Diet-induced changes in hepatic betaine-homocysteine methyltransferase activity are mediated by changes in the steady-state level of its mRNA

Liver betaine-homocysteine methyltransferase (EC 2.1.1.5) activity)fluctuates with changes in the dietary intake of sulfur animo acids, choline, and betaine. The purpose of this study was to determine whether dietary-induced changes in the activity of hepatic betaine-homocysteine methyltransferase are mediated by changes in the level of its mRNA. The hepatic activity and mRNA content of betaine-homocysteine methyltransferase were measured in rats fed one of five amino acid-defined diets: basal (1 g/kg methionine, 3 g/kg cystine, 1.25 g/kg choline bitartrate), control (basal plus 2 g/kg methionine, betaine supplemented (basal plus 3 g/kg betaine), cystine supplemented (basal plus 3 g/kg cystine), or betaine and cystine supplemented (basal plus 3 g/kg betaine plus 3 g/kg cystine. The basal diet was deficient solely in methionine, and the control diet was adequate in all nutrients. When compared with rats consuming the control diet, rats fed the methionine-deficient diet exhibited a 4-fold increase in the steady-state level of betaine-homocysteine methyltransferase mRNA (p < 0.05). Betaine addition to the methionine-deficient diet elevated mRNA level even further, resulting in a nearly 10-fold higher mRNA levels compared with the methionine-adequate control diet (p < 0.05). Dietary cystine had no effect on betaine-homocysteine methyltransferase mRNA levels. Liver betaine-homocysteine methyltransferase activity mirrored its mRNA levels. We conclude that dietary-induced changes of liver betaine-homocysteine methyltransferase activity are mediated by changes in the steady-state levels of its mRNA.