Effects of intravenous betaine on methionine-loading–induced plasma homocysteine elevation in rats

An intravenous methionine-loading model was characterized, and the suppressive effect of betaine on plasma homocysteine elevation induced by methionine loading was examined in rats. The plasma homocysteine concentrations significantly increased 5–120 minutes after 0.34 mmol/kg of methionine loading and then returned to the baseline within 240 minutes. Betaine was then intravenously administered at the same time as the methionine loading. The total increment of plasma homocysteine was assessed using the positive incremental area under the plasma homocysteine concentration curve over the 240-minute post–methionine-loading period (ΔAUC0-240). Betaine reduced ΔAUC0-240 dose-dependently: 81% of the control by 1.7 mmol/kg of betaine and 33% by 3.4 mmol/kg. The effects of glycine and methylglycine, analogues of betaine, were also investigated. As observed for betaine, methylglycine decreased ΔAUC0-240 to 44% of the control, whereas glycine showed no significant effect on ΔAUC0-240, indicating that methyl groups of betaine and dimethylglycine were necessary to suppress plasma homocysteine elevation. These results suggest that betaine contributes to the suppression of plasma homocysteine elevation by promoting homocysteine metabolism, and seems to work as a methyl donor.