Fig. 3. Methionine synthase is a ubiquitous cytosolic enzyme that plays a crucial role in the generation of s-adenosylmethionine and in the folate cycle. Nitrous oxide inhibits cobalamin (vitamin B¹²) from acting as a coenzyme for methionine synthase and thus inhibits the cycle. 5-methyl-THF = 5-methyltetrahydrofolate; 5,10-methylene-THF = 5,10-methylenetetrahydrofolate; THF = tetrahydrofolate. Modified from Weimann  9 ; with permission from Elsevier.