“…4): the methylcobalamin-dependent enzyme, methionine synthetase, (reaction 1) utilises 5-methyl-THF to form methionine from homocysteine; as said before, this reaction is also required to allow 5-methyl-THF, the plasma form of folate, to be converted into folate polyglutamates; methionine adenosyltransferase (reaction 2) converts methionine into S-adenosylmethionine (SAM), a high-energy compound that functions as the methyl donor in a series of vital methylating reactions, including the methylation of myelin in the brain. By inhibiting the irreversible reduction of 5,10-methylene-THF into 5-methyl-THF, catalysed by 5,10-methylene-THF reductase (reaction 3), and by stimulating methionine synthetase (see [27] and references therein), Sadenosylmethionine also avoids the trapping of THF in the form of 5-methyl-THF and allows its recycling and utilisation by the other reactions that require single carbon units. The low plasma concentration of methionine in our patient as well as the presence on the CSF amino acid chromatogram of a component that was tentatively identified as cystathionine, could thus be explained by an impairment of the reconversion of homocysteine into methionine, caused by a deficiency of 5-methyl-THF.…”