Cysta, cystathionine; CBS, cystathionine ß-synthase; CH2THF, methylenetetrahydrofolate; CH3THF, methyltetrahydrofolate; MTHFR, methylenetetrahydrofolate reductase (provision of 5-methylfolate for homocysteine remethylation); MTR, methionine synthase (remethylation of homocysteine to methionine); MTRR, methionine synthase reductase (activiation of methionine synthase); THF, tetrahydrofolate; TCN2, transcobalamin-II (vitamin B12 transport).