Effect of Vitamin B₁₂ Deficiency on S-Adenosylmethionine Metabolism in Rats

The effect of vitamin B₁₂ (B₁₂) deficiency on the levels of S-adenosylmethionine (SAM) in tissues and the activities of hepatic methionine synthase, methionine adenosyltransferase and glycine N-methyltransferase were investigated. The striking depression of methionine synthase activity was observed in all rats fed the B₁₂-deficient diets with or without methionine supplementation for 150 days. The SAM level in liver was decreased by B₁₂ deficiency. However, brain SAM level was not affected. The activities of hepatic methionine adenosyltransferase isozymes, α-form and β-form, were decreased by B₁₂ deficiency. Hepatic glycine N-methyltransferase activity in rats fed the low methionine-B₁₂-deficient diet showed a tendency to lower, although the change the activity was not statistically significant, compared with B₁₂-supplemented rats. It is proposed that the fall in the activity of hepatic methionine adenosyltransferase may be one of the causes of the decreased hepatic SAM level in B₁₂-deficient rats.