Interrelationship between serum concentrations of methionine, vitamin B12 and folate

The metabolism and intracellular recycling of methionine requires vitamin B12 as cofactor and methyltetrahydrofolate as coreactant. To examine the interrelationship between serum levels of methionine, vitamin B12 and folate, serum methionine levels were assayed in patients with reduced levels of these vitamins and in healthy subjects. Approximately half the patients with either vitamin B12 or folate deficiency, and more than half those with combined deficiency, had methionine levels below the reference range (37-136 mumol/l). There was no correlation between vitamin B12 or folate and serum methionine levels within the healthy group. There was also no correlation within the group with low folate levels. Within the vitamin B12 deficient group on the other hand, the correlation was highly significant (r = 0.547, p less than 0.001). The finding that many patients with vitamin B12 or folate deficiency have normal serum levels of methionine suggests a significant role for other factors such as betaine-dependent methionine synthesis in the maintenance of serum methionine concentrations.