distribution of 14C-formate incorporation into the various products was found in folate deficiency states.