Folate trap occurs due to the deficiency of

The conversion of 5,10-methylene-THF into 5-methyl-THF is irreversible. The only way to make fuher use of 5-methyl-THF and to maintain the folate cycle is by the vitamin B12-dependent remethylation of homocysteine to methionine (regenerating THF). The methyl group transfer is therefore greatly dependent on 5-methylTHF and the availability of Vitamin-B12. In cases of vitamin-B12 deficiency, an intracellular deficiency of biologically active THF arises as methyl-THF is not conveed to THF and the folate gets trapped as methyl-THF. This situation is called a 'folate trap' (or methyl group trap). Reference: Harpers illustrated biochemistry 30th edition