The effect of vitamin B
12(B
12)-deficiency on the activities of hepatic methionine synthase, homocysteine methyltransferase, and cystathionine β-synthase was investigated in rats. The rats bred from B
12-deficient dams were fed the B
12-deficient diets for 150 days after weaning. Growth retardation of the B
12-deficient rats was already observed on day 30 and continued through 150 days. But dietary supplementation of 0.5% DL-methionine slightly improved the growth retardation. Urinary excretion of methylmalonic acid increased to about 15 mg/mg creatinine and hepatic B
12 concentration declined to about 2 ng/g liver after a 150-day feeding of the B
12-deficient diets. Hepatic methionine synthase activity in rats fed the B
12-deficient diets supplemented with or without methionine decreased to about 5% of B
12-supplemented controls. Hepatic betaine-homocysteine methyltransferase activity showed no significant change caused by B
12-deficiency. Hepatic cystathionine β-synthase activity in rats fed the B
12-deficient diets supplemented with or without methionine decreased to about 61% and 27% of their B
12-supplemented controls, respectively, but the decrease was partially improved by methionine supplementation. In conclusion, the rats bred from B
12-deficient dams showed a severe B
12-deficiency after a 150-day feeding of the B
12-deficient diets. The decrease of hepatic cystathionine β-synthase activity was supposed to be due to the adaptation by the defect of methionine resynthesis.
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