The effect of vitamin B12 deficiency on methylfolate metabolism and pteroylpolyglutamate synthesis in human cells.

1. The uptake of 14C from [methyl-14C]methyltetrahydrofolate was significantly reduced in the phytohaemagglutinin (PHA)-stimulated lymphocytes from nine patients with untreated pernicious anaemia compared with the uptake in seven normal subjects. 2. The uptake of I4C from [14C]methyltetrahydrofolate by the lymphocytes from seven of the patients with pernicious anaemia was consistently increased by addition of vitamin B,, in vitro. 3. The proportion of 14C taken up from [‘4C]methyltetrahydrofolate transferred to non-folate compounds was found to be significantly reduced in the PHA-stimulated lymphocytes from nine patients with untreated pernicious anaemia compared with the proportion transferred in the PHA-stimulated lymphocytes from seven normal subjects. Addition of vitamin BIZ in vitro consistently increased the transfer in vitamin B,,-deficient cells but had no consistent effect in normal cells. 4. Normal and vitamin B,,-deficient PHA-stimulated lymphocytes took up [3H]folic acid and after 72 h incubation converted this largely into pteroylpolyglutamate forms. 5. The proportion of labelled lymphocyte folate as pteroylpolyglutamate after incubation with [3H]folic acid was the same in vitamin B,,-deficient as in normal lymphocytes and the proportion of pteroylpolyglutamates formed in vitamin BIZdeficient lymphocytes was unaffected by addition of vitamin B,, in uitro. 6. No radioactivity could be decteted in pteroylpolyglutamates after incubating normal PHA-stimulated lymphocytes with [14C]methyltetrahydrofolate for 72 h, suggesting that pteroylpolyglutamate forms of folate cannot be made directly from methyltetrahydrofolate. 7. These results are consistent with the ‘methyltetrahydrofolate trap’ hypothesis