Biosynthesis of methionine in mouse cells cultured in vitro

In the mouse cell-lines cultured in vitro, viz. L-cells and mouse embryo fibroblasts, the methylation of homocysteine to methionine is carried out by vitamin B12-dependent 5-methyltetrahydrofolate:L-homocysteine methyltransferase only. In these cells grown in the standard Eagle medium, the activity of another methyltransferase, which utilizes betaine as the methyl donor, was not detected. The high activity of the vitamin B12-dependent methionine synthetase is typical for mouse cells from the logarithmic phase of growth. In L-cells 60%, and in the mouse fibroblasts 30% of the enzyme exist in the holo-form; the ratio between the holo- and apoenzyme activity remains stable in cells from logarithmic and stationary cultures. The level of the activity of methionine synthetase strongly depends on the presence of vitamin B12, folate and methionine in the culture medium and is greater after prolonged contact of the cells with these agents.