The remethylation of homocysteine to form methionine is catalyzed by the cytoplasmic enzyme 5-methyltetrahydrofolate-homocysteine S-methyltransferase
(EC 220.127.116.11), which is also called methionine synthase. This enzyme requires methylcobalamin (MeCbl), a derivative of cobalamin, or vitamin B12, for activity.The patient was a 6-month-old girl with mental retardation, megaloblastic anemia, and high folate activity in the serum and red cells.
For this reason and also because rodent and human forms of the enzyme were electrophoretically distinguishable, these workers could use binding to identify the presence of human methyltransferase (MTR) in rodent-human somatic cell hybrids. By this approach, they assigned the methyltransferase gene to chromosome 1.